Defect Report #400

Submitter: Nick Stoughton (US)
Submission Date: 2011-10-24
Source: Austin Group
Reference Document: N1559
Version: 1.1
Date: February 2012
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.

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