Issue 0017.07: What is the scope and uniqueness of size_t?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Derek M. Jones, WG14
Date: 1992-12-10
Reference document: X3J11/90-056
Submitted against: C90
Status: Closed
Cross-references: 0047, 0050
Converted from: dr.htm, dr_017.html

Scope and uniqueness of size_t

Subclause 6.3.3.4 on page 45, lines 1-2 says: “... and its type (...) is size_t defined in the <stddef.h> header.” This line could be read as either of the following:

1. “... and its type is size_t which happens to be defined in stddef.h.”
2. “... and its type is the size_t defined in stddef.h.”

(It was probably intended as a helpful piece of information only.) So what does the compiler do?

In (1) the compiler has to define a size_t in some outer scope. This definition does not make size_t visible, but gives a type to the return value of sizeof. Now if the programmer defines a typedef making size_t synonymous with float (say) then the compiler now has to use this new type. This interpretation does not require the programmer to include <stddef.h> in order to use sizeof.

In (2) the compiler picks up the type size_t from <stddef.h> (assuming that the user included this header). Should the compiler give a diagnostic if this header was not included and sizeof was used? A subsequent typedef for size_t does not affect the type of the result of sizeof.

These problems do not arise with int, et al. because they are keywords. Thus “typedef float int” would give a syntax error and need not be considered semantically.

According to subclause 6.3.3.4, page 45, sizeof has type size_t. What happens if the type of size_t does not match what the compiler thinks is the type of sizeof?

Comment from WG14 on 1997-09-23:

Response

The relevant citations are subclause 6.3.3.4

The value of the result is implementation-defined, and its type (an unsigned integral type) is size_t defined in the <stddef.h> header.

and subclause 7.1.6

The types are ...

size_t

which is the unsigned integral type of the result of the sizeof operator; ...

These sections, both separately and together, define the relationship between the result type of sizeof and the type size_t defined in stddef.h. The result type of sizeof and the type size_t defined in stddef.h are an unsigned integral type, and size_t defined in <stddef.h> is identical to the result type of sizeof. To restate, in a conforming implementation, the result type of sizeof will be the same as the type of size_t defined in <stddef.h>.

Since these two types are the same, there need be no mechanism for a compiler to discover the type of size_t defined in <stddef.h>. A compiler's private knowledge of the result type of sizeof is as good as stddef.h's private knowledge of the type of size_t.

Note that the result of sizeof has the same type as not just any size_t, but the size_t defined in <stddef.h>.