Defining _GNU_SOURCE hides the declaration of aligned_alloc

[Ken Brown]

On 2/2/2016 5:27 PM, Ken Brown wrote:
> 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?

Paul Eggert says they should:

> Defining _GNU_SOURCE should make aligned_alloc visible regardless of whether -std=c99 is specified. This is because defining _GNU_SOURCE means, "Make GNU symbols visible even when compiling pedantically." This is OK, since the C standard says the behavior is undefined whenever the user defines a reserved symbol like _GNU_SOURCE.

Ken

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