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Defining _GNU_SOURCE hides the declaration of aligned_alloc


The issue in the Subject line came up in connection with an emacs bug report.

Here's a test case:

$ cat test.c
#define _GNU_SOURCE
#include <stdlib.h>

int
main ()
{
  aligned_alloc (1, 1);
}

$ gcc test.c -Wimplicit-function-declaration
test.c: In function âmainâ:
test.c:7:3: warning: implicit declaration of function âaligned_allocâ

The cause is that the declaration of aligned_alloc in stdlib.h is guarded by #if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L; but defining _GNU_SOURCE causes __ISO_C_VISIBLE to be defined as 1999. Here's an excerpt from /usr/include/sys/cdefs.h showing how this happens:

/* Deal with _GNU_SOURCE, which implies everything and the kitchen sink */
#ifdef _GNU_SOURCE
[...]
#define    _XOPEN_SOURCE        700
[...]
#endif
[...]
#if _XOPEN_SOURCE - 0 >= 700
[...]
#define    _POSIX_C_SOURCE        200809
[...]
#endif
[...]
#if _POSIX_C_SOURCE >= 200809
[...]
#define    __ISO_C_VISIBLE        1999
[...]
#endif /* _POSIX_C_SOURCE */

According to the discussion of the emacs bug I mentioned, Linux and FreeBSD don't have this issue. Should Cygwin's headers be changed to conform to those other platforms?

Ken

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