Defect Report Summary for C11
Version 1.10

DR 400

DR 474 Prev <— Closed —> Next DR 401, or summary at top

Submitter: Nick Stoughton (US)
Submission Date: 2011-10-24
Source: Austin Group
Reference Document: N1559
Subject: realloc with size zero problems

Summary

There are at least three existing realloc behaviors when NULL is returned; the differences only occur for a size of 0 (for non-zero size, all three implementations set errno to ENOMEM when returning NULL, even though that is not required by C99).

AIX

realloc(NULL,0) always returns NULL, errno is EINVAL
realloc(ptr,0) always returns NULL, ptr freed, errno is EINVAL

BSD

realloc(NULL,0) only gives NULL on alloc failure, errno is ENOMEM
realloc(ptr,0) only gives NULL on alloc failure, ptr unchanged, errno is ENOMEM

glibc

realloc(NULL,0) only gives NULL on alloc failure, errno is ENOMEM
realloc(ptr,0) always returns NULL, ptr freed, errno unchanged

The Austin Group raised this matter with WG14 during earlier meetings (most notably during the London 2011 meeting). The direction from WG14 has led to The Austin Group aligning the POSIX text more closely to that in C99 and C1x as a part of TC1.

The behavior now required in POSIX is that implemented by BSD above. However, C99 has a loophole in implementation-defined behavior that still appears to permit AIX and glibc behaviors. The C1x draft carries the same wording loophole, so the planned course of action is to raise a defect against C1x once it completes standardization, where the outcome of that defect will either be that C1x tightens the wording to eliminate the loophole or relaxes the wording to align with existing practice. Therefore, the behavior of errno in Issue 8 should be deferred until after any C1x defect has been resolved.

If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

An implementation should not be allowed to define the behavior of returning a null pointer as a successful reallocation; if a null pointer is returned, then the orignal pointer has not been freed.

Suggested Change

If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned and errno set to indicate the error, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

Note Memory allocated by these functions should be freed via a call to free, and not by means of a realoc(p, 0).

Committee Discussion

• It was pointed out that the AIX behavior was not correct according to Rajan, in fact the behavior is that errno is not set.
• This change came out of a defect report on C99

Committee Discussion

• The current C Rationale needs to be edited.

Proposed Technical Corrigendum

In subsection 7.22.3 paragraph 1, change

If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, ...

to

If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned to indicate an error, ...

In subsection 7.22.3.5 (The realloc function), change the final sentence of paragraph 3 from

If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.

to

If size is non-zero and memory for the new object is not allocated, the old object is not deallocated. If size is zero and memory for the new object is not allocated, it is implementation-defined whether the old object is deallocated. If the old object is not deallocated, its value shall be unchanged.

In subsection 7.22.3.5 (The realloc function), change paragraph 4 from

The realloc function returns a pointer to the new object (which may have the same value as a pointer to the old object), or a null pointer if the new object could not be allocated.

to

The realloc function returns a pointer to the new object (which may have the same value as a pointer to the old object), or a null pointer if the new object has not been allocated.

Add to subsection 7.31.12 a new paragraph (paragraph 2):

Invoking realloc with a size argument equal to zero is an obsolescent feature.

DR 474 Prev <— Closed —> Next DR 401, or summary at top

DR 401

DR 400 Prev <— Closed —> Next DR 402, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject: "happens before" can not be cyclic

Summary

C++11 forbids "happens before" from being cyclic, but this change has not made its way into C11. In order to fix this, the following sentence (taken from C++ N3291, 1.10p12) should be added to 5.1.2.4p18:

The implementation shall ensure that no program execution demonstrates a cycle in the "happens before" relation.

NOTE: This cycle would otherwise be possible only through the use of consume operations.

Suggested Technical Corrigendum
See above.

Committee Discussion

• Seems as if C++ made this change at the last minute and WG 14 had already voted a document for ballot.
• There seems to be consensus to make a change along this line.

Proposed Technical Corrigendum

Add to 5.1.2.4p18:

The implementation shall ensure that no program execution demonstrates a cycle in the "happens before" relation.

NOTE: This cycle would otherwise be possible only through the use of consume operations.

DR 400 Prev <— Closed —> Next DR 402, or summary at top

DR 402

DR 401 Prev <— Closed —> Next DR 403, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject: memory model coherence is not aligned with C++11

Summary

If an operation A that modifies an atomic object M happens before an operation B that modifies M , then A shall be earlier than B in the modification order of M .

NOTE: The requirement above is known as write-write coherence.

If a value computation A of an atomic object M happens before a value computation B of M , and A takes its value from a side effect X on M, then the value computed by B shall either be the value stored by X or the value stored by a side effect Y on M, where Y follows X in the modification order of M.

NOTE: The requirement above is known as read-read coherence.

If a value computation A of an atomic object M happens before an operation B on M, then A shall take its value from a side effect X on M, where X precedes B in the modification order of M.

NOTE: The requirement above is known as read-write coherence.

If a side effect X on an atomic object M happens before a value computation B of M, then the evaluation B shall take its value from X or from a side effect Y that follows X in the modification order of M.

NOTE: The requirement above is known as write-read coherence.

Suggested Technical Corrigendum

Committee Discussion

• The consensus was that this is an oversight, and should be changed as recommended.

Proposed Technical Corrigendum

In 5.1.2.4 Paragraph 22 starting at the third sentence, add:

If an operation A that modifies an atomic object M happens before an operation B that modifies M , then A shall be earlier than B in the modification order of M .

NOTE: The requirement above is known as write-write coherence.

If a value computation A of an atomic object M happens before a value computation B of M , and A takes its value from a side effect X on M, then the value computed by B shall either be the value stored by X or the value stored by a side effect Y on M, where Y follows X in the modification order of M.

NOTE: The requirement above is known as read-read coherence.

If a value computation A of an atomic object M happens before an operation B on M, then A shall take its value from a side effect X on M, where X precedes B in the modification order of M.

NOTE: The requirement above is known as read-write coherence.

If a side effect X on an atomic object M happens before a value computation B of M, then the evaluation B shall take its value from X or from a side effect Y that follows X in the modification order of M.

NOTE: The requirement above is known as write-read coherence.

DR 401 Prev <— Closed —> Next DR 403, or summary at top

DR 403

DR 402 Prev <— Closed —> Next DR 404, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject: malloc() and free() in the memory model

Summary

Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.

Calls to these functions that allocate or deallocate a particular region of memory shall occur in a single total order, and each such deallocation call shall synchronise with the next allocation (if any) in this order.

Suggested Technical Corrigendum

Committee discussion

• The consensus was that this is an oversight, and should be changed along the lines that are recommended.

Proposed Technical Corrigendum

Replace last two sentences in 7.22.3 paragraph 2 with:

Calls to these functions that allocate or deallocate a particular region of memory shall occur in a single total order, and each such deallocation call shall synchronize with the next allocation (if any) in this order.

DR 402 Prev <— Closed —> Next DR 404, or summary at top

DR 404

DR 403 Prev <— Closed —> Next DR 405, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject: joke fragment remains in a footnote

Summary

"Atomic objects are neither active nor radioactive" and "Among other implications, atomic variables shall not decay".

Suggested Technical Corrigendum

Committee discussion

• It is not clear that rewording will make the footnote useful.

Proposed Technical Corrigendum

In 7.17.3 Paragraph 13, remove footnote 256.

DR 403 Prev <— Closed —> Next DR 405, or summary at top

DR 405

DR 404 Prev <— Closed —> Next DR 406, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject: The mutex specification

Summary

For purposes of determining the existence of a data race, these behave as atomic operations (1.10). The lock and unlock operations on a single mutex shall appear to occur in a single total order. [ Note: this can be viewed as the modification order (1.10) of the mutex. — end note ]

Prior calls to mtx_unlock on the same mutex shall synchronize with this operation.

For purposes of determining the existence of a data race, mutex calls behave as atomic operations. The lock and unlock operations on a single mutex shall appear to occur in a single total order.

NOTE: This can be viewed as the modification order of the mutex.

Suggested Technical Corrigendum

Committee discussion

• The quoted text was added to C++11 after WG 14 voted out the FDIS in London.
• The consensus was that this is probably an oversight, and should be changed along the lines that are recommended above.

Committee Discussion

• The wording in the proposed wording is not correct, should read "shall occur" rather than "shall appear to occur."
• No consensus reached on the words, Clark Nelson took action item to furnish words for the next meeting.

Committee Discussion

For purposes of determining the existence of a data race, lock and unlock operations behave as atomic operations. All lock and unlock operations on a particular mutex occur in some particular total order.

NOTE: This total order can be viewed as the modification order of the mutex.

Committee Discussion

Proposed Technical Corrigendum

For purposes of determining the existence of a data race, lock and unlock operations behave as atomic operations. All lock and unlock operations on a particular mutex occur in some particular total order.

NOTE: This total order can be viewed as the modification order of the mutex.

DR 404 Prev <— Closed —> Next DR 406, or summary at top

DR 406

DR 405 Prev <— Closed —> Next DR 407, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject: Visible sequences of side effects are redundant

Summary

The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every subsequent side effect, it is not the case that B happens before it. The value of an atomic object M, as determined by evaluation B, shall be the value stored by some operation in the visible sequence of M with respect to B.

Also propose changing 5.1.2.4p22 to the following:

The value of an atomic object M, as determined by evaluation B, shall be the value stored by some side effect A that modifies M, where B does not happen before A.

NOTE: The set of side effects that a given evaluation might take its value from is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below

NOTE: Although the rules for multi-threaded executions differ here from those of C++11, the executions they allow are precisely the same. Visible sequences of side effects are a redundant restriction.

Suggested Technical Corrigendum

Committee Discussion

• The changes seem reasonable, there is a concern about having C and C++ differ.
• Should be contingent on Defect 402.

Committee Discussion

• There was no work done on this defect between meetings.
• Seems more complicated than thought at the last meeting.
• It is not clear if WG 21 has considered these changes.
• Clark Nelson took an action item to liaise with WG 21.

Committee Discussion

This item has become WG21 Core issue 1466

Committee Discussion

There has been no discussion or action from WG21.

Committee Discussion

These changes have been proposed for the C++ working draft:

• For C 5.1.2.4 paragraph 2, the following C++ discussion is relevant:

  Change 1.10 paragraph 14 as follows:

  The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every side effect, it is not the case that B happens before it. The value of an atomic object M, as determined by evaluation B, shall be the value stored by some operation in the visible sequence of M with respect to B side effect A that modifies M, where B does not happen before A. [Note: It can be shown that the visible sequence of side effects of a value computation is unique given The set of side effects that a given evaluation might take its value from is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below. —end note]

• For C 5.1.2.4 paragraph 24, the following C++ discussion is relevant:

  1.10p20 should be changed as follows:

  [ Note: The visible sequence of side effects value observed by a load of an atomic depends on the "happens before" relation, which depends on the values observed by loads of atomics, which we are restricting here. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the "happens before" relation derived as described above, satisfy the resulting constraints as imposed here. —end note ]

• For C 5.1.2.4 paragraph 27, the following C++ discussion is relevant:

  I think 1.10p22 should be changed as follows:

  [ Note: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the "visible sequence"coherence rules. —end note ]

• For C 7.17.3 paragraph 6, the following C++ discussion is relevant:

  I believe the 29.3p3 wording should change as follows:

  • the result of the last modification A of M that precedes B in S, if it exists, or
  • if A exists, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst and that does not happen before A, or
  • if A does not exist, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst.

Apr 2014 meeting

Committee Discussion

WG21 liaison has been asked to ascertain status of this w.r.t. C++14 and to provide a suggested TC.

Oct 2014 meeting

Committee Discussion

A paper N1856 was provided that discusses the drift between the two Standards and a first cut at some possible wording changes, as follows. It was not, however, discussed, but does provide insight as to the necessary direction for a resolution to this DR.

1. Change 5.1.2.4 paragraph 22 as follows:

   The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every side effect, it is not the case that B happens before it. The value of an atomic object M, as determined by evaluation B, shall be the value stored by some operation in the visible sequence of M with respect to B side effect A that modifies M, where B does not happen before A. [Note: It can be shown that the visible sequence of side effects of a value computation is unique given The set of side effects that a given evaluation might take its value from is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below. —end note]

2. Change 5.1.2.4 paragraph 24 as follows:

   [Note: The visible sequence of side effects value observed by a load of an atomic depends on the “happens before” relation, which depends on the values observed by loads of atomics, which we are restricting here. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the “happens before” relation derived as described above, satisfy the resulting constraints as imposed here. —end note]

3. Change 5.1.2.4 paragraph 27 as follows:

   [Note: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the “visible sequence” coherence rules. —end note]

4. Change 7.17.3 paragraph 6 as follows:

   There shall be a single total order S on all memory_order_seq_cst operations, consistent with the “happens before” order and modification orders for all affected locations, such that each memory_order_seq_cst operation B that loads a value from an atomic object M observes one of the following values:

   • the result of the last modification A of M that precedes B in S, if it exists, or

   • if A exists, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst and that does not happen before A, or

   • if A does not exist, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst.

   [Note:...

Apr 2015 meeting

Committee Discussion

The provided words were accepted, with slight editorial changes, as the Proposed Technical Corrigendum.

Proposed Technical Corrigendum

Change 5.1.2.4 paragraph 22 from:

The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every subsequent side effect, it is not the case that B happens before it. The value of an atomic object M, as determined by evaluation B, shall be the value stored by some operation in the visible sequence of M with respect to B.

to:

The value of an atomic object M, as determined by evaluation B, shall be the value stored by some side effect A that modifies M, where B does not happen before A.

After 5.1.2.4 paragraph 22 add:

Note The set of side effects from which a given evaluation might take its value is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below.

Change 5.1.2.4 paragraph 24 from:

Note 11: The visible sequence of side effects depends on the “happens before” relation, which in turn depends on the values observed by loads of atomics, which we are restricting here. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the “happens before” relation derived as described above, satisfy the resulting constraints as imposed here.

to

Note 11: The value observed by a load of an atomic depends on the “happens before” relation, which in turn depends on the values observed by loads of atomics. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the “happens before” relation derived as described above, satisfy the resulting constraints as imposed here.

Change 5.1.2.4 paragraph 27 from:

Note 13: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the “visible sequence” rules.

to

Note 13: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the coherence requirements.

Change 7.17.3 paragraph 6 from:

There shall be a single total order S on all memory_order_seq_cst operations, consistent with the “happens before” order and modification orders for all affected locations, such that each memory_order_seq_cst operation B that loads a value from an atomic object M observes one of the following values:

• the result of the last modification A of M that precedes B in S, if it exists, or

• if A exists, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst and that does not happen before A, or

• if A does not exist, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst.

to:

There shall be a single total order S on all memory_order_seq_cst operations, consistent with the “happens before” order and modification orders for all affected locations, such that each memory_order_seq_cst operation B that loads a value from an atomic object M observes one of the following values:

• the result of the last modification A of M that precedes B in S, if it exists, or

• if A exists, the result of some modification of M that is not memory_order_seq_cst and that does not happen before A, or

• if A does not exist, the result of some modification of M that is not memory_order_seq_cst.

DR 405 Prev <— Closed —> Next DR 407, or summary at top

DR 407

DR 406 Prev <— Closed —> Next DR 408, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject:

Summary

For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that Ais sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M.

For atomic modifications A and B of an atomic object M, if there is a memory_order_seq_cst fence X such that A is sequenced before X, and X precedes B in S, then B occurs later than A in the modification order of M.

For atomic modifications A and B of an atomic object M, if there is a memory_order_seq_cst fence Y such that Y is sequenced before B, and A precedes Y in S, then B occurs later than A in the modification order of M.

Suggested Technical Corrigendum

Committee Discussion

• The changes are difficult to fully understand - a diagram might help.
• A paper for the next meeting would help the Committee make progress on this.
• Some concern about having C and C++ differ.

Committee Discussion

• No work was done on this defect between meetings, so no additional information has been provided.
• There seems to be two issues here, and this should possibility be two defect reports rather than one.
• No known liaison has taken place with WG 21 on this issue.
• The proposed words are inappropriate for C, still need words that work for C.
• There seems to be editorial issues with the proposed words for C++ as well.
• There is consensus to adopt something along the lines of the first proposed change, with wording that is consistent with C.
• WG 14 needs to stay in step with WG 21.
• Clark Nelson took an action item to liaise with WG 21.

Committee Discussion

• This item has also become WG21 Library Issue 2130.
• Rajan presented a diagram and argued that we should adopt the first part and that the second two parts were redundant.

• After 7.17.3 paragraph 11 add the following:

  For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M.

Committee Discussion

• The corresponding words in proposed working draft for C++ have changed, and the proposal above is no longer appropriate.
• The corresponding paragraph reference for C is still 7.13.3 paragraph 11.
• The following change has been applied to the C++ working draft:

  Change 29.3 [atomics.order] paragraph 7 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]

  -7- For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M. For atomic modifications A and B of an atomic object M, B occurs later than A in the modification order of M if:

  • there is a memory_order_seq_cst fence X such that A is sequenced before X, and X precedes B in S, or
  • there is a memory_order_seq_cst fence Y such that Y is sequenced before B, and A precedes Y in S, or
  • there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S.

Committee Discussion

WG21 liaison has been asked to ascertain status of this w.r.t. C++14 and to provide a suggested TC.

Oct 2014 meeting

Committee Discussion

A paper N1856 was provided that discusses the drift between the two Standards and a first cut at some possible wording changes, as follows. It was not, however, discussed, but does provide insight as to the necessary direction for a resolution to this DR.

1. [Drafting note: The project editor is kindly asked to consider to replace in 1.10 [intro.multithread] p17 the phrase "before an operation B on M" by "before a modification B of M".]

2. Change 7.17.3 paragraph 11 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]

   For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M. For atomic modifications A and B of an atomic object M, B occurs later than A in the modification order of M if:

   • there is a memory_order_seq_cst fence X such that A is sequenced before X, and X precedes B in S, or
   • there is a memory_order_seq_cst fence Y such that Y is sequenced before B, and A precedes Y in S, or
   • there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S.

   [ Note: memory_order_seq_cst ensures sequential consistency only for a program that is free of data races and uses exclusively memory_order_seq_cst operations. Any use of weaker ordering will invalidate this guarantee unless extreme care is used. In particular, memory_order_seq_cst fences ensure a total order only for the fences themselves. Fences cannot, in general, be used to restore sequential consistency for atomic operations with weaker ordering specifications. —end note ]

Apr 2015 meeting

Committee Discussion

The provided words were adopted as the Proposed Technical Corrigendum. The project editor is asked to review and replace the phrase "before an operation B on M" by "before a modification B of M".

Proposed Technical Corrigendum

Change 7.17.3 paragraph 11 from:

For atomic operations A and B on an atomic object M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M.

to:

For atomic modifications A and B of an atomic object M, B occurs later than A in the modification order of M if:

• there is a memory_order_seq_cst fence X such that A is sequenced before X, and X precedes B in S, or
• there is a memory_order_seq_cst fence Y such that Y is sequenced before B, and A precedes Y in S, or
• there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S.

DR 406 Prev <— Closed —> Next DR 408, or summary at top

DR 408

DR 407 Prev <— Closed —> Next DR 410, or summary at top

Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Subject: Should locks provide intra-thread synchronization

Summary

(x = 1)==(x = 2)

(lock; x = 1; unlock; x)==(lock; x = 2; unlock; x)

Suggested Technical Corrigendum

Committee Discussion

• The changes seem reasonable, but without actual text no position can be formed.
• A paper for the next meeting is probably the best way to make progress.

Committee Discussion

• Blaine Garst and the submitter worked on this between meetings, they believe this is not a problem.
• Convener prefers this remain OPEN for now, there could be additional text as discussed in Oct.
• C11 does define the semantics of a lock within a single thread.

Proposed Committee Response

• The expression (x = 1)==(x = 2) has undefined behavior in C11.
• The happens-before relationship imposed by (lock(), x = 1, unlock()) does not constrain the possible interwoven order of evaluation of (lock(), x = 2, unlock()) .

DR 407 Prev <— Closed —> Next DR 410, or summary at top

DR 409

DR 486 Prev <— Review —> Next DR 427, or summary at top

Submitter: Fred J. Tydeman (USA)
Submission Date: 2012-1-11
Source: WG 14
Reference Document: N1593
Subject: f(inf) is inf being a range error

Summary

Several of the functions in <math.h> that compute infinity for f(infinity) have the phrase (or something similar):

A range error occurs if the magnitude of x is too large.

Since infinity is 'too large', one might conclude that f(infinity) is a range error for those functions.

However, 7.12.1#5 has:

A floating result overflows if the magnitude of the mathematical result is finite but so large that the mathematical result cannot be represented without extraordinary roundoff error ...

The key word being 'finite'. So, one could conclude f(infinity) being infinity is not overflow (and therefore, not a range error).

To me, this appears to be a contradiction. I have encountered both kinds of implementations; some treat this case as a range error, and others that do not.

For both LIA and IEEE-754, f(infinity) being infinity is not considered an error.

Suggested Change

1. 7.12.5.4 The cosh functions

   Change to: A range error occurs if the magnitude of finite x is too large.

2. 7.12.5.5 The sinh functions

   Change to: A range error occurs if the magnitude of finite x is too large.

3. 7.12.6.1 The exp functions

   Change to: A range error occurs if the magnitude of finite x is too large.

4. 7.12.6.2 The exp2 functions

   Change to: A range error occurs if the magnitude of finite x is too large.

5. 7.12.6.3 The expm1 functions

   Change to: A range error occurs if the magnitude of finite x is too large.

6. 7.12.6.6 The ldexp functions

   Change to: A range error may occur for finite arguments.

7. 7.12.6.13 The scalbn and scalbln functions

   Change to: A range error may occur for finite arguments.

8. 7.12.7.3 The hypot functions

   Change to: A range error may occur for finite arguments.

9. 7.12.7.4 The pow functions

   Change to: A range error may occur for finite arguments.

10. 7.12.8.2 The erfc functions

    Change to: A range error occurs if finite x is too large.

11. 7.12.8.3 The lgamma functions

    Change to: A range error occurs if finite x is too large.

12. 7.12.8.4 The tgamma functions

    Change to: A range error occurs if the magnitude of finite x is too large and may occur if the magnitude of x is too small.

13. 7.12.12.1 The fdim functions

    Change to: A range error may occur for finite arguments.

14. 7.12.13.1 The fma functions

    Change to: A range error may occur for finite arguments.

Committee Discussion

• The committee rejected the Suggested Change in the main body of this defect report.
• The committee considered the following, but rejected it (as just being a restatement of 7.12.1 paragraphs 4 and 5).

  If the result overflows, a range error shall occur.

