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To:	cygwin <cygwin AT cygwin DOT com>
From:	Ken Brown <kbrown AT cornell DOT edu>
Subject:	Defining _GNU_SOURCE hides the declaration of aligned_alloc
Message-ID:	<56B12D61.7030200@cornell.edu>
Date:	Tue, 2 Feb 2016 17:27:45 -0500
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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 ‘main’:
test.c:7:3: warning: implicit declaration of function ‘aligned_alloc’

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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