• A question arose as to why these range error cases are listed in the individual functions (instead of just being covered by the blanket 7.12.1 paragraphs 4, 5, and 6)

  7.12.1 paragraph 1 has the answer:

  The behavior of each of the functions in <math.h> is specified for all representable values of its input arguments, except where stated otherwise.

• Several other approaches were discussed, without any consensus reached
  1. Add a footnote to 7.12.1 paragraph 5, first sentence:

     In an implementation that supports infinities, a range error may happen for functions that map an infinity argument into an exact infinity or exact zero result.

  2. Add to end of 7.12.1 paragraph 4:

     Recommended practice

     In an implementation that supports infinities, a range error should not happen for functions that map an infinity argument into an exact infinity or exact zero result.

  3. Add to 7.12.1 paragraph 4:

     An implementation may define additional range errors, provided that such errors are consistent with the mathematical definition of the function.

Committee Discussion

• Fred wrote a paper N1629 discussing "Fixing a contradiction" and "Taking care of infinity".
• The definition of range error, however, in 7.12.1 paragraph 4 indicates infinity is excluded (since it has a representation), and as such no change is required.

Committee Discussion

Fred presented another paper N1979 noting an error in the October 2012 committee response, and after discussion, the proposed clarification was adopted, and is as follows

Proposed Committee Response

The definition of range error in 7.12.1 paragraph 4 excludes infinity.

For example, exp(+infinity) is +infinity. Since the input +infinity is representable, then the output +infinity is representable in an object of the specified type. By, 7.12.1 paragraph 4, a range error has not happened. Also, by 7.12.1 paragraph 5, since the result is not finite, overflow has not happened.

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DR 410

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Submitter: Fred J. Tydeman (USA)
Submission Date: 2012-1-11
Source: WG 14
Reference Document: N1595
Subject: ilogb inconsistent with lrint, lround

For the case of converting a large finite value to an integer value, lrint and lround have one set of requirements, while ilogb has another set. This is inconsistent.

Both 7.12.9.5 The lrint and llrint functions and 7.12.9.7 The lround and llround functions have:

If the rounded value is outside the range of the return type, the numeric result is unspecified and a domain error or range error may occur.

While 7.12.6.5 The ilogb functions has:

If the correct value is outside the range of the return type, the numeric result is unspecified.

I believe that the following changes to C11 should be done.

1. 7.12.6.5 The ilogb functions:

   Change to: If the correct value is outside the range of the return type, the numeric result is unspecified and a domain error or range error may occur.

Committee discussion

• Some believe the rationale presented as a reason for doing this is not a valid rationale for the change.

Proposed Technical Corrigendum

In 7.12.6.5 paragraph 2, change the last sentence to:

If the correct value is outside the range of the return type, the numeric result is unspecified and a domain error or range error may occur.

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DR 411

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Submitter: Project Editor (Larry Jones)
Submission Date: 2012-01-18
Source: Project Editor
Reference Document: N/A
Subject: Predefined macro values

Summary

The actual values for the predefined macros __STDC_VERSION__ and __STDC_LIB_EXT1__ should be specified.

Suggested Technical Corrigendum

Change the relevant list entry in 6.10.8.1 to:

__STDC_VERSION__ The integer constant 201112L.

Change the relevant list entry in 6.10.8.3 to:

__STDC_LIB_EXT1__ The integer constant 201112L.

Committee Discussion

• The committee asked the Convener to look into making this an errata if possible.

Proposed Technical Corrigendum

Change 6.10.8.1 from:

__STDC_VERSION__ The integer constant201ymmL.¹⁷⁸⁾

to:

__STDC_VERSION__ The integer constant 201112L.¹⁷⁸⁾

Change 6.10.8.3 from:

__STDC_LIB_EXT1__ The integer constant201ymmL, intended to indicate support for the extensions defined in annex K (Bounds-checking interfaces).¹⁷⁹⁾

to:

__STDC_LIB_EXT1__ The integer constant 201112L, intended to indicate support for the extensions defined in annex K (Bounds-checking interfaces).¹⁷⁹⁾

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DR 412

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Submitter: Edward Diener (comp.std.c)
Submission Date: 2012-01-18
Source: Project Editor (Larry Jones)
Reference Document: N/A
Subject: #elif

Summary

It appears that #elif is not entirely equivalent to #else, #if, and #endif.

Consider the code:

#if 1
...
#else
#if this is not a valid expression
...
#endif
#endif

This is well-defined. Since the controlling expression of the #if evaluates to true, the #else group is skipped and thus the nested #if is only processed through the directive name (6.10.1p6).

However, if this is recast using #elif:

#if 1
...
#elif this is not a valid expression
...
#endif

the #elif is not part of a group that is skipped and thus must be processed completely, including evaluating the controlling condition (even though the resulting value is of no interest).

I do not believe this was the committee's intent.

Suggested Technical Corrigendum

In 6.10.1p6, change:

Only the first group whose control condition evaluates to true (nonzero) is processed.

to:

Only the first group whose control condition evaluates to true (nonzero) is processed; any following groups are skipped and their controlling directives are processed as if they were in a group that is skipped.

Proposed Technical Corrigendum

In 6.10.1p6, change:

Only the first group whose control condition evaluates to true (nonzero) is processed.

to:

Only the first group whose control condition evaluates to true (nonzero) is processed; any following groups are skipped and their controlling directives are processed as if they were in a group that is skipped.

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DR 413

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Submitter: Willem Wakker
Submission Date: 2012-01-27
Source: WG14
Reference Document: N1601
Subject: initialization

Summary

typedef struct {
int k;
int l;
int a[2];
} T;

typedef struct {
int i;
T t;
} S;

T x = {.l = 43, .k = 42, .a[1] = 19, .a[0] = 18 };

void f(void)
{
S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
}


The question is: what is now the value of l.t.k? Is it 42 (due to the initialization of .t = x) or is it 0 (due to the fact that .t.l starts an incomplete initialization of .t?

19 The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject;¹⁵¹) all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

Suggested Technical Corrigendum

Committee Discussion

• It was noted that this is basically similar to dr_253.
• The following was proposed, but there was no consensus for adoption.

  The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject ¹⁵¹⁾. Subsequently, all subobjects that are not initialized explicitly previously shall be initialized implicitly the same as objects that have static storage duration.

Committee Discussion

• The original author intended the result to be 42 by the following reasoning:
• 6.7.9 paragraphs 17-18 specify that each designator list affects only the smallest subobject to which the designator list refers. As a result, the second clause of paragraph 19 occurs once for the greater object as a whole, filling in only those parts of the whole object that were never initialized explicitly.
• gcc and some IBM compilers give the result as 0, although it is not believed that there is code dependent on this interpretation.
• David Keaton proposed N1659.
• This, however, does not clarify the interpreted conflict of paragraph 19 "subobjects that are not initialized explicitly [shall be set to zero]" applied "recursively to subaggregates" (paragraph 20).
• Adding the example is a desired outcome.

Committee Discussion

There was no work performed on this DR.

Although both GCC and six compilers from IBM provide the unintended answer, it is believed to be such a rarely used feature that it is not depended upon to a great degree, and the compiler venders are willing to change their behavior appropriately.

Committee Discussion

There has been considerable discussion and several proposals ( N1659, N1749) to clarify the standard to no avail. Upon reflection, and consultation with the author, we believe that the proper course of action is twofold. First, simply answer the question asked as the committee believes that the standard already specifies correctly. To add clarification to the standard we will also add the examples from N1659 that leads to this answer.

Proposed Committee Response

Proposed Committee Corrigendum

Add the following example to 6.7.9:


    typedef struct {
        int k;
        int l;
        int a[2];
    } T;

    typedef struct {
        int i;
        T t;
    } S;

    T x = {.l = 43, .k = 42, .a[1] = 19, .a[0] = 18 };

    void f(void)
    {
        S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
    }


The value of l.t.k is 42, because implicit initialization does not override explicit initialization.

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Submitter: Tom Plum
Submission Date: 2012-03-20
Source: WG14
Reference Document: N1614
Subject: Typos in 6.27 Threads <threads.h>

The enumeration constants are

mtx_plain

which is passed to mtx_init to create a mutex object that supports neither timeout nor test and return;

The mtx_init function creates a mutex object with properties indicated by type,
which must have one of the six values:
mtx_plain for a simple non-recursive mutex,
mtx_timed for a non-recursive mutex that supports timeout,
mtx_plain | mtx_recursive for a simple recursive mutex, or
mtx_timed | mtx_recursive for a recursive mutex that supports timeout.

Suggested Technical Corrigendum

The enumeration constants are

mtx_plain

which is passed to mtx_init to create a mutex object that does not support timeout;

The mtx_init function creates a mutex object with properties indicated by type,
which must have one of the these values:

Committee Discussion

Adopt the Suggested Technical Corregendum as proposed.

Proposed Technical Corrigendum

The enumeration constants are

mtx_plain

which is passed to mtx_init to create a mutex object that does not support timeout;

The mtx_init function creates a mutex object with properties indicated by type,
which must have one of the these values:

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DR 415

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Submitter: Seacord (PL22.11)
Submission Date: 2012-06-02
Source: WG14
Reference Document: N1617
Subject: Missing divide by zero entry in Annex J

Summary

The undefined behavior defined in paragraph 6 of 6.5.5 is missing in J.2 and should be added.

Suggested Technical Corrigendum

Add a bullet with the following text to J.2 after bullet 45

If the quotient a/b is not representable, the behavior of both a/b and a%b is undefined (6.5.5).

Committee Discussion

Adopt the Suggested Technical Corregendum as proposed.

Proposed Technical Corrigendum

Add a bullet with the following text to J.2 after bullet 45

If the quotient a/b is not representable, the behavior of both a/b and a%b is undefined (6.5.5).

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DR 416

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Submitter: Owen Shepherd
Submission Date: 2012-08-12
Source: WG14
Reference Document: N1627
Subject: tss_t destruction unspecified

Summary

The standard does not specify if or when destructors for thread specific data keys (created with the tss_create function) are invoked.

This proposal suggests to align the behavior with that specified by POSIX for pthread_key_t. This behavior is hoped to both match the intention of the standard, and be possible to implement on other systems (it is noted that a pthreads implementation exists for Windows, for example, while the behavior of POSIX and Windows thread local storage implementations differ greatly)

Suggested Technical Corrigendum

After 7.26.5.1p2, add

Returning from func shall have the same behaviour as invoking thrd_exit with the returned value

Change 7.26.5.5 parts 2 and 3 from

The thrd_exit function terminates execution of the calling thread and sets its result code to res.

The program shall terminate normally after the last thread has been terminated. The behavior shall be as if the program called the exit function with the status EXIT_SUCCESS at thread termination time.

to

For every thread specific storage key which was created with a non-NULL destructor and for which the value is non-NULL, thrd_exit shall set the value associated with the key to NULL and then invoke the destructor with its previous value. The order in which destructors are invoked is unspecified.

If after this process there remain keys with both non-NULL destructors and values, the implementation shall repeat this process up to TSS_DTOR_ITERATIONS times.

If thrd_exit is not being invoked in the last thread in the process, then the implementation shall terminate execution of the calling thread and set its result code to res. Otherwise, the implementation shall behave as if exit(EXIT_SUCCESS) was invoked.

(This change provides application writers guarantees about the identity of the thread executing functions registered with atexit)

After 7.26.6.1p2, add

The value NULL shall be associated with the newly created key in all existing threads. Upon thread creation, the value associated with all keys shall be initialized to NULL

Note that destructors associated thread specific storage are not invoked at process exit.

To 7.26.6.2p2, append

If tss_delete is called while another thread is executing destructors, whether this will affect the number of invocations of the destructor associated with key on that thread is unspecified. If the thread from which tss_delete is invoked is executing destructors, then no further invocations of the destructor associated with key will occur on said thread.

Calling tss_delete will not result in the invocation of any destructors.

After 7.26.6.4p2, add

This action will not invoke the destructor associated with the key on the value being replaced.

Committee Discussion

• This paper was discussed in conjunction with DR424 which covers one additional related issue.
• The key observation is that these papers are about the deliberate underspecification of threads, since that allows the greatest opportunity for implementation on a variety of operating systems.
• Pete Becker, an implementor of the C11 threads library, was asked about these papers and replied in SC22/WG14 message 12813.
• Based on that response the committee is concerned that there could be subtleties in adopting the Proposed Technical Corrigendum and that, as such, these changes are substantial enough to warrant a proposal and to not be considered a defect.

Committee Discussion

• A revised paper N1687 was submitted, although not in the recommended html format.
• The suggested technical corrigendum would specify new instances of undefined behavior as well as new requirements on implementations.
• The committee expressed concern that some of the new requirements may be onerous or impossible to provide above the native platform implementation.

Committee Discussion

After several papers N1750, N1751, revisions, and much discussion, the committee has agreed on the following as the resolution.

Proposed Technical Corrigendum

After 7.26.5.1 paragraph 2, add

Returning from func shall have the same behavior as invoking thrd_exit with the value returned from func.

For every thread-specific storage key which was created with a non-null destructor and for which the value is non-null, thrd_exit shall set the value associated with the key to NULL and then invoke the destructor with its previous value. The order in which destructors are invoked is unspecified.

If after this process there remain keys with both non-null destructors and values, the implementation shall repeat this process up to TSS_DTOR_ITERATIONS times.

Following this, the thrd_exit function terminates execution of the calling thread and sets its result code to res.

The value NULL shall be associated with the newly created key in all existing threads. Upon thread creation, the value associated with all keys shall be initialized to NULL.

Destructors associated with thread-specific storage are not invoked at program termination.

A call to tss_create from within a destructor results in undefined behavior.

A call to tss_delete function results in undefined behavior if the call to tss_create which set key completed after the thread commenced executing destructors.

If tss_delete is called while another thread is executing destructors, whether this will affect the number of invocations of the destructor associated with key on that thread is unspecified.

Calling tss_delete will not result in the invocation of any destructors.

A call to tss_get function results in undefined behavior if the call to tss_create which set key completed after the thread commenced executing destructors.

This action will not invoke the destructor associated with the key on the value being replaced.

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DR 417

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Submitter: John Benito
Submission Date: Oct 2012
Source: WG14
Reference Document: N1628
Subject: Annex J not updated with necessary aligned_alloc entries

Summary

— The order and contiguity of storage allocated by successive calls to the calloc, malloc, and realloc functions (7.22.3).

— The amount of storage allocated by a successful call to the calloc, malloc, or realloc function when 0 bytes was requested (7.22.3).

— A non-null pointer returned by a call to the calloc, malloc, or realloc function with a zero requested size is used to access an object (7.22.3).

— Whether the calloc, malloc, and realloc functions return a null pointer or a pointer to an allocated object when the size requested is zero (7.22.3).

Suggested Technical Corrigendum

— The order and contiguity of storage allocated by successive calls to the calloc, malloc, realloc, and aligned_alloc functions (7.22.3).

— The amount of storage allocated by a successful call to the calloc, malloc, realloc, or aligned_alloc function when 0 bytes was requested (7.22.3).

— A non-null pointer returned by a call to the calloc, malloc, realloc, or aligned_alloc function with a zero requested size is used to access an object (7.22.3).

— Whether the calloc, malloc, realloc and aligned_alloc functions return a null pointer or a pointer to an allocated object when the size requested is zero (7.22.3).

Committee Discussion

Adopt as proposed the Suggested Technical Corrigendum.

Proposed Technical Corrigendum

— The order and contiguity of storage allocated by successive calls to the calloc, malloc, realloc, and aligned_alloc functions (7.22.3).

— The amount of storage allocated by a successful call to the calloc, malloc, realloc, or aligned_alloc function when 0 bytes was requested (7.22.3).

— A non-null pointer returned by a call to the calloc, malloc, realloc, or aligned_alloc function with a zero requested size is used to access an object (7.22.3).

— Whether the calloc, malloc, realloc and aligned_alloc functions return a null pointer or a pointer to an allocated object when the size requested is zero (7.22.3).

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DR 418

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Submitter: Fred J. Tydeman (USA)
Submission Date: 2012-9-13
Source: WG14
Reference Document: N1633
Related: N1497
Subject: Possible defect report: fmod(0.,NaN) and fmod(NaN,infinity)

First question. When Annex F is in effect, what should the value of fmod(0.,NaN) be? The two choices are 0. or NaN.

Annex F.10.7.1 The fmod functions has:

• fmod(+/-0, y) returns +/-0 for y not zero.
• fmod(x, y) returns a NaN and raises the ''invalid'' floating-point exception for x infinite or y zero (and neither is a NaN).

So, that first bullet item says fmod(0.,NaN) is 0.

Elsewhere in that annex (F.10 Mathematics, paragraph 11), we have:

Functions with a NaN argument return a NaN result and raise no floating-point exception, except where stated otherwise.

That says that fmod(0.,NaN) is NaN.

One idea is to explicitly add words about a NaN to the first bullet item in F.10.7.1, such as:

• fmod(+/-0, y) returns +/-0 for y not zero nor NaN.

However, if F.10#11 covers NaN arguments before any other arguments are considered, then words about NaN could be removed from the second case in F.10.7.1, such as:

• fmod(x, y) returns a NaN and raises the ''invalid'' floating-point exception for x infinite or y zero.

I believe that takes us back to before N1497 was done.

Second question: what should fmod(NaN,infinity) be? Must it be the same NaN argument, or may it be any NaN?

Annex F.10.7.1 The fmod functions has:

• fmod(x, +/-infinity) returns x for x not infinite.

Which says fmod(NaN,infinity) must be the same NaN argument.

But, if F.10#11 covers this NaN argument, then this case is just some NaN.

It appears that the third bullet should either be left alone or changed to:

• fmod(x, +/-infinity) returns x for finite x.

Some other functions that discuss NaN arguments in Annex F are: frexp, ilogb, modf, hypot, pow, fmax, fmin, and fma. Of those, only hypot, pow, fmax, and fmin have exceptions on NaN in implies NaN out.

Committee Discussion

• Given that, unless stated otherwise, if the argument is a NaN, the result is a NaN, and there is no floating point exception.
• The parenthetical comment might be causing confusion, and removing it seems to make it more confusing.
• Adding more explicit wording is possible but seems unnecessary.

Proposed Committee Response

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DR 419

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Submitter: Douglas Walls
Submission Date: 2012-09-16
Source:WG14
Reference Document: N1635
Subject: Generic Functions

Summary

I was trying to reconcile the rules in 7.1.4 Use of library functions allowing an implementation to define a function as a macro, and the user suppressing the macro by enclosing the name of the function in parentheses. But, I don't see how to make a function declaration, where a parameter can be any atomic type.

I've convinced myself, generic functions will take compiler magic. There is no way to declare them using C standard conforming code. Just like the type generic macros of <tgmath.h> in C99.

Somehow I missed this. I remember all the discussion of adding atomic operation to operators like += but somehow I missed the fact we were again adding in function specifications that cannot be implemented using standard C. I thought we were adding type generic macros. Sigh ...

I've been told that the discussion included talk about a proposal to recast them as generic macros, but that never happened so we ended up with generic functions through the back door without much explication.

Suggested Technical Corrigendum

Redefine the atomic type generic functions as type generic macros. Define the underlying functions to which the type generic macros expand.

Proposed Technical Corrigendum

7.17.1 add a new paragraph after paragraph 5:

It is unspecified whether any generic function declared in stdatomic.h is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name of a generic function, the behavior is undefined.

J.2 add:

The macro definition of a generic function is suppressed in order to access an actual function (7.17.1)

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DR 420

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Submitter: Jens Gustedt
Submission Date: 2012-10-08
Source: WG14
Reference Document: N1647
Subject: sytax error in specification of for-statement

Summary

The standard lists two different forms for the for-statement in 6.8.5p1:

for ( expression[opt] ; expression[opt] ; expression[opt] ) statement
for ( declaration expression[opt] ; expression[opt] ) statement

whereas later in 6.8.5.3 these two forms are subsumed in a third form by:

for ( clause-1 ; expression-2 ; expression-3 ) statement

Obviously the second form above is a typo and doesn't fit within this third form, the semantic that is given in 6.8.5.3 and to common practice in existing compilers.

Suggested Technical Corrigendum

Replace the second form in 6.8.5p1 and A.2.3 by the intended one:

for ( declaration expression[opt] ; expression[opt] ; expression[opt] ) statement

Proposed Committee Response

The second form of for-statement is not a typo. The syntax for "declaration", in 6.7 paragraph 1, includes an optional init-declarator-list and a trailing semicolon.

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DR 421

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Submitter: Jens Gustedt
Submission Date: 2012-10-08
Source: WG14
Reference Document: N1648
Subject: initialization of atomic_flag

Summary

C11 expresses the intention to have atomic_flag as a primitive that should allow to emulate all other atomic types and operations, 7.17.8 p3 in a note says:

The remaining types can be emulated with atomic_flag, though with less than ideal properties.

With the current semantic for the initialization of atomic_flag this goal cannot be achieved.

Details

This is a very good concept as far as I can see, but I have one problem to achieve this, initialization. The phrasing for atomic types in general and for atomic_flag are different. For atomic_flag we have:

An atomic_flag that is not explicitly initialized with ATOMIC_FLAG_INIT is initially in an indeterminate state.

The problem is how to emulate an atomic type with atomic_flag during initialization. Say we emulate with something like

struct atomic_int_struct {
  atomic_flag cat;
  int val;
};

Then the ATOMIC_VAR_INIT macro could simply look like:

#define ATOMIC_VAR_INIT(V) { .cat = ATOMIC_FLAG_INIT, .val = (V), }

But if I’d have a variable of atomic_int_struct with static storage duration

struct atomic_int_struct v;

is supposed to do the right thing, namely to guarantee that v has a valid state at startup, so it should contain a 0 for .val, and .cat must be in a determinate state. Since all atomic operations should work without problems on v, the state of .cat can’t be anything else than “clear”.

Now looking into the possible implementations of atomic_flag in assembler I didn’t yet meet a processor where the “clear” state would not be naturally represented by an all 0 value. So I guess in any reasonable implementation we would just have

#define ATOMIC_FLAG_INIT { 0 }

(or some equivalent formulation.)

If this is so, why ATOMIC_FLAG_INIT at all? Why not phrasing the same as for the other atomic types

Suggested Technical Corrigendum

Eliminate the mention of ATOMIC_FLAG_INIT in 7.17.1p3, B.16 and the index.

Proposed change for the initialization of atomic_flag, 7.17.8p4:

The default initializer { 0 } may be used to initialize an atomic_flag to the clear state. An atomic_flag object with automatic storage duration that is not explicitly initialized using { 0 } is initially in an indeterminate state; however, the default (zero) initialization for objects with static or thread-local storage duration initializes an atomic_flag to the clear state.
EXAMPLE
atomic_flag guard = { 0 };

If the committee would want to keep the macro ATOMIC_FLAG_INIT arround, a partial alternative to the above text would be to modify the text in 7.17.1

ATOMIC_FLAG_INIT
which expands to a default initializer ({ 0 } or equivalent) for an object of type atomic_flag.

Committee Discussion

• The standard deliberately does not specify values for the clear and set states of atomic_flag objects in order to support the widest possible set of architectures.
• The change suggested could be viewed as a proposal, but should be submitted as such, and since C11 shares the same memory model as C++11 a corresponding proposal should be submitted to WG21 as well.

Proposed Committee Response

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Submitter: Jens Gustedt
Submission Date: 2012-10-08
Source: WG14
Reference Document: N1649
Subject: initialization of atomic types

Summary

The current version of the standard doesn't specify to which value an atomic object should be initialized if it is initialized by default.

An atomic object with automatic storage duration that is not explicitly initialized using ATOMIC_VAR_INIT is initially in an indeterminate state; however, the default (zero) initialization for objects with static or thread-local storage duration is guaranteed to produce a valid state.

The mentioned valid state (in contrast to the indeterminate state mentioned before) is thus a determinate state, but the value that is stored is not mentioned explicitly. In the introduced language of the standard it is no definition of a "determinate state". It could be an "implementation-defined value", just an "unspecified value" or a default (zero) initialization. Everything suggests the later, that this would be the same value as for initializing a variable of the underlying base type by { 0 }. But I think it would have helped to make that explicit.

Suggested Technical Corrigendum

Proposed change for the initialization of atomic objects, 7.17.2.1p2:

An atomic object with automatic storage duration that is not explicitly initialized using ATOMIC_VAR_INIT is initially in an indeterminate state; however, the default (zero) initialization for objects with static or thread-local storage duration is guaranteed to produce a valid state that corresponds to the value of a zero initialized object of the unqualified base type.
EXAMPLE All three of the following objects initially have an observable value of 0.
_Atomic(unsigned) A = { 0 };
_Atomic(unsigned) B = ATOMIC_VAR_INIT(0u);
static _Atomic(unsigned) C;

Committee Discussion

• ATOMIC_VAR_INIT is required to initialize an atomic object to a known value.
• The default value for an atomic object is defined to be valid but is unspecified.
• The committee does not see this as a defect.
• N1649 proposes that the default value be specified to be the same as the non-atomic type’s default value.
• Such a proposal should be submitted as such and may also need to be submitted to and addressed by WG21 (C++) as well.

Proposed Committee Response

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Submitter: Jens Gustedt
Submission Date: 2012-10-08
Source: WG14
Reference Document: N1650
Subject: Defect Report relative to n1570: underspecification for qualified rvalues

Summary

The dealing of rvalues with qualified types is largely underspecified in all versions of the C standard. This didn't surface as a problem until C11, since until then the type of an expression was not observable but only its value.

With C11 now a problem arises for type generic primary expressions; with _Generic type qualifications of values have become observable.

The standard in any of its versions has not much to say when it comes to qualified types for rvalues. They definitively do exist, since the cast operator (6.5.4p2) explicitly specifies that the type could be qualified. That section on casts also has the only indication that relates to rvalues. There is a footnote (thus not normative) that says

89) A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the unqualified version of the type.

That could mean two things:

1. the effective type of the resulting rvalue is unqualified
2. all operators that will accept the rvalue as an operand will act all the same whether the type is qualified or not

doing some tests I have found that clang and gcc disagree on this point. (gcc doesn't have _Generic, yet, but other builtins to observe types)

clang seems to implement 1., gcc 2. They agree for lvalues like this

_Generic((double const){ 0 },
         default: 0,
         double const: 1)

both evaluate it to 1.

They disagree on the outcome for rvalues

_Generic((double const)0,
         default: 0,
         double const: 1)

clang gives 0, gcc gives 1.

(for gcc all with caution that it doesn't have _Generic yet, but that this was obtained using an emulation of it by means of other gcc builtins)

So that situation can easily lead to simple programs that have a behavior that depends on an undocumented choice and thus observe unspecified behavior.

Discussion

Importance of observability of qualifiers

This is not a defect of the _Generic construct itself. The intention is clearly to distinguish all types (with the exception of VM types) that are not compatible, thus to allow to distinguish all 8 different forms of qualifications of a type (resp. 16 for pointer types) that can be obtained from the qualifiers _Atomic, const, volatile (and restrict).

For type generic expressions that are intended to operate on lvalues, such distinction can be crucial for any of the four qualifiers:

• For const or volatile qualified lvalues there might be situations where application code might want to use an unqualified compound literal in place of the controlling expression.
• For _Atomic qualified lvalues there might be situations where application code might want to select a different function than for expressions with same base type but without such a qualifier.
• restrict (or not) qualified pointers may enable an application to select different algorithms or functions (e.g memcopy versus memmove).

Lvalue conversion of the controlling expression of the generic selection is not a solution

Up to now, the conversions of 6.3.2.1 do not apply to primary expressions but only to operators. E.g in the following

double A[5];
double (*B)[5] = &A;
double (*C)[5] = &(A);

B and C should be initialized to the same value, the address of A. If in (A) the primary expression () would enforce a decay of the array to a pointer (and thus to an rvalue) the initialization expression for C would be a constraint violation.

So it seems obvious that the conversions in 6.3.2 ("Other Operands") are not intended to be applied to primary expressions.

Also the conversions in 6.3.2 are not consistent with respect to qualifiers. The only conversion that explicitly drops qualifiers is lvalue conversion (6.3.2.1p2). Array to pointer conversion (6.3.2.1p3) doesn't change qualifiers on the base type. Pointer conversion then, in 6.3.2.3, may add qualifiers to the base type when converting.

Origin

Two different constructs can be at the origin of a qualification of an rvalue:

• casts, but only for scalar types
• function evaluation, resulting in any type but an array or function type.

Both constructs explicitly allow for qualifiers to be applied to the type. In particular 6.7.6.3p15 emphasizes (and constrains) the return type of function specifications to have compatible types, thus indicating that the qualification of the return type bares a semantic meaning.

Operations

If we suppose that any rvalue expression carries its qualification further, other operations (e.g unary or binary +) could or could not result in qualified rvalues. The conversion rules in 6.3 and in particular the usual arithmetic conversions in 6.3.1.8 that allow to determine a common real type don't specify rules to deal with qualifiers.

It seems that a lot of compilers already warn on such "superfluous" qualifications, but in view of type generic primary expression it is not clear that such warnings are still adequate.

Comparison to C++

C++ had to resolve this problem since its beginnings, because the feature of function overloading together with references of rvalues had made rvalue types and their qualifications observable.

Interestingly, to solve the problem the C++ refers to the C standard, claiming that C would drop all qualifiers for rvalue expressions that have scalar base type. It does this without refering to a particular text in the C standard, and in fact it can't since there doesn't seem to be such text.

The actual solution in C++ is thus that all rvalue expressions of non-scalar types are const-qualified and that those of scalar types are unqualified. In view that scalar types are exactly those types that are allowed to have cast operators that qualify the type, all of this looks like a useless additional complication of the issue.

Suggested Technical Corrigendum

There doesn't seem to be an easy solution to this defect, and the proposed solutions (as below or even differently) probably will need some discussion and investigations about their implications on existing code before a consensus could be reached.

Proposal 1: Require the implementation to specify its choice

This is (to my opinion) the worst solution, because the potential different code paths that an application code could take are numerous. There are 4 different qualifiers to handle and code that would have to rely on enumerating all combinations of different generic choices can quickly become a maintenance nightmare.

Also, implementations that chose to keep qualifiers on rvalues would have to decide (and document) by their own what the rules would be when operators are applied to such qualified rvalues.

Proposal 2: Keep all qualifiers on types of rvalue expressions

For this solution in should be then elaborated how operators handle qualifiers. A natural way would be to accumulate qualifiers from operands with different qualifiers.

An important issue with this approach is the rapidly increasing number of cases, in particular 16 for pointer types. To keep the number of cases low when programming with type generic expressions we would need a generic tool for the following:

How to drop qualifiers for type generic expressions? Or alternatively add all qualifiers?

For arithmetic types with base type other than _Bool, char, or short something like the following would be useful:

+(X)                                 // if unary plus drops all qualifiers
(X) + (int const volatile _Atomic)0  // if qualifiers accumulate

Proposal 3: Require the implementation to provide a feature test macro

This solution would already be a bit better than the previous one, since applications that compose type generic macros could select between two (or several) implementations. But the main problems (complexity and underspecification of operations) would remain.

Proposal 4: Drop all qualifiers from the controlling expression of the generic selection

This is not an ideal solution, since it would remove a lot of expressiveness from the generic selection construct. Lvalues could no be distinguished for their qualifiers:

void f(double*);
#define F(X) _Generic((X), double: f)(&(X))

double const A = 42;
F(A);

Here dropping the qualifiers of A would result in a choice of f and in the evaluation of f(&A). In case that f modifies its argument object (which we can't know) this would lead to undefined behavior.

Not dropping the qualifiers would lead to a compile time constraint violation, because none of the types in the type generic expression matches. So here an implementation would be forced to issue a diagnostic, whereas if qualifiers are dropped the diagnostic is not mandatory.

Proposal 5: Drop all qualifiers of rvalues

This solution seems the one that is chosen by clang. It is probably the easiest to specify. As mentioned above it has the disadvantage that the two very similar expressions (int const){0} and (int const)0 have different types.

Some clarification should be added to the standard, though.

6.5.1.1, modify as follows:
EXAMPLE The cbrt type-generic macro could be implemented as follows. Here the prefix operator + in the selection expression ensures that lvalue conversion on arithmetic types is performed such that e.g lvalues of type float const select cbrtf and not the default cbrt.

#define cbrt(X) _Generic(+(X), \
long double: cbrtl,            \
default: cbrt,                 \
float: cbrtf                   \
)(X)

6.5.2.2, add after p1: The type of a function call is the return type of the function without any qualifiers.

6.5.4, add after p2: The type of a cast expression of a qualified scalar type is the scalar type without any qualifiers.

6.7.63, change p15, first sentence: For two function types to be compatible, the unqualified versions of both return types shall be compatible.

C11: When introduced like this, this will invalidate some valid C11 programs, since some type generic expression might behave differently. The faster such corrigendum is adopted the less likely it is that such programs exist.

Proposal 6: Add a const qualifier to all types for rvalues

Analogous as in the case above it has the disadvantage that the two very similar expressions (int){0} and (int)0 have different types.

This is my favorite solution, since it also "repairs" another issue that I am unconfortable with: the problem of array decay in objects with temporary lifetime:

  struct T { double a[4]; } A;
  struct T f(void) { return (struct T){ 0 }; }
  double g0(double* x) { return *x; }
  ...
  g0(f().a);

Here f() is an rvalue that results in an object of temporary lifetime struct T and then f().a decays to a double*. Semantically a better solution would be that it decays to a double const* since a modification of the value is not allowed (undefined behavior). Already with C99 it would be clearer to declare g1 as:

  double g1(double const* x) { return *x; }

If f() would be of type struct T const, f().a would decay to a double const*. Then a call of g0 would be a constraint violation and g1 would have to be used.

The necessary changes to the standard would be something like:

6.5.1.1, modify as follows:
EXAMPLE The cbrt type-generic macro could be implemented as follows. Here the prefix operator + in the selection expression ensures that lvalue conversion on arithmetic types is performed such that e.g lvalues of type float select cbrtf and not the default cbrt.

#define cbrt(X) _Generic(+(X), \
long double const: cbrtl,      \
default: cbrt,                 \
float const: cbrtf             \
)(X)

6.5.2.2, add after p1: The type of a function call is the const-qualified return type of the function without any other qualifiers.

6.5.4, add after p2: The type of a cast expression of a qualified scalar type is the const-qualified scalar type without any other qualifiers.

6.7.63, change p15, first sentence: For two function types to be compatible, the unqualified versions of both return types shall be compatible.

C11: When introduced like this, this will invalidate some valid C11 programs, since some type generic expression might behave differently. The faster such corrigendum is adopted the less likely it is that such programs exist.

C99: When introduced like this, this will invalidate some valid C99 programs that pass rvalue pointers as presented above to function parameters that are not const-qualified but where the called function then never modifies the object of temporary lifetime behind the pointer. Unless for very old legacy functions (from before the introduction of const to the language) such interfaces should be able to use the "correct" const-qualification, or they could be overloaded with a type generic interface that takes care of that issue.

Committee Discussion

• This paper is new enough that a thorough examination of its contents has not been made. It is not clear whether it is a DR or a proposal.
• If implementers do not know what to do, it is a defect.
• Handling of the atomic type qualifier may be the most likely defect, if there is one.

Committee Discussion

• The intent of the Standard is most clearly reflected in the author's Proposal 5.
• Clark Nelson provided an in-depth analysis of Proposal 5.
• The suggested changes to 6.5.1.1 are unnecessary. The controlling expression of a generic selection was very carefully not added to the list of contexts in which lvalue conversion is not done and type qualification is discarded; see 6.3.2.1p2. As such, the controlling expression of a generic selection can not have qualified type. It was thought that a note to that effect might be useful in 6.5.1.1p3.
• The suggested addition to 6.5.4 is useful, but a better change would be to change 6.5.4p5 to:

  Preceding an expression by a parenthesized type name converts the value of the expression to the unqualified version of the named type. This construction is called a cast. A cast that specifies no conversion has no effect on the type or value of an expression.

  Also, footnote 104 should be reduced to just its first sentence.
• The suggested changes to 6.5.2.2 and 6.7.63p15 are desirable, but a simpler change would be to remove any qualifier from the declared return type of a function. So, in 6.7.6.3p5, change to:

  and the type specified for ident in the declaration "T D" is "derived-declarator-type-list T", then the type specified for ident is "derived-declarator-type-list function returning the unqualified version of T".

• Atomic types may or may not be subject to distinct generic selection and this needs to be resolved.

Committee Discussion

• The committee notes that in reflector email 13037 (2013-08-13) that the issue w.r.t. atomics applies to casts, and also needs addressing. The standard was thought to be clear that during conversion to non-atomic type that any implementation size differences are resolved to the non-atomic value, and such conversions would be expected for casts. Such does not seem to be the case, however.
• Further refinement of when and where and how to express the treatment of lvalues and non-lvalues has been made. Specifically, the controlling expression of a generic selection "was very carefully not added" to the list of cases where lvalue conversion is not done.
• Specifically, change 6.5.4.p5 to:

  Preceding an expression by a parenthesized type name converts the value of the expression to the unqualified version of the named type. This construction is called a cast. A cast that specifies no conversion has no effect on the type or value of an expression. (and delete footnote 104)

• Also, change 6.7.63 paragraph 5:

  . . . then the type specified for ident is "derived-declaration-type-list function returning the unqualified version of T".

• A paper has been solicited to resolve the atomic issues that would argue that type generic selection should apply distinctly to atomic types, as seems to be the direction sought by the submitter.

Apr 2014 meeting

Committee Discussion

• The proposed resolution found in N1803 was withdrawn.
• A paper has been solicited to provide a technical corrigendum. It should reflect that the first sentence of footnote 104, "A cast does not yield an lvalue" should be kept.

Apr 2015 meeting

Committee Discussion

The paper N1863 was provided and its Suggested Technical Suggestion was adopted.

Proposed Technical Corrigendum

Change 6.5.4.p5 from

Preceding an expression by a parenthesized type name converts the value of the expression to the named type. This construction is called a cast¹⁰⁴. A cast that specifies no conversion has no effect on the type or value of an expression.

104) A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the unqualified version of that type.

Preceding an expression by a parenthesized type name converts the value of the expression to the unqualified version of the named type. This construction is called a cast¹⁰⁴. A cast that specifies no conversion has no effect on the type or value of an expression.

104) A cast does not yield an lvalue.

... then the type specified for ident is “derived-declarator-type-list function returning T”.

... then the type specified for ident is “derived-declarator-type-list function returning the unqualified version of T”.

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Submitter: Jens Gustedt
Submission Date: 2012-10-08
Source: WG14
Reference Document: N1651
Subject: underspecification of tss_t

Summary

Section 7.26.6 “Thread-specific storage functions” of C11 is severely underspecified since it uses terms that are not introduced (so far) in the context of C. This is really a pity, since POSIX also has pthread_key_t that is completely feature equivalent and for which the specification is much more complete.

Jacob Navia had observed that at several occasions in comp.std.c but it seems that he had not got enough attention such that this had made it in a defect report.

The tss_create function creates a thread-specific storage pointer with destructor dtor, which may be null.

The main problem is that it is nowhere explained/defined

• what a destructor should be,
• when such a destructor should be called,
• in which thread context this constructor is executed
• what happens when new destructors are added while others are executed
• what the meaning of the value of TSS_DTOR_ITERATIONS would be.

Suggested Technical Corrigendum

I think several paragraphs should be added after the one above:

The effect is that for each thread that has the thread specific storage corresponding to key set to a value x that is not null, the destructor function *dtor is called with dtor(x) before the thread exits.

This call to dtor is executed in the context of the same thread; it is sequenced after the return statement or the call to thrd_exit that terminates the thread and before any return from thrd_join of a waiter for this same thread. If there are several thread specific storages for the same thread their destructor functions are called in an unspecific order but with a sequence point between each of these function calls.

If a destructor function for key issues calls to tss_set, tss_get or tss_delete with the same key the behavior is undefined.
tss_set can be used to set the value of a thread specific storage for a different key key2 that had not been set before or that has been processed with a call to the corresponding destructor.

By that the set of thread specific storages for a given thread may change during the execution of the corresponding destructors.

If after processing all tss that are active at the return of the thread function or at the end of thrd_exit there are still tss that are active the procedure of calling destructors is iterated. An implementation may bind the maximum number such of supplementary iterations by TSS_DTOR_ITERATIONS.

A second problem is that there are two functionalities that are easily mixed up and which interrelationship should be clarified: the destructor that is called (let us suppose this) at exit of a thread, and tss_delete that deletes a thread specific storage for all running threads. I think something like the following should be added in 7.26.6.2 after para 2:

The deletion of key will not change the observable behavior of any of the active threads. If tss_delete is called for key and there is a thread that has a non-null value for key that has passed a terminating return statement or call to thrd_exit but not yet finished the processing of all its tss destructors, the behavior is undefined.

Committee Discussion

These issues are covered under DR 416. See discussion there.

Apr 2013 meeting

Committee Discussion

In addition to DR 416 this report suggests defining as undefined behavior the interaction of thrd_exit and tss_delete.

Proposed Committee Response

The issues raised herein have been considered in conjunction with DR 416 and are jointly resolved in that DR's Proposed Technical Corrigendum.

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Submitter: Jens Gustedt
Submission Date: 2012-10-08
Source: WG14
Reference Document: N1653
Subject: no specification for the access to variables with temporary lifetime

Summary

Section 6.2.4 in p4 and p5 requires implementation defined behavior for accessing objects with thread local or automatic storage from different threads than where they are defined. No such mention is done for objects with temporary lifetime in p8. Can they be accessed by other threads? Is this property handled similar to the property for automatic storage duration? Or should this simply be forbidden?

Suggested Technical Corrigendum

Add to the end of 6.2.4 p8:

The result of attempting to indirectly access an object with temporary lifetime from a thread other than the one with which the object is associated is implementation-defined.

Add to 7.26.1p3:

__STDC_THREAD_TEMPORARY_VISIBLE__
which expands to 1 if objects of temporary lifetime are visible to other threads and to 0 otherwise.

Proposed Committee Response

Objects with temporary lifetime are defined in 6.2.4 paragraph 8 to be those with automatic storage duration, and so inter-thread access is implementation defined. No change needed.

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DR 426

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Submitter: Fred J. Tydeman
Submission Date: 2013-01-07
Source: WG 14
Reference Document: N1670
Subject: G.5.1: -yv and -x/v are ambiguous

Summary

The tables in G.5.1 have the mathematical formulas -yv and -x/v. I believe that they are ambiguous as they could have two meanings:

• (-y)/v and (-x)/v
• -(y/v) and -(x/v)

I believe it matters for at least these cases:

1. The two operands are different NaNs, negate flips the sign of a NaN, and the result of * and / depends upon the sign and value of the NaN.
2. The result is a NaN from non-NaN operands, negate does not flip the sign of a NaN, while both * and / set the sign of the result as the XOR of the signs of the operands.
3. All operands are non-NaN, the result is inexact and non-NaN, and a rounding that is not symmetric about zero is in effect.

Suggested Technical Corrigendum

Committee Discussion

• The committee sees the mathematical formulae as unambiguous since the regroupings presented in the paper are mathematically equivalent, and should not be construed as C expressions. As such, there was considerable skepticism expressed that this was indeed a defect.
• The author promised to provide a supplemental paper that would substantiate his concern about this as a defect.

Committee Discussion

• N1738 clarifies a typo in the original document.
• A very long discussion ensued, starting with the fact that C11 notation was used in the clarifying example rather than mathematical notation.
• The concerns raised about NaNs were deemed moot by the fact that multiply and divide are not required by either C11 or IEEE-754 to honor the sign of NaNs.
• The intended mathematical result seems obvious to most on the committee, and the arguments about mathematical notations were not persuasive.
• The third case, however, has modest merit if one considers an implementation that transcribes the provided notation into C11 directly, in which case when asymmetric rounding mode is selected an incorrect result will be formed if negation is applied after the division (or multiplication). More exactly, in the third case, all operands are non-Nan, the result is inexact and non-NaN, and a rounding that is not symmetric about zero is in effect.
• As such, the committee, with great reluctance, narrowly formed a consensus for change, and is concerned that this issue is almost certainly not significant enough to warrant the effort already expended, let alone further discussion, lest the entire matter be returned with no changes proposed as a Record of Response.

Proposed Technical Corrigendum

In the table in G.5.1 #2, change

-yv

to

(-y)v

in three places.

In the table in G.5.1 #3, change

-x/v

to

(-x)/v

in two places.

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Submitter: Shao Miller <sha0.miller@gmail.com>
Submission Date: 2013-01-24
Source: WG 14
Reference Document: N1671
Subject: Function Parameter and Return Value Assignments

Summary
The wording for the the assignments of function arguments to function parameters and for the assignment of a return statement's expression to the value of the function call can potentially be confused.

6.5.2.2p2:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

The appearance of "may be assigned" can lead to the question (#1) of whether or not the constraints and semantics under both 6.5.16 and 6.5.16.1 might apply. The Forward references indicate 6.5.16.1, so this question might be unwarranted.

The appearance of "unqualified version of the type of its corresponding parameter" does not match 6.9.1p10, which doesn't use "unqualified" (see below).

6.5.16p2:

An assignment operator shall have a modifiable lvalue as its left operand.

If 6.5.2.2p2's mention of "assigned" implies this constraint as a secondary constraint, it is not clear which "modifiable lvalue" or even "lvalue" would ever satisfy the constraint. The "modifiable lvalue" does not appear to be the parameter, because:

• the mention of "unqualified version" suggests a theoretical object, while the parameter might be const-qualified, or
• the parameter might have an unqualified structure or union type having at least one const-qualified member (possibly via recursion)

6.7.3p4:

The properties associated with qualified types are meaningful only for expressions that are lvalues.132)

132) The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.

This can suggest that 6.5.2.2p2's "an object with the unqualified version of the type" implies an lvalue, but (question #2) is it a modifiable lvalue? Question #3: If the type is a structure or union type with a const-qualified member (possibly via recursion), are the members considered to be unqualified, too? If so, this is an important difference from pointer types where the referenced type (or its referenced type, recursively) would not be considered unqualified. Also worth consideration would be an array object (which is not qualified) having elements matching such a structure or union type (possibly via recursion).

The return type of a function might be const-qualified, or might be a structure or union type having such a member (possibly via recursion). Question #4: Should the return type of a function be adjusted to be an unqualified version of the type? Such an adjustment might have implications for type compatibility and composite type and might be better off left alone. (const is being used for illustrative purposes, but all type qualifiers can equally be considerations.)

6.8.6.4p3:

If a return statement with an expression is executed, the value of the expression is returned to the caller as the value of the function call expression. If the expression has a type different from the return type of the function in which it appears, the value is converted as if by assignment to an object having the return type of the function.160)

160) The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of function return. The representation of floating-point values may have wider range or precision than implied by the type; a cast may be used to remove this extra range and precision.

If the return type of a function is const-qualified, or is a structure or union type having such a member (possibly via recursion), then "as if by assignment" works for 6.5.16.1, but the constraint of 6.5.16p2 requires a "modifiable lvalue".

The footnote reminds us that a return statement with an expression is not an assignment, but it is not clear that only 6.5.16.1 applies for the "as if by assignment" case.

6.9.1p10:

On entry to the function, the size expressions of each variably modified parameter are evaluated and the value of each argument expression is converted to the type of the corresponding parameter as if by assignment. (Array expressions and function designators as arguments were converted to pointers before the call.)

6.9.1p11:

After all parameters have been assigned, the compound statement that constitutes the body of the function definition is executed.

A const-qualified lvalue cannot normally be assigned-to. An lvalue for an object having a structure or union type containing a const-qualified member (possible via recursion) cannot normally be assigned-to.

6.9.1p10 doesn't match the use of "unqualified" in 6.5.2.2p2 (see above).

Suggested Technical Corrigendum

Sun c99 and GCC disagree on the return statement's semantics.

Change 6.5.2.2p2 to:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall be such that it satifies the constraints of simple assignment when considering the argument to be the right operand and considering the left operand to have the unqualified version of the type of the corresponding parameter.

(Loosely establishes an example for "as if by simple assignment".)

Change 6.8.6.4p3 to:

If a return statement with an expression is executed, the value of the expression is returned to the caller as the value of the function call expression. If the expression has a type different from the return type of the function in which it appears, the value is converted as if by simple assignment to an object having the unqualified version of the return type of the function.160)

Change 6.9.1p10 to:

On entry to the function, the size expressions of each variably modified parameter are evaluated in an unspecified order, the value of each argument expression is converted to the unqualified version of the type of the corresponding parameter as if by simple assignment, then each converted value becomes the initial value for the corresponding parameter. (Array expressions and function designators as arguments were converted to pointers before the call.)

Change 6.9.1p11 to:

After all parameters have initial values, the compound statement that constitutes the body of the function definition is executed.

Add bullet to J.1

- The order in which the size expressions of variably modified parameters are evaluated upon function entry (6.9.1).

Apr 2013 meeting

Committee Discussion

• No one on the committee understood the problem that this paper was trying to discuss.
• In particular, the actual difference in behavior referenced between Sun c99 and GCC is not elaborated upon.
• The committee will solicit further input from the author.

• As a result of further correspondence with the author, a proposed resolution was made on the WG14 reflector in message 13024 and refined in 13035.
• The defect is that the initial values of parameters to function calls are specified in the standard in terms of assignment instead of initialization, such that const and aggregates containing const would be excluded since they cannot be the subject of assignments!
• A Suggested Technical Corrigendum was composed, and follows. It is not yet a Proposed Technical Corrigendum due to the following unresolved discussion points.

• In 6.5.2.2p2 change:

  If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

  to

  If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be used to initialize an object having the type of its corresponding parameter.

  In 6.5.2.2p4 change

  An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is assigned to the value of the corresponding argument.
  

  to
  

  An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is initialized to the value of the corresponding argument.

• It is not clear yet whether there are issues around conversions with respect to this change since this is actual practice that we are intending to simply reflect and to not introduce new constraints.
• This change might be augmented by a new example along the lines of, say, 6.5.16.1 example 3.

Committee Discussion

The issue of conversion has to do with whether there are differing promotions and type conversions that would apply when constructing an argument list that would not occur if these expressions were used as initializers in a declaration.

Oct 2014 meeting

Committee Discussion

The committee concluded after a discussion that there were no promotion or type conversion issues raised by the proposed wording above, and that the following should be adopted as a Proposed Technical Corrigendum.

Proposed Technical Corrigendum (superceded)

In 6.5.2.2p2 change:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

to

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be used to initialize an object having the type of its corresponding parameter.

In 6.5.2.2p4 change

An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is assigned the value of the corresponding argument.

to

An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is initialized to the value of the corresponding argument.

Apr 2015 meeting

Committee Discussion

The goal of preserving conversions as if by assignment is fulfilled by the definition of initialization found in 6.7.9 Initialization paragraph 11. Another instance of assignment that should be changed was found in 6.9.1 Function Definitions paragraph 11.

It was noted that implicit conversion is described only in terms of assignment (6.5.16.1). There was broad agreement that committee members and implementors are unconfused by the intent of the standard here despite the inconsistencies. It was also noted that initialization is distinct from assignment and, in the case of non-lock free atomic implications, this requires operational differences and as such that it is worth further consideration. As such, the following should be regarded as a possible direction.

In 6.5.2.2p2 change:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

to

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be used to initialize an object having the type of its corresponding parameter.

In 6.5.2.2p4 change

An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is assigned the value of the corresponding argument.

to

An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is initialized to the value of the corresponding argument.

In 6.9.1 paragraph 11 change:

After all parameters have been assigned,

to

After all parameters have been initialized,

Oct 2015 meeting

Committee Discussion

• The committee no longer believes that any of the issues raised in this report warrant changes to the standard.
• The difference in behavior on return value semantics cited in gcc has been resolved by a bug fix to gcc
• Qualifiers on the return type of a function are superfluous since by resolution to DR 423 the committee affirms that qualifiers are dropped as part of the evaluation of expressions including function calls.
• The committee does agree that the use of “assignment” to describe the initialization of function parameters can be misleading, but that in fact there is not an implementation that has ever been misled. Careful reading of the Standard notes that initialization is described fundamentally “as if by assignment”, and that it would take a comprehensive review and edit of the Standard to possibly achieve a more consistent treatment of the topics. There is a danger of circularity that must be avoided as well.

Proposed Committee Response

The committee believes that the primary issue of return value semantics was a consequence of a mistake in the implementation of gcc which has been rectified, and that further the treatment of qualifiers has been clarified in the Proposed Technical Corrigendum of DR 423. The treatment of initialization in the Standard is clear enough that no errors have been observed in implementations, and as such further clarification is unwarranted at this time.

DR 409 Prev <— Review —> Next DR 439, or summary at top

DR 428

DR 426 Prev <— Closed —> Next DR 429, or summary at top

Submitter: Douglas Walls
Submission Date: 2013-02-11
Source: WG 14
Reference Document: N1672
Subject: runtime-constraint issue with sprintf family of routines in Annex K

Summary

Suggested Technical Corrigendum

snprintf_s
Replace K.3.5.3.5p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  snprintf_s function sets s[0] to the null character.

sprintf_s
Replace K.3.5.3.6p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  sprintf_s function sets s[0] to the null character.

vsnprintf_s
Replace K.3.5.3.12p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  vsnprintf_s function sets s[0] to the null character.

vsprintf_s
Replace K.3.5.3.13p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  vsprintf_s function sets s[0] to the null character.

Committee Discussion

• The committee agrees with the assessment and the suggested changes.
• There are, however, other places where similar changes are needed.

Committee Discussion

• Further investigation confirmed that there were several other functions that also need similar corrections to their runtime constraints. All of these additional functions also need additional corrections as specified in DR433, and the full resolution of both this defect and the additional issue will be found in the Proposed Technical Corrigendum of DR433.
• That list is:
  • K.3.9.1.3 The snwprintf_s function
  • K.3.9.1.4 The swprintf_s function
  • K.3.9.1.8 The vsnwprintf_s function
  • K.3.9.1.9 The vswprintf_s function
  • K.3.9.3.2.1 The mbsrtowcs_s function
  • K.3.9.3.2.2 The wcsrtombs_s function
• It is noted that with these changes that K.3.5.1.2 tmpname_s will have wording inconsistent with respect to these modifications.
• Consistent wording would be, in K.3.5.1.2p2 replace "less than or equal to RSIZE_MAX" with "not greater than RSIZE_MAX".
• As such, the committee continues to accept unchanged the Proposed Technical Corrigendum as partial fulfillment of this defect, and that full resolution of the other similar defects will be found in DR433.

Proposed Technical Corrigendum

snprintf_s
Replace K.3.5.3.5p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  snprintf_s function sets s[0] to the null character.

sprintf_s
Replace K.3.5.3.6p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  sprintf_s function sets s[0] to the null character.

vsnprintf_s
Replace K.3.5.3.12p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  vsnprintf_s function sets s[0] to the null character.

vsprintf_s
Replace K.3.5.3.13p3 with:

  If there is a runtime-constraint violation, then if s is not a null
  pointer and n is greater than zero and not greater than RSIZE_MAX, then the
  vsprintf_s function sets s[0] to the null character.

DR 426 Prev <— Closed —> Next DR 429, or summary at top

DR 429

DR 428 Prev <— Closed —> Next DR 430, or summary at top

Submitter: Douglas Walls
Submission Date: 2013-02-11
Source: WG 14
Reference Documents: N1673  N1748
Subject: Should gets_s discard next input line when (s == NULL) ?

Summary

Suggested Technical Corrigendum

Committee Discussion

• The issue is found in Annex K.3.5.4.1p3, not p2.
• The Microsoft implementation appears to treat this as a runtime-constraint violation.
• The Dikumware implementation leaves this behavior unspecified.

Committee Discussion

• Given that footnote 404 already provides guidance on this issue, the author and the committee agree that the answer to the question posed is indeed yes.
• The other issue with respect to s[0] cannot be set to the null character when s==NULL has been determined to be resolved by the following changes.

Proposed Technical Corrigendum

If there is a runtime-constraint violation, characters are read and discarded from stdin until a new-line character is read, or end-of-file or a read error occurs, and if s is not a null pointer s[0] is set to the null character.

DR 428 Prev <— Closed —> Next DR 430, or summary at top

DR 430

DR 429 Prev <— Closed —> Next DR 431, or summary at top

Submitter: Douglas Walls
Submission Date: 2013-02-11
Source: WG 14
Reference Document: N1674
Subject: getenv_s, maxsize should be allowed to be zero

Summary

Suggested Technical Corrigendum

Replace Annex K.3.6.2.1p2 second sentence with:

maxsize shall not be greater than RSIZE_MAX.

K.3.6.2.1p2 would then read thusly:

name shall not be a null pointer.  maxsize shall not be greater than
RSIZE_MAX.  If maxsize is not equal to zero, then value shall not be a null pointer.

Committee Discussion

• The proposed change seems appropriate and is consistent with the existing rationale.
• The committee notes that some implementations may need to change to avoid treating a value of zero for maxsize as a constraint violation.

Proposed Technical Corrigendum

Replace Annex K.3.6.2.1p2 second sentence with:

maxsize shall not be greater than RSIZE_MAX.

K.3.6.2.1p2 would then read thusly:

name shall not be a null pointer.  maxsize shall not be greater than
RSIZE_MAX.  If maxsize is not equal to zero, then value shall not be a null pointer.

DR 429 Prev <— Closed —> Next DR 431, or summary at top

DR 431

DR 430 Prev <— Closed —> Next DR 432, or summary at top

Submitter: Douglas Walls
Submission Date: 2013-02-21
Source: WG14
Reference Document: N1675
Subject: atomic_compare_exchange: What does it mean to say two structs compare equal?

Summary

Suggested Technical Corrigendum

Either define equality of objects of struct type, add a restriction disallowing
use of atomic structs as arguments for the atomic_compare_exchange generic functions,
or note that atomic_compare_exchange generic functions for objects of atomic
struct type results in undefined behavior.

Committee Discussion

• There is no sentiment to define equality for structs.
• 7.17.6 lists atomic types required to have the same size and alignment as the corresponding direct type. Other atomic types may have differing size and alignment as per 6.2.5p27.
• 6.5.2.3p5 states that any access to an atomic struct or union member is undefined behavior and as such so would atomic_compare_exchange since it requires access.

Committee Discussion

• There are several points that need addressing.
• The original intention of the atomics design was to allow memcmp and memcpy (with suitable synchronization) be a common implementation for all _Atomic objects.
• This practice, however, would lead to undefined behavior for implementations that have padding bits for integer representations.
• For implementations that choose larger representations for some _Atomic types, there would need to be at least one larger type specific implementation of atomic_compare_exchange compared to what might be possible to implement in common for all types. This seems to imply that an implementation must supply something akin to a type generic implementation of atomic_compare_exchange. Type generic macros as applied to _Atomic is the subject of DR423.
• A proposal to address these issues has been solicited.

Apr 2014 meeting

Committee Discussion

• The proposed resolution from N1803 was withdrawn since there are implementations choosing to represent atomic non-lock-free types with extra state and hence a larger size.
• Structure compare will result in undefined behavior.
• A new paper to address this DR has been solicited

Oct 2014 meeting

Committee Discussion

As requested, the paper N1864 was written and provided. From our C++ liaison, however, it was learned that corresponding behavior is well defined and is in use. Further investigation revealed that atomic_compare_exchange in C++ is and has been explicitly defined to be that of bit comparison. C11 defines it as value comparison.

It was noted that bit comparison for atomic bool would not give the expected answer if differing non-zero "true" values were compared. It was also noted that bit comparison exposes padding bits, whereas lock bits would be required to be discarded, leading to code that might work on one implementation of an architecture but fail on another.

A new paper was solicited.

Apr 2015 meeting

Committee Discussion

The paper N1906 was provided and discussed and its Proposed Technical Corrigendum was adopted. This resolution clarifies that

Proposed Committee Response

atomic_compare_exchange is now aligned with C++11 as operating on bit representations. Where these representations are unpadded integer or structure values, the operation is well defined. The type bool is padded in many implementations.

Proposed Technical Corrigendum

In 7.17.7.4p2 replace

Atomically, compares the value pointed to by object for equality with that in expected, and if true, replaces the value pointed to by object with desired, and if false, updates the value in expected with the value pointed to by object

Atomically, compares the contents of the memory pointed to by object for equality with that in expected, and if true, replaces the contents of the memory pointed to by object with expected, and if false, updates the value in expected with the value pointed to by object.

DR 430 Prev <— Closed —> Next DR 432, or summary at top

DR 432

DR 431 Prev <— Closed —> Next DR 433, or summary at top

Submitter: Fred J. Tydeman (USA)
Submission Date: 2013-03-07
Source: WG 14
Reference Document: N1677
Subject: Possible defect report: Is 0.0 required to be a representable value?

There are many places in C11 that assume a floating-point zero value, e.g., 0.0, is representable.

• 6.3.1.7 Real and complex #1 has:

  When a value of real type is converted to a complex type, the real part of the complex result value is determined by the rules of conversion to the corresponding real type and the imaginary part of the complex result value is a positive zero or an unsigned zero.

• 6.7.9 Initialization, #10 (second bullet) has (for static and thread storage initialzation):

  if it has arithmetic type, it is initialized to (positive or unsigned) zero;

• 7.22.1.3 The strtod, strtof, and strtold functions, #10 has:

  If no conversion could be performed, zero is returned.

• 7.29.4.1.1 The wcstod, wcstof, and wcstold functions, #10 has:

  If no conversion could be performed, zero is returned.

• Annex F and Annex G, being based upon IEEE-754, require signed floating-point zeros.

There are many places in C11 that allow for a representable floating-point zero value.

• 5.2.4.2.2 Characteristics of floating types <float.h> #4 has:

  An implementation may give zero and values that are not floating-point numbers (such as infinities and NaNs) a sign or may leave them unsigned.

• 7.3.3 Branch cuts, #1 and #2 talk about signed zeros.
• 7.6 Floating-point environment <fenv.h>, #6, lists FE_DIVBYZERO (divide-by-zero) as a possible floating-point exception. And, 7.12.1 Treatment of error conditions gives more details.
• 7.12 Mathematics <math.h> #6 says that zero, subnormal, normal, NaN, and infinite are mutually exclusive kinds of floating-point values. And 7.12.3.1 The fpclassify macro uses those classification macros.
• 7.12.3.2 The isfinite macro, #2 says that zero is a finite value.
• 7.12.3.5 The isnormal macro, #2 says that zero is not a normal value.
• 7.12.*; many of the math functions (atan2, frexp, ilogb, log, log10, log2, logb, pow, lgamma, tgamma, fmod, remainder, remquo, copysign) mention zero as special cases.

The C Rationale in its discussion of 5.2.4.2.2 has:

Note that the floating-point model adopted permits all common representations, including sign-magnitude and two's-complement, but precludes a logarithmic implementation.

The C89 Committee also endeavored to accommodate the IEEE 754 floating-point standard by not adopting any constraints on floating-point which were contrary to that standard.

However, if one carefully reads 5.2.4.2.2 Characteristics of floating types <float.h>, #2 and #3, one finds that zero is not required to be representable. As I read those paragraphs, normalized floating-point numbers are the only things required to be contained in floating types. Subnormal floating-point numbers, unnormalized floating-point numbers, infinities, and NaNs are additional (optional) things that may be contained in floating types. Zero is not mentioned explicitly.

So, it appears that some parts of C11 require that floating-point zeros be representable, while other parts do not require that they be representable.

I think that the first sentance in 5.2.4.2.2 #3 should be changed to:

Floating types shall be able to represent normalized floating-point numbers (f₁ > 0 if x != 0) and (positive or unsigned) zero. In addition, floating types may be able to contain other kinds of floating-point numbers, such as negative zero and subnormal ...

Suggested Technical Corrigendum

Committee Discussion

• The committee agrees that the standard is missing an explicit requirement for floating point zero.
• There is no need for the clause if x != 0.

Proposed Technical Corrigendum

The first sentance in 5.2.4.2.2 #3 should be changed to:

Floating types shall be able to represent normalized floating-point numbers (f₁ > 0) and (positive or unsigned) zero. In addition, floating types may be able to contain other kinds of floating-point numbers, such as negative zero and subnormal ...

DR 431 Prev <— Closed —> Next DR 433, or summary at top

DR 433

DR 432 Prev <— Closed —> Next DR 434, or summary at top

Submitter: Douglas Walls
Submission Date: 2013-03-12
Source: WG 14
Reference Document: N1683 N1771
Subject: Issue with constraints for wide character function arguments involving RSIZE_MAX

Summary

Suggested Technical Corrigendum

Committee Discussion

• RSIZE_MAX is intended to be the maximum size, in bytes, permitted by an implementation for an object.
• The constraints for wide character functions appear to be incorrect.
• For example, K.3.9.2.1.2p8 second sentence should read something like "Neither s1max*sizeof(wchar_t) nor n*sizeof(wchar_t) shall be greater than RSIZE_MAX."

Committee Discussion

• The list of functions cited was not entirely correct, and upon further review the Suggested Technical Corrigendum from N1771 were adopted.
• These changes also address, where noted, the defect reported in DR428.

Apr 2014 meeting

Committee Discussion

A "nor nor" typo in the Suggested Technical Corrigendum for K.3.9.3.2.2p12 was noticed and corrected in the Proposed Technical Corrigendum below.

Proposed Technical Corrigendum

K.3.6.5.1 The mbstowcs_s function

In K.3.6.5.1p2, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.6.5.1p3, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX/sizeof(wchar_t)".

K.3.6.5.2 The wcstombs_s function

In K.3.6.5.2p2, replace "then neither len nor dstmax shall be greater than RSIZE_MAX" with
"then neither len shall be greater than RSIZE_MAX/sizeof(wchar_t) nor dstmax shall be greater than RSIZE_MAX".
In K.3.6.5.2p3, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX".

K.3.9.1.3 The snwprintf_s function

In K.3.9.1.3p2, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.1.3p3, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX/sizeof(wchar_t)".  See DR 428

K.3.9.1.4 The swprintf_s function

In K.3.9.1.4p2, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.1.4p3, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX/sizeof(wchar_t)".  See DR 428

K.3.9.1.8 The vsnwprintf_s function

In K.3.9.1.8p2, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.1.8p3, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX/sizeof(wchar_t)".  See DR 428

K.3.9.1.9 The vswprintf_s function

In K.3.9.1.9p2, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.1.9p3, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX/sizeof(wchar_t)".  See DR 428

K.3.9.2.1.1 The wcscpy_s function

In K.3.9.2.1.1p2, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.2.1.1p3, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".

K.3.9.2.1.2 The wcsncpy_s function

In K.3.9.2.1.2p8, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.2.1.2p9, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".

K.3.9.2.1.3 The wmemcpy_s function

In K.3.9.2.1.3p15, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.2.1.3p16, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".

K.3.9.2.1.4 The wmemmove_s function

In K.3.9.2.1.4p20, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.2.1.4p21, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".

K.3.9.2.2.1 The wcscat_s function

In K.3.9.2.2.1p3, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.2.2.1p4, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".

K.3.9.2.2.2 The wcsncat_s function

In K.3.9.2.2.2p10, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.2.2.2p11, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".

K.3.9.2.3.1 The wcstok_s function

In K.3.9.2.3.1p2, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".

K.3.9.3.2.1 The mbsrtowcs_s function

In K.3.9.3.2.1p3, replace "RSIZE_MAX" with "RSIZE_MAX/sizeof(wchar_t)".
In K.3.9.3.2.1p4, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX/sizeof(wchar_t)".  See DR 428

K.3.9.3.2.2 The wcsrtombs_s function

In K.3.9.3.2.2p12, replace "then neither len nor dstmax shall be greater than RSIZE_MAX" with
"then neither len shall be greater than RSIZE_MAX/sizeof(whcar_t) nor dstmax shall be greater than RSIZE_MAX".
In K.3.9.3.2.2p13, replace "less than RSIZE_MAX" with "not greater than RSIZE_MAX".

DR 432 Prev <— Closed —> Next DR 434, or summary at top

DR 434

DR 433 Prev <— Closed —> Next DR 435, or summary at top

Submitter: Fred J. Tydeman (USA)
Submission Date: 2012-10-26
Source: WG 14
Reference Document: N1660 N1660
Subject:Possible defect report: Missing constraint w.r.t. Atomic

Summary

6.7.2.4 Atomic type specifiers, has in paragraph 2:

Atomic type specifiers shall not be used if the implementation does not support atomic types (see 6.10.8.3).

But, 6.7.3 Type qualifiers, has no similar constraint with respect to _Atomic.

Also, 7.17.6 Atomic integer types, has no similar constraint. Aside: The only constraints I see in the library are in <float.h> and <limits.h>, so it is not clear if this case should be a constraint.

Suggested Technical Corrigendum

Add to 6.7.3 Type qualifiers, a new paragraph after paragraph 3,

Atomic type qualifiers shall not be used if the implementation does not support atomic types (see 6.10.8.3).

Add to 7.16.6 Atomic integer types, a new paragraph before paragraph 1:

Constraints

Atomic type names shall not be used if the implementation does not support atomic types (see 6.10.8.3).

Committee Discussion

• The committee agrees to the first additional constraint.
• The second constraint is not required due to 7.17 paragraph 2.

Proposed Technical Corrigendum

Add to 6.7.3 Type qualifiers, a new paragraph after paragraph 3,

Atomic type qualifiers shall not be used if the implementation does not support atomic types (see 6.10.8.3).

DR 433 Prev <— Closed —> Next DR 435, or summary at top

DR 435

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Submitter: Fred J. Tydeman (USA)
Submission Date: 2012-10-26
Source: WG 14
Reference Document: N1661
Subject:Possible defect report: Missing constraint w.r.t. Imaginary

Summary

The type specifier _Complex shall not be used if the implementation does not support complex types (see 6.10.8.3).

But, G.2 Types, has no similar constraint with respect to _Imaginary.

Suggested Technical Corrigendum

Add to G.2 Types, a new sentence in paragraph 1:

The _Imaginary type specifier shall not be used if the implementation does not support imaginary types (see 6.10.8.3).

Committee Discussion

This is not actually a defect.

Proposed Committee Response

Annex G requires _Imaginary be supported, so there is no need to cite a requirement for when it is not supported.

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DR 436

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Submitter: Willem Wakker (NL)
Submission Date: 2013-05-08
Source: WG 14
Reference Document: N1713
Subject: Request for interpretation of C11 6.8.5#6

C11, section 6.8.5 paragraph 6 reads:

An iteration statement whose controlling expression is not a constant expression,156) that performs no input/output operations, does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation to terminate.157)

Question: to what does the that refers back to: to the controlling expression or to the constant expression?

Committee Discussion

This is indeed an ambiguity, and after considering various proposals, the following was approved.

Apr 2014 meeting

Committee Discussion

The committee noted a typo in the Suggested Technical Corrigendum where "its expression *157" was intended to be "its expression-3 *157", and so has been corrected below.

Proposed Technical Corrigendum

An iteration statement may be assumed by the implementation to terminate if its controlling expression is not a constant expression *156), and none of the following operations are performed in its body, controlling expression or (in the case of a for statement) its expression-3 *157):

• input/output operations
• accessing a volatile object
• synchronization or atomic operations.

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DR 437

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Submitter: Nick Stoughton (US)
Submission Date: 2013-06-19
Source: Austin Group
Reference Document: N1719
Subject: clock overflow problems

Summary

C11 (and C99 before it) state for clock() that

If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t)(-1).

In order to measure the time spent in a program, the clock function should be called at the start of the program and its return value subtracted from the value returned by subsequent calls.

The normative requirement implies that if more processor time has passed than can be fit into a variable of type clock_t the function must fail and return (clock_t)-1.

However, existing implementations almost exclusively ignore this requirement and if more ticks pass than can fit into a clock_t the implementation simply truncates the value and return the lowermost bits of the actual value. In programming environments where clock_t is a 32-bit integer type and CLOCKS_PER_SEC is one million (a very common implementation), clock() will start misreporting in less than 36 minutes of processor time for signed clock_t, or 72 minutes for unsigned clock_t.

Question 1: Are such implementations conforming? If not, should the standard be altered in any way to permit this de-facto standard implementation?

Question 2: Should the standard define some limit macros for clock_t (effectively defining new values in limits.h for CLOCK_MAX, the minimum maximum value for a clock_t)?

Question 3: If the value is truncated and clock_t is a signed type, the recommended application usage n footnote 319 (subtracting clock_t values to measure intervals) can cause the application to invoke undefined behavior via integer overflow. In particular, if the initial call to clock() returned A > 0 (by virtue of some processor time having been consumed before the start of main() or the point of first measurement), and a subsequent call returned B=INT_MIN just after overflow, then the recommended practice of computing B-A invokes undefined behavior. Should there be any warning of this included in the footnote?

Suggested Change

Given that the vast majority of surveyed implementations appear to have implemented clock with a simple incrementing counter with no check for overflow, the requirement for clock() to return (clock_t)-1 when the number of clock ticks cannot be represented in a variable of type clock_t should be relaxed:

At 7.27.2.1 paragraph 3, change:

If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t)(-1).

If the processor time used is not available, the function returns the value (clock_t)-1.

In order to measure the time spent in a program, the clock function should be called at the start of the program and its return value subtracted from the value returned by subsequent calls. Note, however, that such a subtraction may result in undefined behavior if clock_t is an unsigned integer type.

Committee Discussion

• This issue is tied to Austin Group Defect #686.
• The committee feels that the core issue regards required behavior under the condition that overflow occurs in the implementations choice for the representation of clock_t.
• It is noted that many and possibly most implementations have chosen an integer clock_t size that overflows early and often.
• The standard does seem to require that the overflow condition is detected and that the corresponding return value shall be that of failure (clock_t)-1.
• Such detection runs counter to the desired behavior of the clock counter as being that of highest implementation efficiency, and to subsequent uses across durations that do not exhibit overflow after the first occurrence.
• The suggested technical corrigendum does not appear to discuss or address this core issue and warrants further discussion. The following suggestion for the footnote is offered:

  Implementations commonly use an integer clock_t type that can overflow in as little as 36 minutes. All uses of clock() to measure a duration of time must address the issue of possible overflow.

Apr 2014 meeting

Committee Discussion

The author will be solicited for a revised technical corrigendum.

Oct 2014 meeting

Committee Discussion

There was no paper submitted on this topic, and the committee will again solicit the Austin Group for direction.

Apr 2015 meeting

Committee Discussion

The paper N1895 was provided and discussed. The general sentiment in the committee is that clock_t is underspecified and that this function should be deprecated and replaced in a revision to the standard with something that uses, perhaps, struct timespec. In particular, no implementations are known to implement the -1 return value on overflow.

The committee reviewed the following words and approved them as the Proposed Technical Corrigendum.

Proposed Committee Response

To question 1, such programs are not conforming and, no, the standard should not be altered to accept this behavior.

To question 2, no, this is not the direction.

To question 3, the committee does not agree that this invokes undefined behavior. The value returned under such conditions is unspecified.

Proposed Technical Corrigendum

In 7.27.2.1p3 change:

If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t)(-1)³¹⁹. ...

319) In order to measure the time spent in a program, the clock function should be called at the start of the program and its return value subtracted from the value returned by subsequent calls.

If the processor time used is not available, the function returns the value (clock_t)(-1). If the value cannot be represented, the function returns an unspecified value³¹⁹. ...

319) This may be due to overflow of the clock_t type.

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DR 438

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Submitter: Nick Stoughton (US)
Submission Date: 2013-06-19
Source: Austin Group
Reference Documents: N1720, Austin Group Defect #701
Subject: ungetc / ungetwc and file position after discarding push back problems

Summary

C11 (and C99 before it) state for both ungetc() and ungetwc() that

A successful intervening call (with the stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards any pushed-back characters for the stream. ... The value of the file position indicator for the stream after reading or discarding all pushed-back characters shall be the same as it was before the characters were pushed back.

Suggested Change

The value of the file position indicator for the stream after reading or discarding all pushed-back characters shall be the same as it was before the characters were pushed back.

The value of the file position indicator for the stream after all pushed-back characters have been read shall be the same as it was before the characters were pushed back.

Oct 2013 meeting

Committee Discussion

• This issue arises because of the different architectures of a buffered stream which can hold pushed back characters, and that of the underlying filesystem which honors seek independent of the buffering.
• The committee sentiment is that the suggested changes are nearly correct and have requested that the project editor suggest better wording, subject to further discussion.

Apr 2014 meeting

Committee Discussion

The Standard is correct as written because the intent is that the specified file position indicator is an intermediate state inside the file positioning function after the pushed-back characters are discarded but before the actual seek. That gives you a reliable file position from which to do the seek. It's not intended that the file positioning function doesn't set the file position indicator.

Proposed Technical Corrigendum

Add a footnote to 7.21.7.10 paragraph 5, second sentence:

Note that a file positioning function may further modify the file position indicator after discarding any pushed-back characters.

Note that a file positioning function may further modify the file position indicator after discarding any pushed-back wide characters.

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DR 439

DR 427 Prev <— Review —> Next DR 453, or summary at top

Submitter: Clark Nelson
Submission Date: 2013-07-16
Source: WG 14
Reference Document: N1729
Subject: Issues with the definition of “full expression”

Summary

I have discovered several issues in 6.8p4, which defines “full expression” and points out the major implications for an expression that is a full expression. In this paper I present the issues, along with my recommendations. For the issues for which it makes sense, I will later submit defect reports.

Here is the text of 6.8p4, with clause numbering added for convenience of reference:

1. A full expression is an expression that is not part of another expression or of a declarator.
2. Each of the following is a full expression:
   1. an initializer that is not part of a compound literal;
   2. the expression in an expression statement;
   3. the controlling expression of a selection statement (if or switch);
   4. the controlling expression of a while or do statement;
   5. each of the (optional) expressions of a for statement;
   6. the (optional) expression in a return statement.
3. There is a sequence point between the evaluation of a full expression and the evaluation of the next full expression to be evaluated.

And here are the issues.

1. The phrase “not part of another expression or of a declarator” (sentence 1) is rather difficult to understand. It probably means: not part of another expression, nor part of a declarator. (But DeMorgan's law is hard on the brain.)

   I believe this could be fixed as a simple editorial issue.

2. The status of an initializer expression depends on whether the context is a declaration or a compound literal (clause 2.1). That would seem to imply different sequencing guarantees in those contexts. As it turns out, it does, but the implication is quite subtle. Consider 6.7.9p23:

   The evaluations of the initialization list expressions are indeterminately sequenced with respect to one another and thus the order in which any side effects occur is unspecified.

   And consider this example:

   #include <stdio.h>
   
   
   
   #define ONE_INIT	'0' + i++ % 3
   
   #define INITIALIZERS	[2] = ONE_INIT, [1] = ONE_INIT, [0] = ONE_INIT
   
   
   
   int main()
   
   {
   
   	int i = 0;
   
   	char x[4] = { INITIALIZERS }; // case 1
   
   	puts(x);
   
   	puts((char [4]){ INITIALIZERS }); // case 2
   
   	puts((char [4]){ INITIALIZERS } + i % 2); // case 3
   
   }

   In every use of the INITIALIZERS macro, the variable i is incremented three times. In cases 1 and 2, there is no undefined behavior, because the increments are in expressions that are indeterminately sequenced with respect to one another, not unsequenced. There is no guarantee in what order the evaluations are done, so there is no guarantee in what order they will appear, but the initial values are guaranteed to be '0', '1' and '2'.

   (It's not perfectly clear whether that guarantee was provided by C99, which instead said:

   The order in which any side effects occur among the initialization list expressions is unspecified.

   In any event, as a data point, that guarantee was not honored by GCC until release 4.6, in 2011.)

   On the other hand, because case 3 contains an unsequenced evaluation of i in the same full expression, it has undefined behavior.

   Considering the number of hours it took me to finally reach this conclusion, I thought it would be worthwhile to bring it to the full committee to make sure everyone understands and agrees with it. If so, an addition to the rationale might be in order.

3. Consider 6.7.6p3 (emphasis added):

   A full declarator is a declarator that is not part of another declarator. The end of a full declarator is a sequence point. …

   Also 6.2.4p8:

   A non-lvalue expression with structure or union type, where the structure or union contains a member with array type (including, recursively, members of all contained structures and unions) refers to an object with automatic storage duration and temporary lifetime.³⁶⁾ Its lifetime begins when the expression is evaluated and its initial value is the value of the expression. Its lifetime ends when the evaluation of the containing full expression or full declarator ends. Any attempt to modify an object with temporary lifetime results in undefined behavior.

   It is clear from these passages that the sequence of evaluations includes not only full expressions, but also full declarators – whatever sense it makes to talk about “evaluating” a full declarator. But sentence 3 does not acknowledge that reality.

   My inclination is to adopt a bit of terminology from Ada, and start talking about the “elaboration” of a declarator, which, for a variably modified type, involves the run-time evaluation of array sizes, and then to re-draft sentence 3 and the other paragraphs cited here, to make it clear that sequence points separate elaborations of full declarators as well as evaluations of full expressions. In any event, I think there's a problem in sentence 3 that needs to be fixed.

4. Expressions in abstract declarators are not mentioned at all (compare to sentence 1). The logical inference is that such an expression is not a full expression by itself, but part of the containing full expression. But there are cases where there is no containing full expression. For example:

   typedef _Atomic(int (*)[rand()]) T;
   
   _Alignas(int [rand()]) int i;

   In these examples, not only is there no containing full expression, there isn't even any containing full declarator, because these expressions appear in the declaration specifiers, not the declarator.

   Probably the simplest approach here would be to disallow variably modified types with _Atomic and _Alignas, at least until the next revision of the standard.

5. The list of full expression contexts (sentence 2) is not logically complete. According to the definition (sentence 1), an expression appearing in a constant-expression context is (often) a full expression. Of course there are no sequencing implications relevant for constant expressions, but it's not clear that makes it important for a constant expression not to be counted as a full expression. In any event, it's not clear how the list normatively interacts with the definition.

   I think we should consider moving the list into a note, so it's clear that the definition is, well, definitive. The note could also point out that sequencing is irrelevant to constant expressions.

Committee Discussion

• These are subtle issues upon which the committee wishes to continue discussing.
• The committee is seeking concurrence with the project editor that Point A may be viewed as editorial.
• To Point B the committee is in agreement that there is a "guarantee" of the distinct values 0, 1, and 2, and that this is not new to C11. The committee requires further discussion as to whether and how these subtleties could be expressed in a non-normative fashion, such as "in the rationale", "in a footnote", or simply as an example.
• To Point C the committee agrees that some change is needed and has solicited a suggested resolution from the author. The fact that VLAs can have several dimensions and hence several expressions gave pause to all.
• To Point D the committee agrees that this has been left undefined and deserves a committee response but no normative or other change.
• To Point E, the committee agreed that the list of full expressions named in 6.8 paragraph 4 second sentence should likely be regarded as non-normative rather than allowing it to appear exhaustive. To achieve this, the list could be transformed into a note or footnote.

Apr 2014 meeting

Committee Discussion

The committee solicits the author for any suggested technical corrigenda.

Oct 2014 meeting

Committee Discussion

There was no paper submitted on this topic, and the committee will again solicit the author for suggested technical corrigenda.

Apr 2015 meeting

Committee Discussion

There was no paper submitted on this topic, and the committee has solicited the author for suggested technical corrigenda.

Oct 2015 meeting

Committee Discussion

• The paper N1965 was submitted and discussed.
• From that, the editorial suggestion from Part A was incorporated into Part C which was approved by the committee as a normative element in the following Proposed Technical Corrigendum, and Part E was approved as a non-normative footnote.
• The Part D section had already met with some some skepticism from the committee and was mostly provided as an idea to consider. Discussion revealed that there are some implementations of VLA that would be broken by this change, and so the committee preferred to not try to address this issue from this DR at this time. Another new paper on this point would be welcomed by the committee.
• The sequencing issues raise in Part B are subtle and the Standard potentially confusing, but there is no reported evidence that this is causing any issues in practice. The new paper offered no Suggested Technical Corrigendum in this area, and the committee did not request further elaboration of what might be needed in this area, and so was content to accept just the changes resolving issues A, C, and E in the following Proposed Technical Corrigendum.

Proposed Technical Corrigendum

Change 6.2.4p8 sentence 3 from:

Its lifetime ends when the evaluation of the containing full expression or full declarator ends.

Its lifetime ends when the evaluation of the containing full expression ends.

Delete 6.7.6p3 sentence 2:

The end of a full declarator is a sequence point.

Change 6.8p4 from:

A full expression is an expression that is not part of another expression, or of a declarator. Each of the following is a full expression: an initializer that is not part of a compound literal; the expression in an expression statement; the controlling expression of a selection statement (if or switch); the controlling expression of a while or do statement; each of the (optional) expressions of a for statement; the (optional) expression in a return statement. There is a sequence point between the evaluation of a full expression and the evaluation of the next full expression to be evaluated.

to:

A full expression is an expression that is not part of another expression, nor part of a declarator or abstract declarator. There is also an implicit full expression in which the non-constant size expressions for a variably modified type are evaluated; within that full expression, the evaluation of different size expressions are unsequenced with respect to one another. There is a sequence point between the evaluation of a full expression and the evaluation of the next full expression to be evaluated.

Add after 6.8p4:

NOTE: Each of the following is a full expression:

• a full declarator for a variably modified type;
• an initializer that is not part of a compound literal;
• the expression in an expression statement;
• the controlling expression of a selection statement (if or switch);
• the controlling expression of a while or do statement;
• each of the (optional) expressions of a for statement;
• the (optional) expression in a return statement.

While a constant expression satisfies the definition of a full expression, evaluating it does not depend on nor produce any side effects, so the sequencing implications of being a full expression are not relevant to a constant expression.

• The end of a full declarator; declarators (6.7.6);

Change the Annex C bullet from:

• Between the evaluation of a full expression and the next full expression to be evaluated. The following are full expressions: an initializer that is not part of a compound literal; the expression in an expression statement; the controlling expression of a selection statement (if or switch); the controlling expression of a while or do statement; each of the (optional) expressions of a for statement; the (optional) expression in a return statement.

to:

Between the evaluation of a full expression and the next full expression to be evaluated. The following are full expressions: a full declarator for a variably modified type; an initializer that is not part of a compound literal; the expression in an expression statement; the controlling expression of a selection statement (if or switch); the controlling expression of a while or do statement; each of the (optional) expressions of a for statement; the (optional) expression in a return statement.

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DR 440

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Submitter: Joseph Myers
Submission Date: 2013-07-21
Source: WG 14
Reference Document: N1730
Subject: Floating-point issues in C11 from PDTS 18661-1 UK review, Issue 1

Summary

Some issues with floating point in C11 have been identified as part of the UK review of the N1711 draft of TS 18661-1. While such issues relate to the general area of C bindings to IEC 60559:2011, and so could be addressed in the TS on that basis, since the issues also apply to C11 as-is it may be more appropriate to address some or all of these issues as Defect Reports with a view to having a normative fix in a future TC to C11 rather than only having a fix in conjunction with the new bindings.

Issue 1: Choice of long double in Annex F

Annex F provides various choices for the long double type (which may or may not be an IEC 60559 type), but no method is provided for an application to determine which choice has been made.

If a macro is provided to say whether the type is an IEC 60559 type, all the other properties can be determined from the existing <float.h> macros. So, a sufficient fix would be:

In 5.2.4.2.2, insert a new paragraph after paragraph 10: Whether a type matches an IEC 60559 type is characterized by the implementation-defined values of FLT_IS_IEC_60559, DBL_IS_IEC_60559, and LDBL_IS_IEC_60559:

• 0 type does not match an IEC 60559 format
• 1 type's values and operations are those of an IEC 60559 basic, interchange or extended type

Committee discussion led to a proposed committee response.

Correspondence with the author led the committee to augment the proposed committee response.

Proposed Committee Response

To do as suggested, distinguish whether float, double, and long double are IEC or not, requires the addition of new macros, which is a feature, which is not allowed by the mechanism of defect reports.

The defect originator notes that the underlying issue that needs consideration in any further comprehensive publication of the Standard is that all implementation defined behaviors need to be strictly called out in the Standard, and moreover that they be done so in a manner that is accessible to a client of the implementation to allow proper choice of algorithms. It has been asserted that leaving implementation defined behaviors formally undefined in the Standard has led to significant and unnecessary divergences in implementations.

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DR 441

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Submitter: Joseph Myers
Submission Date: 2013/07/21
Source: WG 14
Reference Document: N1730
Subject: Floating-point issues in C11 from PDTS 18661-1 UK review, Issue 2

Summary

Issue 2: Definition of FLT_ROUNDS

The C11 definition of FLT_ROUNDS is inadequate in that it refers to floating-point addition but does not say addition of what type. If long double is not an IEC 60559 type, it may not fully support all rounding modes even though they are supported by other types. (This is an actual issue with real implementations using non-IEC 60559 types for long double.) A suitable fix would be:

In 5.2.4.2.2#8, insert "for type float" after "floating-point addition". At the end of F.2#1, insert "The value of FLT_ROUNDS applies to all IEC 60559 types supported by the implementation, but may not apply to non-IEC 60559 types.".

The committee adopted a Proposed Committee Response that has been substantially revised.

Oct 2014 meeting

Committee Discussion

Proposed Committee Response (obsolete)

The committee regards the existing definition of FLT_ROUNDS as intended to apply to types float, double and long double. However, if all three types cannot support the same set of rounding modes, the implementation needs to set FLT_ROUNDS to -1 meaning indeterminable.

As has been pointed out, in Annex F, only the types float and double need be IEC 60559 types. If long double is not an IEC 60559 type (for example, a pair of doubles), it may not support the same set of rounding modes as float and double. In this case, having FLT_ROUNDS apply to float and double (but not long double) would result in a value of 0, 1, 2, or 3 and would provide new and useful information to the programmer.

However, this behavioral change could also break existing programs, and as such the committee prefers to leave as is for this revision of the Standard.

Apr 2015 meeting

Committee Discussion

Proposed Committee Response

The implementation of long double, for example, may significantly differ from IEC floating types and may not support the same choices as would otherwise be possible for FLT_ROUNDS. All known implementations define FLT_ROUNDS as the value 1 (round to nearest). and as such exempting non-IEC long double behavior allows the potential for implementations to provide the full range of possible values for IEC floating types.

Proposed Technical Corrigendum

At the end of F.2#1, insert

The value of FLT_ROUNDS applies to all IEC 60559 types supported by the implementation, but need not apply to non-IEC 60559 types.

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DR 442

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Submitter: Joseph Myers
Submission Date: 2013-07-21
Source: WG 14
Reference Document: N1730
Subject: Floating-point issues in C11 from PDTS 18661-1 UK review, Issue 3

Summary

Issue 3: Floating-point exceptions and 6.5#5

C11 6.5#5 says "If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined.". When the Annex F bindings are in effect, it must be intended that floating-point exceptions do not produce such undefined behavior (instead, behavior such as evaluating to a NaN must be defined). But no normative text actually says that.

A suitable fix would be:

Append to 6.5#5: For implementations defining __STDC_IEC_559__, this does not apply to exceptional conditions where the behavior (such as raising a floating-point exception and returning a NaN) is defined by Annex F, directly or by reference to IEC 60559.

Committee Discussion

• The following words were discussed extensively and thought appropriate as appropriate for inclusion in F.3, rather than a new F.4:

  If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), and the behaviour is not defined in this annex or by reference to IEC 60559, 6.5p5 applies and the behaviour is undefined.

• The committee also suggested adding an example or footnote suggesting that this would apply to a non-IEC 60550 long double.

Apr 2014 meeting

Committee Discussion

Further correspondence with the author and excerpted in N1804 has identified the core issue as being a simple misunderstanding of the applicability of normative annexes to the standard.

Proposed Committee Response

WG14 treats normative annexes such as Annex F as if they were linear extensions of the standard itself. When Annex F is requested via definition of __STDC_IEC_559__ then 6.5#5 is superseded and floating point exceptions become well defined.

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DR 443

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Submitter: Joseph Myers
Submission Date: 2013-07-21
Source: WG 14
Reference Document: N1730
Subject: Floating-point issues in C11 from PDTS 18661-1 UK review, Issue 4

Summary

Issue 4: Floating-point state not being an object

The description of the floating-point environment in C11 fails to make sufficiently clear what is or is not an object (C11 footnote 205 is not normative, and so cannot be used to that effect); it uses terms such as "system variable" without saying what that is. Simply moving that footnote to normative text would fix this issue:

Move the contents of footnote 205 (C11 subclause 7.6) to the end of 5.1.2.3#2.

Committee Discussion

• The committee agrees that the floating point environment is not an object and must be stated in normative terms instead of in non-normative footnote 205.
• The environment may be modified more than once in an expression without inducing undefined behavior, this is just "how it works".
• The floating-point environment is described in the standard using an otherwise undefined term "system variable" having 7.6p2 thread storage duration, implying that it is an object, and so this is perhaps the actual manifestation of the defect that requires correction.
• The floating-point environment is technically an actor, not an object, having a model of its behavior defined purely by messages as its internal "piece of state" is not directly visible. In this case the messages are the macros and functions, as are the electronic signals from the floating-point computational units to consult and honor the desired floating point modal behaviors, and the electronic signals that record "auxiliary information" in the model for floating point exceptions.
• In any case, the floating point environment implementation is not described by the abstract C machine, only its operations.
• The committee prefers that this be cleared up in 7.6, by moving the second sentence and possibly the first sentence into normative text.

Apr 2014 meeting

Committee Discussion

The committee discusses this issue further and could not see an actual defect: there are no misinterpretations stated or implied.

Proposed Committee Response

Since operations on the floating point environment are well defined there is no need to normatively define anything further about its implementation. The footnote adds clarity and should remain as is.

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DR 444

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Submitter: Joseph Myers
Submission Date: 2013-07-23
Source: WG 14
Reference Document: N1731
Subject: Issues with alignment in C11, part 1

Summary

Issue 1: Existence of over-aligned types

6.2.8#3 defines the concept of an over-aligned type, with a footnote saying "Every over-aligned type is, or contains, a structure or union type with a member to which an extended alignment has been applied.". But there is no way in the syntax to apply such an alignment to a member. _Alignas appears in the syntax for alignment-specifier, which in turn appears in that for declaration-specifiers (6.7#1). But structure and union members instead use struct-declaration which uses specifier-qualifier-list which doesn't include a case for alignment-specifier at all. So for the reference to over-aligned types, and the reference in 6.7.5#6 to the "declared object or member", to be meaningful, something needs adding to the syntax for struct-declaration. (Note that specifier-qualifier-list is also used in the syntax for type-name, and it seems less likely that a type-name was intended to be able to include alignment-specifiers.)

Committee Discussion

• The desired and intended syntax is indeed missing.
• Whereas one could achieve the desired effect by placing the directive on the aggregate itself and controlling it by the maximum alignment of each of its members, this is far from the intended goal, and we do consider this a defect.
• We solicit a suggested technical corrigendum from the author.
• Modifying the definition of type name will require considerable thought.
• Applying the directive beyond aggregate members, such as a simple scalar declaration, is not well defined, and brings in the various storage durations and how or whether all or any are supported.

Apr 2014 meeting

Committee Discussion

• The author provided the following technical corrigendum as input to N1804:
  6.7.2.1#1: Move the definition of specifier-qualifier-list to 6.7.7#1. Insert a new definition in 6.7.2.1#1:

  specifier-qualifier-alignment-list:

  type-specifier specifier-qualifier-alignment-list_opt
  type-qualifier specifier-qualifier-alignment-list_opt
  alignment-specifier specifier-qualifier-alignment-list_opt

  Change the use of specifier-qualifier-list in the syntax for struct-declaration to specifier-qualifier-alignment-list.

  6.7.2#2:

  Change "and in the specifier-qualifier list in each struct declaration and type name" to ", in the specifier-qualifier-alignment list in each struct declaration and in the specifier-qualifier list in each type name".

  6.7.3#5:

  Change "specifier-qualifier-list" to "specifier-qualifier-list, specifier-qualifier-alignment-list or declaration-specifiers" (twice).

  (That the wording about duplicate qualifiers and qualifiers used with _Atomic doesn't deal with declaration-specifiers, the syntax production relevant to normal declarations, is a pre-existing problem noticed in the course of preparing this wording.)

  (I believe all the semantics and constraints required for alignment specifiers on members are in place, including 6.2.7#1 dealing with cross-translation-unit type compatibility and references to bit-fields and members in 6.7.5.)

• The committee agrees with this as the proper direction and will solicit further review from implementors.

Oct 2014 meeting

Committee Discussion

There still has not been adequate review of these changes. The Project Editor and others have been asked to examine these changes closely prior to our next meeting.

Apr 2015 meeting

Committee Discussion

The suggested syntax provided by the author has been adopted on a trial basis in at least one implementation. It does not, however, provide for compound literals.

A simpler syntax change was discussed, to wit

specifier-qualifier-list:

type-specifier specifier-qualifier-list_opt

type-qualifier specifier-qualifier-list_opt

alignment-specifier specifier-qualifier-list_opt

where specifier-qualifier-list is used in the grammar in only two productions: struct-declaration (which relates to the primary purpose of this DR), and type-name, which is used only in the definitions of these constructs:

• generic association (generic selection)
• compound literal
• sizeof expression
• _Alignof expression
• cast expression
• atomic type specifier
• alignment specifier

A constraint could be added to 6.7.7 type-name after paragraph 1 disallowing the use of alignment-specifier in a type-name except in the case of compound literal which was deemed useful by the committee. The following principles were elucidated:

_Alignas needs to be applied wherever objects are laid out in memory. On modern architectures page and cache line alignment of data structures is increasingly critical for performance.

Alignment is incorporated into the type system for structure (and union) members, but otherwise is not considered part of the type.

In 6.7.3p5, there are two references to specifier-qualifier-list, which should also reference declaration specifiers.

In 6.7.5 paragraphs 2 and 4, there are occurrences of the phrase “alignment attribute” which should instead read “alignment specifier”

Oct 2015 meeting

Committee Discussion

The committee did not discuss the direction from the last meeting in any substantial manner. It has solicited a paper from the author of the direction expressing these ideas as a Suggested Technical Corrigendum.

Apr 2016 meeting

Committee Discussion

A new paper N2028 was submitted that embodied the direction above and the committee accepted it.

Proposed Technical Correigendum

Change 6.7.2.1p1 from

specifier-qualifier-list:

type-specifier specifier-qualifier-list_opt

type-qualifier specifier-qualifier-list_opt

to

specifier-qualifier-list:

type-specifier specifier-qualifier-list_opt

type-qualifier specifier-qualifier-list_opt

alignment-specifier specifier-qualifier-list_opt

Change 6.7.5p2 from

An alignment attribute shall not be specified in a declaration of a typedef, or a bit-field, or a function, or a parameter, or an object declared with the register storage-class specifier.

to

An alignment specifier shall appear only in the declaration specifiers of a declaration, or in the specifier-qualifier list of a member declaration, or in the type name of a compound literal. An alignment specifier shall not be used in conjunction with either of the storage-class specifiers typedef or register, nor in a declaration of a function or bit-field.

Change 6.7.3p5 from

If the same qualifier appears more than once in the same specifier-qualifier-list, either directly or via one or more typedefs, the behavior is the same as if it appeared only once. If other qualifiers appear along with the _Atomic qualifier in a specifier-qualifier-list, the resulting type is the so-qualified atomic type.

to

If the same qualifier appears more than once in the same specifier-qualifier-list or as declaration-specifiers, either directly or via one or more typedefs, the behavior is the same as if it appeared only once. If other qualifiers appear along with the _Atomic qualifier the resulting type is the so-qualified atomic type.

Change 6.7.5p4 from

The combined effect of all alignment attributes in a declaration shall not...

to

The combined effect of all alignment specifiers in a declaration shall not...

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DR 445

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Submitter: Joseph Myers
Submission Date: 2013-07-23
Source: WG 14
Reference Document: N1731
Subject: Issues with alignment in C11, part 2

Summary

Issue 2: Contexts in which alignments are supported

6.2.8#2 defines "fundamental alignment": "A fundamental alignment is represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts, which is equal to _Alignof (max_align_t)."

6.2.8#3 defines "extended alignment": "An extended alignment is represented by an alignment greater than _Alignof (max_align_t). It is implementation-defined whether any extended alignments are supported and the contexts in which they are supported. A type having an extended alignment requirement is an over-aligned type."

6.2.8#4 defines "valid alignment", saying "Alignments are represented as values of the type size_t. Valid alignments include only those values returned by an _Alignof expression for fundamental types, plus an additional implementation-defined set of values, which may be empty. Every valid alignment value shall be a nonnegative integral power of two.".

max_align_t is specified in 7.19#2 as "an object type whose alignment is as great as is supported by the implementation in all contexts".

The memory management functions in 7.22.3 are defined to return a pointer "suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and then used to access such an object or an array of such objects in the space allocated". In the case of aligned_alloc, there may be a stricter requirement given by the alignment passed to the function, but the alignment passed to the function can't result in memory any less-aligned than a fundamental alignment requirement. The alignment requirement still applies even if the size is too small for any object requiring the given alignment (see the response to C90 DR#075).

There are various problems with the above:

• The term "fundamental type" is not defined in C11.
• There is also no definition of what a "context" is in which an alignment might or might not be supported. In common implementation practice, separate contexts might be by the storage duration of the object (static, thread, automatic, allocated, with the last referring to the alignments guaranteed by calloc, malloc and realloc).
• A "valid alignment" may not be a "fundamental alignment". Thus, whatever interpretation is adopted for "fundamental type", nothing in the standard requires the alignment of a "fundamental type" to be a "fundamental alignment". For example, say "long double" is a "fundamental type"; it would seem nonsensical if declaring "long double" objects (in any context) failed to work, but nothing seems to require malloc to return objects sufficiently aligned for long double.
• Given these gaps in the definition, nothing in the normative text appears to imply footnote 57 "Every over-aligned type is, or contains, a structure or union type with a member to which an extended alignment has been applied.", although no doubt it reflects the intent.
• If "fundamental type" is interpreted to mean "basic type", that is not sufficient to resolve these lacunae. For example, if

  struct s { long double ld; }

  has an alignment requirement bigger than long double, it should still be possible to allocate memory for it with malloc, and the same applies to any typedef from a standard header that might also have a bigger alignment requirement than any basic type.

The following principles seem natural for any fix for this issue:

• C99 referred to "any type of object" in the alignment requirements for calloc, malloc and realloc. As a matter of compatibility, this means that any type that could be constructed within C99 (including one using types from standard headers) should have an alignment required by C11 to be supported in all contexts, and the same applies to types from C extensions originally specified as extensions to C99. Most of the following principles follow to a greater or lesser extent from this compatibility principle.
• All basic types have alignments supported in all contexts.
• All enumerated types have alignments supported in all contexts.
• All pointer types have alignments supported in all contexts (even if the type pointed to does not).
• All types from standard headers specified as complete object types in the definitions of those headers have alignments supported in all contexts. (This includes both types specified as typedefs and types specified as structs or unions with a given tag.)
• If a type has an alignment supported in all contexts, so do arrays of that type, qualified versions of that type, and atomic versions of that type.
• If all the members of a structure or union have types with alignments supported in all contexts, and none of them use an _Alignas specifier specifying an alignment bigger than supported in all contexts, then that structure or union has an alignment supported in all contexts.
• Where C extensions such as TS 18661-2 and 18661-3 are proposed that define new types, or existing such extensions such as TR 18037 are revised and updated for C11, care should be taken that the new types are covered under the above, whether through being basic types or through being defined in standard headers. (If SIMD vector types, as mentioned at http://www.open-std.org/pipermail/cplex/2013-June/000010.html, were to end up in any such extension, it would probably be appropriate to define them in a way that does *not* require calloc, malloc and realloc to return memory suitably aligned for them; such types often require alignments bigger than needed for any other type, so imposing such requirements on memory allocation functions would result in undue inefficiency.)

Committee Discussion

• It is indeed the case that 6.2.8p3 "fundamental type" is not defined. The intention is that it be "basic type", such that "fundamental alignment" is well defined, and yet this is not the case, and "fundamental alignment" also needs a definition. It is likely that the definition of fundamental type can be made editorially, but that when and where fundamental alignment is required is substantive.
• It is also the case that 6.2.8p3 "contexts" is not defined. It is intended to mean the full set of storage durations. Perhaps we should say "storage duration contexts".
• It is also the case that "valid alignment" may not be a "fundamental alignment".
• It was the case in C99 that malloc was required to return memory appropriate for "any type", and this should not be lost in the new wording regarding types that are not over-aligned. All other C99 behaviors must also be preserved as he describes as guidance.
• All types declared without the _Alignas directive are intended to have fundamental alignment, which must be tied somehow to basic type. A consequence is that the only manner in which one can produce an over-aligned type is through _Alignas. malloc has been defined in terms of fundamental alignment, yet we do not anywhere strongly state that, say, int has such fundamental alignment.
• These are several interwoven definitions and missing definitions that need a comprehensive review for adequate correction. We solicit the author for a suggested technical corrigendum. A copy of the audio discussion can be provided.

Apr 2014 meeting

Committee Discussion

• The author provided input to N1804 as a proposed technical corrigendum:
• Change 6.2.8#2 to:

  A fundamental alignment is a valid alignment less than or equal to _Alignof (max_align_t). Fundamental alignments shall be supported by the implementation for objects of all storage durations. The alignment requirements of the following types shall be fundamental alignments:

  • all atomic, qualified or unqualified basic types;
  • all atomic, qualified or unqualified enumerated types;
  • all atomic, qualified or unqualified pointer types;
  • all array types whose element type has a fundamental alignment requirement;
  • all types specified in clause 7 as complete object types;
  • all structure or union types all of whose elements have types with fundamental alignment requirements and none of whose elements have an alignment specifier specifying an alignment that is not a fundamental alignment.

  In 6.2.8#3, change "the contexts in" to "the storage durations of objects for which".

  In 6.2.8#4, change "those values returned by an _Alignof expression for fundamental types" to "fundamental alignments".

  (Note that the above admits the possibility that e.g. 1 and 4 are fundamental alignments, but 2 isn't, in which case it can't be an extended alignment either. If we want to ensure that 2 isn't a valid alignment at all in that case, rather than possibly having valid alignments that are neither fundamental nor extended, also change "additional implementation-defined set of values" to "additional implementation-defined set of extended alignments" in 6.2.8#4.) In 6.7.5#3, change "in the context in which it appears" to "for an object of the storage duration, if any, being declared". Add a new constraint at the end of 6.7.5#3: "An object shall not be declared with an over-aligned type with an extended alignment requirement not supported by the implementation for an object of that storage duration.".

  (This deals with the point that if an over-aligned struct is defined, then objects with that type may be supported with some storage durations but not others, so it is the declaration of an object with that type that can violate a constraint rather than the declaration of the struct.)

  In 7.19#2, change "whose alignment is as great as is supported by the implementation in all contexts" to "whose alignment is the greatest fundamental alignment".

  (I'm trying to avoid an undesirable implication that if an alignment of e.g. 1MB is supported in all contexts - and it's quite plausible that an implementation can support more or less arbitrary alignments - then max_align_t must have such an alignment and so such an alignment must count as fundamental and so malloc must return memory suitable aligned for it.)

• This is a difficult area and further study is required.

Oct 2014 meeting

Committee Discussion

The proposed changes have raised no concerns and so the committee has agreed to use them as the following Proposed Technical Corrigendum.

Proposed Technical Corrigendum

Change 6.2.8#2 to:

A fundamental alignment is a valid alignment less than or equal to _Alignof (max_align_t). Fundamental alignments shall be supported by the implementation for objects of all storage durations. The alignment requirements of the following types shall be fundamental alignments:

• all atomic, qualified or unqualified basic types;
• all atomic, qualified or unqualified enumerated types;
• all atomic, qualified or unqualified pointer types;
• all array types whose element type has a fundamental alignment requirement;
• all types specified in clause 7 as complete object types;
• all structure or union types all of whose elements have types with fundamental alignment requirements and none of whose elements have an alignment specifier specifying an alignment that is not a fundamental alignment.

In 6.2.8#3, change

"the contexts in"

"the storage durations of objects for which".

In 6.2.8#4, change

"those values returned by an _Alignof expression for fundamental types"

"fundamental alignments".

In 6.7.5#3, change

"in the context in which it appears"

"for an object of the storage duration, if any, being declared".

Add a new constraint at the end of 6.7.5#3:

"An object shall not be declared with an over-aligned type with an extended alignment requirement not supported by the implementation for an object of that storage duration.".

In 7.19#2, change

"whose alignment is as great as is supported by the implementation in all contexts"

"whose alignment is the greatest fundamental alignment".

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DR 446

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Submitter: Blaine Garst
Submission Date: 2013-07-31
Source: WG 14
Reference Document: N1736
Subject:Use byte instead of character for memcmp, memcpy

Summary

Although it is clear by 7.24.1 String function conventions paragraph 3

For all functions in this sub clause, each character shall be interpreted as if it had the type unsigned char

It would be simpler and more to the point if the three memory functions describe their count parameter n in terms of bytes.

Suggested Technical Corrigendum

memcpy

Change 7.24.2.1 p 2 first sentence from

The memcpy function copies n characters from the object pointed to by s2 into the object pointed to by s1.

to

The memcpy function copies n bytes from the object pointed to by s2 into the object pointed to by s1.

memmove

Change 7.24.2.2 p 2 from

The memmove function copies n characters from the object pointed to by s2 into the object pointed to by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and then the n characters from the temporary array are copied into the object pointed to by s1.

to

The memmove function copies n bytes from the object pointed to by s2 into the object pointed to by s1. Copying takes place as if the n bytes from the object pointed to by s2 are first copied into a temporary array of n bytes that does not overlap the objects pointed to by s1 and s2, and then the n bytes from the temporary array are copied into the object pointed to by s1.

memcmp

Change 7.24.4.1 p 2 from

The memcmp function compares the first n characters of the object pointed to by s1 to the first n characters of the object pointed to by s2.

to

The memcmp function compares the first n bytes of the object pointed to by s1 to the first n bytes of the object pointed to by s2.

Proposed Committee Response

After reviewing the original motivation and suggestion for change, it was noted by the project editor that "character" is used in several distinct contexts, and that it would be inappropriate to simply improve one area without a comprehensive review of all uses such that the existing consistency of uses of character be replaced in a consistent new manner, as yet undetermined. As it stands, although careful reading is strictly required, it is correct and as such this is not a defect.

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DR 447

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Submitter: Fred J. Tydeman
Submission Date: 2013-08-20
Source: WG 14
Reference Document: N1739, DR 285
Subject: Boolean from complex

Summary

What is the value of: _Bool b = 0.0 + 3.0*I;

I believe that there is a contradiction between 6.3.1.2 Boolean type and 6.3.1.7 Real and complex in that one requires that the value to be 1 and the other requires the value to be 0.

6.3.1.2 Boolean type

1 When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1.

6.3.1.7 Real and complex

2 When a value of complex type is converted to a real type, the imaginary part of the complex value is discarded and the value of the real part is converted according to the conversion rules for the corresponding real type.

DR 285 against C99 had a similar issue on conversion from imaginary to boolean. That resulted in:

G.4.2 Real and imaginary

1 When a value of imaginary type is converted to a real type other than _Bool,376) the result is a positive zero.

376) See 6.3.1.2.

Suggested Technical Corrigendum

I believe that 6.3.1.7 Real and complex, paragraph 2 should be changed to:

2 When a value of complex type is converted to a real type other than _Bool(footnote), the imaginary part of the complex value is discarded and the value of the real part is converted according to the conversion rules for the corresponding real type.

(footnote) See 6.3.1.2.

Committee Discussion

The committee agrees.

Proposed Technical Corrigendum

Change 6.3.1.7, paragraph 2 to:

When a value of complex type is converted to a real type other than _Bool(footnote), the imaginary part of the complex value is discarded and the value of the real part is converted according to the conversion rules for the corresponding real type.

(footnote) See 6.3.1.2.

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Submitter: Fred J. Tydeman
Submission Date: 2013-08-20
Source: WG 14
Reference Document: N1740, DR 231, DR 250
Subject: What are the semantics of a # non-directive?

Summary

What is a directive name? What are the semantics of a # non-directive?

In particular, what should happen for a translation unit with these four lines:

# non-directive
# "Long string"
# 'Many characters are implementation defined'
# 1234


The syntax in 6.10 Preprocessing directives has in group-part:

# non-directive

The C standard section 6.10, paragraph 3 has:

A non-directive shall not begin with any of the directive names appearing in the syntax.

I find that confusing as directive name is only used in the C standard in 6.10 paragraphs 3 and 4 (without any definition). So, what is a directive name?

Assuming directive name is one of:

• if
• ifdef
• ifndef
• elif
• else
• endif
• include
• define
• undef
• line
• error
• pragma

then my four line program contains lines that are

# non-directive

so should be valid. However, almost every C compiler I have tried considers them errors that end translation. I did find at least one C compiler that ignored them (treated them as comments). I did find at least one C compiler that considered them errors even inside of:

#if 0
#endif


where they should have been ignored.

I believe that gcc treats

# 1234

# line 1234

I see no semantics for non-directive. So, what is supposed to happen with them? Is it implicitly undefined?

Since preprocessing directives (which includes non-directive) are deleted at the end of translation phase 4, these non-directives could act as comments.

DRs 231 and 250 appear to contradict each other on what happens with a non-directive and neither refers to the other.

DR 231 Says that text-line and non-directive are not implementation defined. They are placeholders in the phases of translation and are subject to normal processing in subsequent phases of translation. And that words were supposed to be added to the Rationale.

DR 250 Says that non-directive is a preprocessing directive. And, it added that as a footnote in 6.10.3#11

Neither DR added normative words.

In answering this, we should consider what happens with mis-spellings, such as:

• #endf
• #deifine

Should # non-directive be a comment (and ignored)? Implementation defined? An error that ends translation (like #error)? Undefined behaviour?

Suggested Technical Corrigendum

Since I do not know what should happen, I have none. But, if we decide on undefined behaviour, I would like that as explicit words.

Committee Discussion

There is intentional vagueness in this area such that implementations have and exhibit unspecified and useful additional behaviors. This has been a source of historical confusion and should be addressed.

Oct 2014 meeting

Committee Discussion

Proposed Technical Corrigendum

Add new paragraph 6.10 paragraph 9:

The execution of a non-directive preprocessing directive results in undefined behavior.

Add to Annex J.2:

The execution of a non-directive preprocessing directive (6.10)

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DR 449

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Submitter: Douglas Walls
Submission Date: 2013-08-24
Source: WG 14
Reference Document: N1744
Subject:What is the value of TSS_DTOR_ITERATIONS for implementations with no maximum?

Summary

Suggested Technical Corrigendum

Oct 2013 meeting

Committee Discussion

• The committee notes that existing practice in all cases has provided a small integer value for this definition.
• Allowing no restrictions is likely to break existing practice.

Committee Discussion

The question posed should be answered.

Proposed Committee Response

The standard intentionally does not define a value of TSS_DTOR_ITERATIONS for implementations with no maximum.

The TSS_DTOR_ITERATIONS macro is used to limit recursion at thread termination. The issue is that existing practice allows the creation of new tss bindings during the destructor call, and one destructor might reincarnate the original, thus forming an infinite recursive destructor loop. This loop may appear non-deterministically and is difficult to detect. The purpose of TSS_DTOR_ITERATIONS is as a bound to such recursion.

It is possible monitor the recursion depth with careful defensive programming and in those cases the value of TSS_DTOR_ITERATIONS is useful as that bound.

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DR 450

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Submitter: Martin Sebor
Submission Date: 2013-09-02
Source: WG 14
Reference Document: N1752
Subject: tmpnam_s clears s[0] when maxsize > RSIZE_MAX

Summary

The majority of bounds checking functions are specified to set the first element of the destination buffer, s[0], to the NUL character when a constraint violation occurs and the s pointer is non-null and the size of the buffer is greater than zero and less than or equal to SIZE_MAX.

However, the tmpnam_s function sets s[0] to NUL even when maxsize is greater than RSIZE_MAX, making its behavior on constraint violation inconsistent with the rest.

Suggested Technical Corrigendum:

Change paragraph 8 in the Returns section of tmpnam_s to read:

• If no suitable string can be generated, or if there is a runtime-constraint violation and s is not null and maxsize is greater than zero and not greater than RSIZE_MAX, the tmpnam_s function sets s[0] to the null character and returns a nonzero value.

Committee Discussion

The committee agrees with the issue, and requests that the suggested technical corrigendum be broken into more parts for both clarity and consistency.

Apr 2014 meeting

Committee Discussion

The committee did not receive revised words and will again solicit them from the author.

Oct 2014 meeting

Committee Discussion

The paper N1873 was provided and discussed, and after several revisions the following proposal were approved.

Proposed Technical Corrigendum

Change K.3.5.1.2 paragraph 8 (the Returns section of tmpnam_s) from:

If no suitable string can be generated, or if there is a runtime-constraint violation, the tmpnam_s function writes a null character to s[0] (only if s is not null and maxsize is greater than zero) and returns a nonzero value.

to:

If no suitable string can be generated, or if there is a runtime-constraint violation, the tmpnam_s function:

• if s is not null and maxsize is both greater than zero and not greater than RSIZE_MAX, writes a null character to s[0]
• returns a nonzero value.

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DR 451

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Submitter: Freek Wiedijk and Robbert Krebbers (Radboud University Nijmegen, The Netherlands)
Submission Date: 2013-08-30
Source: WG 14
Reference Document: N1747
Subject: Instability of uninitialized automatic variables

Summary

The standard is unclear about the following questions:

1. Can an uninitialized variable with automatic storage duration (of a type that does not have trap values, whose address has been taken so 6.3.2.1p2 does not apply, and which is not volatile) change its value without direct action of the program?
2. If the answer to question 1 is "yes", then how far can this kind of "instability" propagate?
3. If "unstable" values can propagate through function arguments into a called function, can calling a C standard library function exhibit undefined behavior because of this?

Specifically, consider:

unsigned char x[1]; /* intentionally uninitialized */
printf("%d\n", x[0]);
printf("%d\n", x[0]);

Does the standard allow an implementation to let this code print two different values? And if so, if we insert either of the following three statements

x[0] = x[0];
x[0] += 0;
x[0] *= 0;

between the declaration and the printf statements, is this behavior still allowed? Or alternatively, can these printf statements exhibit undefined behavior instead of having to print a reasonable number.

Motivation and discussion

The standard is unclear about these questions.

On the one hand the committee response to Defect Report #260 strongly suggests that the committee decided that the standard implies the answer to question 1 to be "yes". (Although Defect Report #260 applies to the C99 standard and hence has been superseded by the C11 standard, no modification to the standard text was deemed necessary at the time, and all relevant text in the C11 standard is identical to that in the C99 standard.) The relevant quote from the committee response to Defect Report #260 is:

In the case of an indeterminate value [...] the actual bit-pattern may change without direct action of the program.

A subtlety is that Defect Report #260 talks about bit-patterns and not about values, but for variables of type unsigned char there is a one-to-one correspondence between bit-patterns and values.

Another argument in favor of "instability" of indeterminate values is that values can "become indeterminate" (e.g. 5.1.2.3p5, 6.2.4p2, and 6.2.4p6). In these cases the value of an object may also change without an explicit store (and can keep changing?)

On the other hand, 6.7.9p10 states that the kind of uninitialized variables that we are discussing get an indeterminate value. From 3.19.2 it follows that if a type has no trap values, then indeterminate and unspecified values are the same. And in 3.19.3, it is stated explicitly that an unspecified value is chosen. Which implies that the value - after having been chosen - cannot change anymore.

Another argument against "instability" is that 6.8p3 states that "the values are stored in the objects (including storing an indeterminate value in objects without an initializer) each time the declaration is reached in the order of execution", and that 6.2.4p2 states that "An object [...] retains its last-stored value throughout its lifetime." The only way that one could read this in light of Defect Report #260 is if "retaining an indeterminate value" is read as meaning that the indeterminateness of the value is retained, without the value having a specific value.

It seems attractive to make a distinction between indeterminate values that are allowed to change without direct action of the program in the way that Defect Report #260 interpreted the standard, and unspecified values that do not have this property. However the current text of 3.19.2 does not allow for this interpretation. Also, probably some instances of "indeterminate" and "unspecified" would need to be changed for such an interpretation to make sense. (For example in 6.2.6.1p6 "the bytes of the object representation that correspond to any padding bytes take unspecified values." should probably become "... take indeterminate values.")

The reason for question 3 is that if the kind of "instability" that questions 1 and 2 ask for is allowed to propagate maximally, then it becomes impossible to implement printf in C itself. When converting an indeterminate value to a string of output characters, the value can keep changing underneath, and the code cannot protect itself against this.

On the other hand, if library functions exhibit undefined behavior on these kinds of "unstable" uninitialized values, then an fwrite of a struct with uninitialized padding bytes would also give undefined behavior. The fact that one wants to be able to copy uninitialized padding bytes in structs using memcpy without undefined behavior is the reason that using the value of an uninitialized object is not undefined behavior. This seems to suggest that an fwrite of a struct with uninitialized padding bytes should not exhibit undefined behavior.

Possible Resolutions

We see three reasonable sets of answers to these questions:

Resolution (a)

1. no
2. not applicable
3. not applicable

Advantage

Easy to repair the unclarity in the standard. Just add text that explicitly states that indeterminate values cannot change without direct action from the program. This will prevent people from invoking the response to Defect Report #260 from then on.

Disadvantage

Restricts the kind of optimizations compilers are allowed to perform.

Resolution (b)

1. yes
2. any operation performed on indeterminate values will have an indeterminate value as its result
3. no

Specifically, "unstable" values will also propagate through function calls. Also, after

x[0] *= 0;

the value of x[0] still will be "unstable" and hence still can be any byte, and will not necessary be 0.

Advantage

Gives compilers more freedom to perform optimizations.

Is Defect Report #260-compliant (i.e., "the committee did not change its mind").

Disadvantage

Needs more modifications to the text of the standard. It will then be necessary to make an explicit distinction between "indeterminate non-trap value" and "unspecified value".

Resolution (c)

1. yes
2. any operation performed on indeterminate values will have an indeterminate value as its result
3. yes, library functions will exhibit undefined behavior when used on indeterminate values (probably functions like memcpy and maybe fwrite should be immune from this)

Advantage

Restricts program behaviors least, giving compilers even more freedom.

Disadvantage

Needs even more modification to the text of the standard.

Needs a decision on what library functions will not have undefined behavior when working on indeterminate values.

This is certainly not compatible with the current version of the standard, as no undefined behavior of this kind related to library functions is described there.

Suggested Technical Corrigendum

For resolution (a)

In 6.2.4p2, change "An object exists, has a constant address, and retains its last-stored value throughout its lifetime." to "An object exists and has a constant address throughout its lifetime. The value of an object is retained, including the object representation, until some other value is stored into it, or until the moment when the value becomes indeterminate (at which moment it is replaced with an indeterminate value, and after which that value is retained again)."

For resolution (b)

In 3.19.2 change "either an unspecified value or a trap representation" to "either an unspecified value or a trap representation, which can change arbitrarily without direct action from the program".

In 6.2.4p2, change "An object exists, has a constant address, and retains its last-stored value throughout its lifetime." to "An object exists, has a constant address, and retains its last-stored value (provided this value is not indeterminate), throughout its lifetime."

At the end of 6.5p1 add "If at least one of the operands of an operator is indeterminate, the result of the operator is also indeterminate."

Some instances of "indeterminate" and "unspecified" (to be determined) should be replaced by respectively "unspecified" and "indeterminate". See for example the instance in 6.2.6.1p6 mentioned earlier.

For resolution (c)

The changes for resolution (b), and also:

In 7.1.4p1 add: "If a function is called with an indeterminate value, the behavior is undefined."

In a selection (to be determined) of functions from the library, add text that counters this general statement added to 7.1.4p1.

Committee Discussion

• The committee asserts that the answer to the first question is "yes", an uninitialized value under the conditions described can appear to change its value.
• Since question 2 is applicable, the answer is that any operation performed on indeterminate values will have an indeterminate value as its result.
• The answer to the applicable question 3 is that indeed library functions will exhibit undefined behavior when used on indeterminate values.
• The committee notes that the problem applies to all types that do not have trap representations as per the implementation.
• Strong sentiment formed, in part based on prior experience in developing Annex L, that a new category of "wobbly" value is needed. The underlying issue is that modern compilers track value propagation, and uninitialized values synthesized for an initial use of an object may be discarded as inconsequential prior to synthesizing a different value for a subsequent use. To require otherwise defeats important compiler optimizations. All uses of "wobbly" values might be deemed undefined behavior.
• The principle of Resolution C is desired.
• This requires careful thought.

Apr 2014 meeting

Committee Discussion

The author provided N1793 and an accompanying presentation N1818 in which his position changed to believing that "wobbly" values are not actually defined by the standard, and after discussion agreed that the following committee response would be an acceptable resolution.

Proposed Committee Response

• The answer to question 1 is "yes", an uninitialized value under the conditions described can appear to change its value.
• The answer to question 2 is that any operation performed on indeterminate values will have an indeterminate value as a result.
• The answer to question 3 is that library functions will exhibit undefined behavior when used on indeterminate values.
• These answers are appropriate for all types that do not have trap representations.
• This viewpoint reaffirms the C99 DR260 position.
• The committee agrees that this area would benefit from a new definition of something akin to a "wobbly" value and that this should be considered in any subsequent revision of this standard.
• The committee also notes that padding bytes within structures are possibly a distinct form of "wobbly" representation.

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DR 452

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Submitter: Shao Miller
Submission Date: 2013-09-29
Source: WG 14
Reference Document: N1762
Subject: Effective Type in Loop Invariant

Summary

6.5p6:

The effective type of an object for an access to its stored value is the declared type of the object, if any.87) If a value is stored into an object having no declared type through an lvalue having a type that is not a character type, then the type of the lvalue becomes the effective type of the object for that access and for subsequent accesses that do not modify the stored value. If a value is copied into an object having no declared type using memcpy or memmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one. For all other accesses to an object having no declared type, the effective type of the object is simply the type of the lvalue used for the access.

Given the following code:

union u1 {
    int x;
    long y;
  };

int func1(void) {
    union u1 o1 = { 42 };

    return (0, o1).x;
  }

The o1 sub-expression in the return statement's expression accesses the stored union value of the object. The comma operator's result has that value, but it is not an lvalue and so "effective type" does not appear to apply. While the access to o1 involves an access to a stored value, the membership operator can be said to access an object whose value is available, but perhaps not exactly "stored." o1.x is an lvalue, but (0, o1).x is not.

6.5.2.3p3:

A postfix expression followed by the . operator and an identifier designates a member of a structure or union object. The value is that of the named member,95) and is an lvalue if the first expression is an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type of the designated member.

6.5p7:

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:88)

• a type compatible with the effective type of the object,
• a qualified version of a type compatible with the effective type of the object,
• a type that is the signed or unsigned type corresponding to the effective type of the object,
• a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
• an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or
• a character type.

Given:

union u2 {
    int x;
    long y;
    char ca[2];
  };

int func2(void) {
    union u2 o2 = { 42 };

    return (0, o2).x;
  }

We have a similar situation, even though (0, o2) yields an object with temporary lifetime. (Side question: Should the expression (0, o2).ca == o2.ca yield zero, non-zero, or should it be implementation-defined?)

Suppose we have a portable strategy to determine whether or not the object representations of int and long are the same. If they are and if we have the following code:

union u3 {
    int x;
    long y;
  };

long func3(void) {
    union u3 o3;

    o3.x = 42;
    return (0, o3).y;
  }

Are we violating the effective type rules? We might expect type-punning to be relevant here and the membership operator to be accessing a member value of a union value.

If the answer is yes, then does the Standard define the effective type of the non-lvalue expression 0, o3 ?

If the answer is no, then this can cause the loss of an optimization opportunity in the following code:

struct s4 {
    int x;
    float f;
  };

void func4(long * lp, struct s4 * s4p) {
    int c;

    for (c = 0; c < (0, *s4p).i; ++c)
      --*lp;
  }

We do not expect *lp to alias into *s4p, so we might optimize this loop such that (0, *s4p).i is only computed once. If, in another translation unit, it turned out that these did alias, the optimization would normally be justified based on a violation of the effective type rules. If there isn't a violation because of the non-lvalue nature of the comma operator's expression, then the optimization would not appear to be justified.

Suggested Technical Corrigendum

None.

Committee Discussion

The committee did not have adequate time to consider these issues and intends that these issues be further refined through consultation with the author.

Apr 2014 meeting

Committee Discussion

Further input was not received from the author and will again be solicited.

Oct 2014 meeting

Committee Discussion

Discussion with the author clarified these issues, and the paper N1888 was discussed. From that, we extract the following example

union u2 {
    int x;
    long y;
    char ca[2];
};

int func2(void) {
    union u2 o2 = { .ca = "a" };

and question, what is the result of (0,o2).ca == o2.ca?

Given that the comma operator doesn't yield an lvalue (6.5.17), and from 6.2.4p8 such a non-lvalue expression is stated to have automatic storage duration, this seems to require that the answer is false, even though this defeats compiler optimizations.

The effective type rule 6.5.p6 also does not seem to apply to objects with temporary lifetime, and has undesirable consequences.

The direction the committee would like to go is something like:

In 6.2.4p8, append

An object with temporary lifetime behaves as if it had the declared type of its value. Such an object is known as a temporary object. A temporary object need not have a unique address.

Apr 2015 meeting

Committee Discussion

The following words were drafted and approved by the committee as the Proposed Technical Corrigendum.

Proposed Committee Response

To the question "Should the expression (O, o2).ca == o2.ca yield zero, non-zero, or should it be implementation defined?" the answer is "implementation defined".

With the following changes, the effective type of O, o3 is defined.

Proposed Technical Corrigendum

In 6.2.4p8, append

An object with temporary lifetime behaves as if it were declared with the type of its value for the purposes of effective type. Such an object need not have a unique address.

(add forward reference to 6.5p6 to this section)

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DR 453

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Submitter: Fred J. Tydeman (USA)
Submission Date: 2013-10-22
Source: WG14
Reference Document: N1776
Subject: Atomic flag type and operations

Summary

It appears to me that there is a wording problem in 7.17.8.*

7.17.8 Atomic flag type and operations
#1: The atomic_flag type provides the classic test-and-set functionality. It has two states, set and clear.

7.17.8.1 The atomic_flag_test_and_set functions
#2: Atomically sets the value pointed to by object to true.

#3: Atomically, the value of the object immediately before the effects.

7.17.8.2 The atomic_flag_clear functions
#2: Atomically sets the value pointed to by object to false.

An issue is states (set, clear) versus values (true, false).

Does an atomic_flag structure have both states (set, clear) and values (true, false)? Can it have all four combinations?

Another issue is 'set' is used both as a verb and a noun.

Another issue is: While the test is atomic, and the set is atomic, it is not clear that both test and set are part of the same atomic operation.

I have been told that the same issues exists in the C++ standard (29.7 [atomics.flag]).

There was discussion of these topics on the WG14 reflector (around messages 13067 to 13073)

One person in the discussion would like the value zero (from default static initialization) to be the clear state. They also mentioned DR 421.

Based upon the email discussion, the intent was that flags logically have exactly two states: "set" and "clear". The test_and_set operation returns true if it was "set", and false if it was "clear". Test_and_set sets the state to "set", and the clear operations set the state to "clear". The value zero need not be the "clear" state.

Suggested Technical Corrigendum

Replace

7.17.8.1 The atomic_flag_test_and_set functions
#2: Atomically sets the value pointed to by object to true.

#3: Atomically, the value of the object immediately before the effects.

7.17.8.2 The atomic_flag_clear functions
#2: Atomically sets the value pointed to by object to false.

with:

7.17.8.1 The atomic_flag_test_and_set functions
#2: Tests the state of the flag pointed to by object and then sets the flag, as a single atomic operation.

#3: Returns true if the flag was set when tested or false otherwise.

7.17.8.2 The atomic_flag_clear functions
#2: Atomically clears the flag pointed to by object.

Add to the rationale in the section on atomic flag:

The atomic flag type is defined in terms of states, not values, as the value zero (false) need not be the "clear" state. The committee knows of one implementation where zero is the "set" state.

Apr 2014 meeting

Committee Discussion

• The committee agrees that the use of true and false are inaccurate.
• The committee does not feel that the proposed words are a sufficient resolution, however, and has solicited a new paper to resolve the issue.

Oct 2014 meeting

Committee Discussion

The paper N1853 was provided and discussed, revised, discussed further, and a further paper was solicited. In particular, the committee did not like "converted to a _Bool" because it implies some unspecified arithmetic conversion.

Apr 2015 meeting

Committee Discussion

The paper N1908 was provided and discussed. "Clears" the flag was vaguely troubling and a new approach was offered:

In 7.17.8.1p2, change:

Atomically sets the value pointed to by object to true.

to:

Atomically places the atomic flag pointed to by object in the set state and returns the value corresponding to the immediately preceding state.

In 7.17.8.1p3, change:

Atomically, the value of the object immediately before the effects.

to:

The atomic_flag_test_and_set functions return the value that corresponds to the state of the atomic flag immediately before the effects. The return value true corresponds to the set state and the return value false corresponds to the clear state.

In 7.17.8.2p2, change:

Atomically sets the value pointed to by object to false.

to:

Atomically places the atomic flag pointed to by object into the clear state.

Oct 2015 meeting

Committee Discussion

The direction developed late at the last meeting is adopted as the Proposed Technical Corrigendum.

Proposed Technical Corrigendum

In 7.17.8.1p2, change:

Atomically sets the value pointed to by object to true.

to:

Atomically places the atomic flag pointed to by object in the set state and returns the value corresponding to the immediately preceding state.

In 7.17.8.1p3, change:

Atomically, the value of the object immediately before the effects.

to:

The atomic_flag_test_and_set functions return the value that corresponds to the state of the atomic flag immediately before the effects. The return value true corresponds to the set state and the return value false corresponds to the clear state.

In 7.17.8.2p2, change:

Atomically sets the value pointed to by object to false.

to:

Atomically places the atomic flag pointed to by object into the clear state.

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DR 454

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Submitter: Fred J. Tydeman (USA)
Submission Date: 2013-10-22
Source: WG14
Reference Document: N1777
Subject: ATOMIC_VAR_INIT (issues 3 and 4)
Related: DR 422 and DR 427

Summary

I see several issues with ATOMIC_VAR_INIT. They could be turned into one combined defect report, or separate defects, or folded into DR 422.

Consider the following code:

#include <stdatomic.h>
int main(void){
 atomic_int guide1 = ATOMIC_VAR_INIT(42); /* known value(42); WHAT STATE? */
 atomic_int guide2;        /* indeterminate value; indeterminate state */
 atomic_int guide3 = 42;   /* known value(42); indeterminate state */
static atomic_int guide4;  /* known value(0); valid state */
static atomic_int guide5 = 42; /* known value(42); valid state */
 atomic_int guide6;
 atomic_init(&guide6, 42); /* known value(42); initialized state */
 return 0;
}

What is the status of the additional state carried for guide1?

• Implicitly undefined
• Indeterminate
• Valid
• Initialized
• Something else

Is the state of guide1 the same as what guide6 has? If yes, does "initialization-compatible" mean do the same thing as if atomic_init() of the same object with the same value?

• (Issue 3 from N1777)

  ATOMIC_VAR_INIT is not usable in assignment to an atomic object.

  I see no difference between:

  atomic_int guide = ATOMIC_VAR_INIT(42);
  

  and

  atomic_int guide;
  guide = ATOMIC_VAR_INIT(42);
  

  I would hope that initialization (which looks like an assignment in a declaration) and a simple assignment would be equivalent and ATOMIC_VAR_INIT could be used in either context.

• (Issue 4 from N1777)

  What should happen if ATOMIC_VAR_INIT(value) is used in context other than initializing an atomic object of the same type as the value?

  Should it be undefined behaviour? A constraint violation? Just the value value converted to the type of the object?

  atomic_float f = ATOMIC_VAR_INIT(42); /* type mis-match */
  
  int nonAtomic = ATOMIC_VAR_INIT(42); /* non-atomic object */
  
  if( ATOMIC_VAR_INIT(42) ){...};
  guide1 = 1729 + ATOMIC_VAR_INIT(42) * 3;
  
  void func( atomic_int ai ); /* function parm/arg */
  func( ATOMIC_VAR_INIT(42) ); /* DR 427 is now making this
                               initialization (not assigment) */
  

  DR 427 is changing how a function parameter is getting its value from the actual argument from assignment to initialization (to get around const). Would this initialization be a valid context for ATOMIC_VAR_INIT?

Suggested Technical Corrigendum

• In the first sentence of 7.17.2.1#2, after

  suitable for initializing

  add the words

  or assigning to

• Add to 7.17.2.1 as a constraint or a new paragraph between 3 and 4:

  If ATOMIC_VAR_INIT is used in a context other than initialization [or assignment] of an atomic object of a compatible type of the value, the behaviour is undefined.

Proposed Committee Response

The ATOMIC_VAR_INIT macro prepares an atomic value that includes any extra state necessary for a non-lock-free type. Initialization, by definition, ignores all previous state. Assignment must honor the extra state that would indicate another atomic operation in progress; such an assignment takes the non-atomic corresponding value resulting from removing all qualifiers including atomic from the value expression, and will manipulate the extra state held in the object to assure proper atomic assignment semantics. ATOMIC_VAR_INIT produces a value appropriate for initialization because it will have any necessary extra state, whereas a value suitable for assignment is the non-qualified version of the assignment expression.

All uses of ATOMIC_VAR_INIT other than for initialization result in implicitly undefined behavior.

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Submitter: Fred J. Tydeman (USA)
Submission Date: 2013-10-22
Source: WG14
Reference Document: N1777
Subject: ATOMIC_VAR_INIT issue 5

Summary

I see several issues with ATOMIC_VAR_INIT. They could be turned into one combined defect report, or separate defects, or folded into DR 422.

Consider the following code:

#include <stdatomic.h>
int main(void){
 atomic_int guide1 = ATOMIC_VAR_INIT(42); /* known value(42); WHAT STATE? */
 atomic_int guide2;        /* indeterminate value; indeterminate state */
 atomic_int guide3 = 42;   /* known value(42); indeterminate state */
static atomic_int guide4;  /* known value(0); valid state */
static atomic_int guide5 = 42; /* known value(42); valid state */
 atomic_int guide6;
 atomic_init(&guide6, 42); /* known value(42); initialized state */
 return 0;
}

What is the status of the additional state carried for guide1?

• Implicitly undefined
• Indeterminate
• Valid
• Initialized
• Something else

Is the state of guide1 the same as what guide6 has? If yes, does "initialization-compatible" mean do the same thing as if atomic_init() of the same object with the same value?

• (Issue 5 from N1777)

  Zero initialization of static atomic objects in C requires more than in C++.

  I have been told that C's 7.17.2.1#2:

  ...; however, the default (zero) initialization for objects with static or thread-local storage duration is guaranteed to produce a valid state.

  is not in C++. If true and assuming that the two languages should be the "same" here, should this be deleted from C? Added to C++?

  DR 422 is somewhat related to this issue.

Suggested Technical Corrigendum

• No suggestion on the requirements mis-match between C and C++.

Apr 2014 meeting

Committee Discussion

• The 7.17.2.1#2 words should not be deleted.
• Interoperability with C++ atomics must be done by macros that use C++'s declarative syntax for atomic variables. As such there is no direct compatibility issue as is asserted.

Oct 2014 meeting

Committee Discussion

There were was no substantiative further discussion.

Proposed Committee Response

Interoperability with C++ atomics must be done by macros that use C++'s declarative syntax for atomic variables. As such there is no direct compatibility issue as is asserted, and 7.17.2.1#2 shall remain.

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DR 456

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Submitter: Rajan Bhakta
Submission Date: 2014-03-05
Source: WG14
Reference Document: N1798
Subject: Compile time definition of UINTN_C(value)

Summary

With reference to ISO/IEC WG14 N1569, subclause 7.20.4.1: The macro UINTN_C(value) shall expand to an integer constant expression corresponding to the type uint_leastN_t.

7.20.4 p1 imposes a stricter requirement on the form of the expansion; it must be an integer constant (for which paragraph 2 points to 6.4.4.1).

The type described in 7.20.4 p3 for the result of the expansion has an interesting property; we observe this for uint_least16_t without reference to the UINT16_C macro by using u'\0' in a context where it will be first promoted as part of the usual arithmetic conversions:

#include <assert.h>

#if u'\0' - 1 < 0
  // Types: #if (uint_least16_t) - (signed int) < (signed int)
  // Due to 6.10.1 p4, near the reference to footnote 167,
  // after applying the integer promotions as part of 6.3.1.8 p1
  // to the operands of the subtraction, the expression becomes:
  // Types: #if (unsigned int) - (signed int) < (signed int)
  // Following 6.3.1.8 p1 through to the last point gives:
  // Types: #if (unsigned int) - (unsigned int) < (signed int)
  // Result: false
# error Expected large unsigned value.
#endif

int main(void) {
  // Types: assert((uint_least16_t) - (signed int) < (signed int))
  // Assuming that signed int can represent all values of uint_least16_t,
  // after applying the integer promotions as part of 6.3.1.8 p1
  // to the operands of the subtraction, the expression becomes:
  // Types: assert((signed int) - (signed int) < (signed int))
  // Result: true
  assert(u'\0' - 1 < 0);
  return 0;
}

The code presented should neither fail to compile nor abort when executed (for example) on a system using two's complement and 8, 16 and 32 bits (respectively) for char, short and int with no padding bits.

Consider the case for N = 8 or 16 on systems with INT_MAX as +2147483647, UCHAR_MAX as 255 and USHRT_MAX as 65535: it is unclear how a macro can be formed such that it expands to an integer constant that has the promoted signed int type in phase 7 of translation and also the promoted unsigned int type in phase 4 of translation without special (non-standard) support from the compiler.

Even if the requirement for an integer constant is relaxed to only require an integer constant expression, the case for N = 8 on systems with INT_MAX as +32767 and UCHAR_MAX as 255 remains a problem without the use of casts (since uint_least16_t, for which we can form a literal, has different promotion behaviour from uint_least8_t).

Implementations seen:

1. #define UINT8_C(c) c ## U
2. #define UINT8_C(c) c

DR 209 seemed to try to address the issue of needing special compiler support in order to define the macros for integer constants; however, the problem seems to remain.

Suggested Technical Corrigendum

1. Add in suffixes for char and short literals.
2. Remove the UINT{8,16}_C macros from the standard.

Apr 2014 meeting

Committee Discussion

• The committee believes that DR209 is still appropriate in that "compiler magic" must be used for the implementation of these macros.
• As such, both proposed resolutions were found inappropriate.

Oct 2014 meeting

Proposed Committee Response

The committee believes that DR209 is still appropriate in that "compiler magic" must be used for the implementation of these macros. The committee does not consider this a defect.

As such, both proposed resolutions were found inappropriate.

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DR 457

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Submitter: David Keaton (suggested by Jens Gustedt)
Submission Date: 2014-03-13
Source: WG14
Reference Document: N1802
Subject: The ctime_s function in Annex K defined incorrectly

Summary

The ctime_s function in Annex K was defined analogously to ctime, and some of the text from the definition of ctime was copied and modified slightly.

K.3.8.2.2p4 states that ctime_s is equivalent to the following.

asctime_s(s, maxsize, localtime_s(timer))

In this case, the text from the original ctime definition was not quite modified enough.  The localtime_s function takes two arguments and the above code only supplies one.

Suggested Technical Corrigendum

In K.3.8.2.2p4, replace

asctime_s(s, maxsize, localtime_s(timer))

with the following.

asctime_s(s, maxsize, localtime_s(timer, &(struct tm){ 0 }))

Proposed Technical Corrigendum

In K.3.8.2.2p4, replace

asctime_s(s, maxsize, localtime_s(timer))

with the following.

asctime_s(s, maxsize, localtime_s(timer, &(struct tm){ 0 }))

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DR 458

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Submitter: Martin Sebor
Submission Date: 2014-03-18
Source: WG14
Reference Document: N1806
Subject: ATOMIC_XXX_LOCK_FREE macros not constant expressions

Summary

Section 7.17.1 Introduction (to section 7.17 Atomics <stdatomic.h>) specifies that the <stdatomic.h> header define a number of macros having the form ATOMIC_XXX_LOCK_FREE that indicate the lock-free property of the corresponding atomic types. No further description of the macros is provided here.

Section 7.17.5 Lock-free property, then goes on to specify that the atomic lock-free macros (presumably the same ones as those listed in 7.17.1) expand to one of three values: 0, 1, or 2.

Neither of the two sections above, nor any other in the standard, specifies whether or not the macros are required to expand to constant expressions usable in preprocessor #if directives. This is in contrast to some other standard macros such as those defined in <limits.h> which are typically so specified using language such as:

The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives.

As discussed in the thread starting with SC22WG14.13216, the only purpose for the existence of the ATOMIC_XXX_LOCK_FREE macros is to be able to write more efficient code by relying on their use in preprocessor #if conditionals. Thus, the absence of the requirement that they expand to constant expressions makes the macros unsuitable for that purpose.

In section 7.17.1, modify paragraph 3 as indicated below:

...which expand to constant expressions suitable for use in #if preprocessing directives and which indicate the lock-free property of the corresponding atomic types (both signed and unsigned); and

Proposed Technical Corrigendum

In section 7.17.1 paragraph 3 change:

...which indicate the lock-free property of the corresponding atomic types (both signed and unsigned); and

...which expand to constant expressions suitable for use in #if preprocessing directives and which indicate the lock-free property of the corresponding atomic types (both signed and unsigned); and

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DR 459

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Submitter: Martin Sebor
Submission Date: 2014-03-22
Source: WG14
Reference Document: N1807
Subject: atomic_load missing const qualifier

Summary

The synopsis of the atomic_load pair of generic functions specified in 7.17.7.2 shows that they accept pointers to a volatile- (bot not const-) qualified type:

          #include <stdatomic.h>

          C atomic_load(volatile A *object);
          C atomic_load_explicit(volatile A *object,
                                 memory_order order);
      

The absence of the const qualifier implies that the functions cannot be called with an argument of type const A* since there is no such conversion.

However, since neither function modifies its argument, there is no need to prevent it from being called with an argument of type const A*. And, in fact, the latest draft C++ standard as of this writing, N3936, does provide an overload of each function that takes a const volatile pointer.

In section 7.17.7.2, paragraph 1, Synopsis, modify the declarations of the atomic_load pair of generic functions as indicated below:

          #include <stdatomic.h>

          C atomic_load(const volatile A *object);
          C atomic_load_explicit(const volatile A *object,
                                 memory_order order);
      

Proposed Technical Corrigendum

In section 7.17.7.2, paragraph 1, Synopsis, modify the declarations of the atomic_load pair of generic functions from:

          #include <stdatomic.h>

          C atomic_load(volatile A *object);
          C atomic_load_explicit(volatile A *object,
                                 memory_order order);
      

          #include <stdatomic.h>

          C atomic_load(const volatile A *object);
          C atomic_load_explicit(const volatile A *object,
                                 memory_order order);
      

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DR 460

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Submitter: Martin Sebor
Submission Date: 2014-03-22
Source: WG14
Reference Document: N1808
Subject: aligned_alloc underspecified

Summary

The aligned_alloc function specifies the following constraints on its arguments, alignment and size:

The value of alignment shall be a valid alignment supported by the implementation and the value of size shall be an integral multiple of alignment.

Therefore, the behavior of the function is undefined when either constraint is violated.

According to section 6.2.8, paragraph 1, the greatest alignment a conforming implementation is required to support (known as fundamental alignment) is _Alignof(max_align_t). Furthermore, according to paragraph 2 of the same section, whether alignments greater than the fundamental alignment (known as extended alignments) are supported and in what contexts is implementation-defined.

The standard specifies no mechanism by which programs could determine whether an extended alignment is supported by an implementation, or whether the aligned_alloc function is among the contexts where an extended alignment is supported.

As a result, there is no way for strictly conforming programs to use the aligned_alloc function with an alignment argument greater than the result of _Alignof(max_align_t). Since the malloc function returns objects that meet the same alignment requirement, this restriction makes aligned_alloc useless in portable programs.

This restriction is unnecessary since it's possible, and in fact nearly trivial given access to the internal details of the memory allocator, to implement an efficient aligned_function that fails when its arguments don't meet the specified requirements.

As a data point, the POSIX Advanced Realtime function posix_memalign, as well as the historical BSD memalign function, are both required to return a null pointer when either of their arguments don't meet the specified requirements (in addition to setting errno to EINVAL.

The proposed corrigendum below changes the standard to require aligned_alloc to fail by returning a null pointer when either of its constraints is violated.

In section 7.22.3.1, modify paragraph 2 as indicated below:

The aligned_alloc function allocates space for an object whose alignment is specified by alignment, whose size is specified by size, and whose value is indeterminate. TIf the value of alignment shall be is not a valid alignment supported by the implementation andor the value of size shall beis not an integral multiple of alignment the function shall fail by returning a null pointer.

In addition, in section J.2 Undefined behavior, remove the following bullet:

— The alignment requested of the aligned_alloc function is not valid or not supported by the implementation, or the size requested is not an integral multiple of the alignment (7.22.3.1).

If the proposal above isn't acceptable, then an alternative solution to consider that would allow aligned_alloc to be used even in strictly conforming programs is to add a new function to determine whether a given alignment is supported by an implementation. For example:

_Bool alignment_is_valid (size_t alignment);

Returns

The alignment_is_valid function returns non-zero if the value specified by alignment is a valid alignment argument to the aligned_alloc function, and zero otherwise.

Apr 2014 meeting

Committee Discussion

The committee agrees that the first proposal in the suggested technical corrigendum should be adopted.

Proposed Technical Corrigendum

In section 7.22.3.1, change paragraph 2 from:

The aligned_alloc function allocates space for an object whose alignment is specified by alignment, whose size is specified by size, and whose value is indeterminate. The value of alignment shall be a valid alignment supported by the implementation and the value of size shall be an integral multiple of alignment.

The aligned_alloc function allocates space for an object whose alignment is specified by alignment, whose size is specified by size, and whose value is indeterminate. If the value of alignment is not a valid alignment supported by the implementation or the value of size is not an integral multiple of alignment the function shall fail by returning a null pointer.

In addition, in section J.2 Undefined behavior, remove the following bullet:

— The alignment requested of the aligned_alloc function is not valid or not supported by the implementation, or the size requested is not an integral multiple of the alignment (7.22.3.1).

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DR 461

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Submitter: Martin Sebor
Submission Date: 2014-03-25
Source: WG14
Reference Document: N1812
Subject: problems with references to objects in signal handlers

Summary

We believe there are two problems in section 7.14.1.1 The signal function, paragraph 5, which specifies the constraints under which signal handlers can access objects declared in other scopes. The problems are summarized in the following two subsections. The section titled Suggested Technical Corrigendum then proposes a correction to both.

Section 7.14.1.1 The signal function, paragraph 5, specifies the following constraints. Note, in particular, to use of the word "refers," and the reference to objects with "static or thread storage duration" underscored in the text below.

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal handler calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler.

The standard doesn't formally define the term refer but its uses in the text suggest that it denotes any use of an object, including one that doesn't involve accessing it. The term access is defined in 3.1 to mean an <execution-time action> to read or modify the value of an object.

Preventing signal handlers from accessing objects is necessary in order to avoid data races between accesses (reads and writes) to the same object in the rest of the program that are in progress but not completed at the time the signal is delivered.

However, by making use of the word "refers," the sentence in 7.14.1.1 quoted above implies that even mentioning the name of an object in an unevaluated context such as the sizeof expression, or taking its address is undefined in a signal handler. This restriction is unnecessary, since such references are safe because they cannot introduce any sort of a data race between the signal handler and the rest of the program. Thus, referring to such objects without accessing them should be permitted in conforming programs.

Furthermore, accessing a const object to read (but not modify) its value also cannot introduce a data race and is safe as well. Thus, the restriction can be relaxed even further to allow signal handlers to read constant objects. Note that const objects are those that are declared const. In particular, accessing an object that was not declared const via a pointer to a const-qualified type does not change the fact that the object itself is not const. This distinction is important to understand that relaxing this constraint cannot introduce the potential for a data race when such a non-const object is modied in the program while it's accessed via a const-qualified pointer in a signal handler.

The comments in the following example should make this distinction clear:

const int safe = (1 << SIGINT) | (1 << SIGQUIT);
      int unsafe = (1 << SIGHUP) | (1 << SIGTERM);

volatile sig_atomic_t sigcount [2];

void handler (int signo) {

    const int *pmask;   // pointer to const int

    // taking the address of any object is safe and should be allowed
    pmask = &safe;

    // access to safe should be allowed since it's a const object
    if ((1 << signo) & *pmask)
        ++sigcount [0];

    // safe and should be allowed
    pmask = &unsafe;

    // access to unsafe remains undefined since it's not a const object
    if ((1 << signo) & *pmask)
        ++sigcount [1];
}

The sentence from paragraph 5 quoted above specifically singles out objects with static or thread storage duration, but permits signal handlers to access objects with automatic storage duration without a similar restriction. However, a signal handler that has access to a local variable defined in another function whose execution is interrupted by the delivery of a signal resulting in the invocation of the signal handler contains the same potential data race as if the two functions both accessed the same object with static storage duration.

To make clear how this condition could arise, consider the following program which, when atomic_intptr_t is a lock-free type, is strictly conforming according to the letter of the standard despite the data race.

atomic_intptr_t p;   // assume atomic_intptr_t is lock-free

void handler (int signo) {
    // the following write access should be undefined since it modifies
    // an object with automatic storage duration declared in f
    ++*(int*)p;
}

void f (void) {
    int i = 0;
    p = (atomic_intptr_t)&i;

    signal (SIGINT, handler);

    while (i < 7)
        printf ("%i\n", i);
}

The proposed corrigendum below changes the standard to remove the unnecessary constraints discussed above, and to add the missing restriction to prevent accessing local variables defined elsewhere in the program. The reference to the lifetime of auto objects makes sure that accesses to local variables defined in signal handlers themselves as well as in functions called from them remain well defined.

In section 7.14.1.1, modify the first sentence of paragraph 5 as indicated below:

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers toaccesses any non-const object with static or thread storage duration, or any non-const object with automatic storage duration whose lifetime started before the signal handler has been entered, that is not a lock-free atomic object other than by...

In addition, make the corresponding change to section J.2 Undefined behavior.

Apr 2014 meeting

Committee Discussion

• The committee agrees that clarifying "refers" would be beneficial.
• Allowing access to const qualified objects would be a feature and cannot be accomplished by the mechanism of a defect report. Such a feature can be proposed in a separate paper.
• Access to objects of allocated storage duration should also be addressed.
• It is suggested that automatic storage duration objects should also be addressed.
• Further revisions have been solicited from the author.

Oct 2014 meeting

Committee Discussion

The paper N1874 was submitted and discussed, again, as a defect, rather than as a new proposal, and the suggested changes to allow new behavior were again rejected. It was noted that a const volatile object implemented in hardware, such as random number generator, might not provide a consistent value if accessed from a signal handler, and so there was general agreement that any changes in this area warrant very careful consideration.

Proposed Committee Response

Extending the behavior as requested is a feature and appropriate as input to the next revision of this Standard. It was noted that a const volatile object that might seem acceptable to reference from a signal handler might not be if it were implemented in hardware (e.g. a hardware random number generator).

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DR 462

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Submitter: Robert Seacord
Submission Date: 2014-03-25
Source: WG14
Reference Document: N1813
Subject: Clarifying objects accessed in signal handlers

Summary

It appears the intent of the committee in Subclause 5.1.2.3 paragraph 5 was to allow lock-free atomic objects or objects of type volatile sig_atomic_t to be accessed from a signal handler.  Objects of type atomic_flag are an obvious choice operations on an object of type atomic_flag are required to be lock free. However, objects of type atomic_flag can only be meaningfully accessed by a call to a function, and calls to these functions from a signal handler are undefined behavior according to subclause 7.14.1.1 paragraph 5.

Suggested Technical Corrigendum

Change subclause 7.14.1.1 paragraph 5 from:

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal handler calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate.²⁵²⁾

to:

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal handler calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, the atomic_flag_test_and_set functions, the atomic_flag_clear functions, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate.²⁵²⁾

Sublcause J.2 Undefined behavior. Page 566

Change:

A signal occurs other than as the result of calling the abort or raise function, and the signal handler refers to an object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, or the signal function (for the same signal number) (7.14.1.1).

to:

A signal occurs other than as the result of calling the abort or raise function, and the signal handler refers to an object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, the atomic_flag_test_and_set functions, the atomic_flag_clear functions, or the signal function (for the same signal number) (7.14.1.1).

Apr 2014 meeting

Committee Discussion

The Suggested Technical Corrigendum was accepted as the Proposed Technical Corrigendum.

Oct 2014 meeting

Committee Discussion

Upon further consideration, since by implication 5.1.2.3p5 allows by implication any of the atomic functions on lock-free atomic objects, the following revision to the Suggested Technical Corrigendum was substantially adopted from the new paper N1887

Proposed Technical Corrigendum (superceded)

Change subclause 7.14.1.1 paragraph 5 from:

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal handler calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate.²⁵²⁾

to:

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal handler calls any function in the standard library other than

• the abort function,

• the _Exit function,

• the quick_exit function,

• the atomic functions from stdatomic.h, when the atomic arguments are lock-free,

• the atomic_is_lock_free function with any atomic argument,

• or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate.²⁵²⁾

In subclause J.2 Undefined behavior, change:

A signal occurs other than as the result of calling the abort or raise function, and the signal handler refers to an object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, or the signal function (for the same signal number) (7.14.1.1).

to:

A signal occurs other than as the result of calling the abort or raise function, and the signal handler refers to an object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, the atomic functions from stdatomic.h (when the atomic arguments are lock-free) , the atomic_is_lock_free function with any atomic argument, or the signal function (for the same signal number) (7.14.1.1).