[Bug libc/21552] XPG4 bsd_signal namespace

["pponnuvel at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21552

PP <pponnuvel at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pponnuvel at gmail dot com

--- Comment #4 from PP <pponnuvel at gmail dot com> ---
Request a clarification about how to get bsd_signal's prototype in glibc 2.26.

Man page of bsd_signal says:
>        bsd_signal():
>           Since glibc 2.26:
>               _XOPEN_SOURCE >= 500
>                   && ! (_POSIX_C_SOURCE >= 200112L)
>          Glibc 2.25 and earlier:
>              _XOPEN_SOURCE

Defining those two macros as such works. But when I define _GNU_SOURCE
additionally, it doesn't work. The following demonstrates the problem:

```
$ cat x.c
#include <signal.h>
int main(void)
{
    bsd_signal(0, 0);
}
$ gcc -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=199506L -std=gnu99 x.c
$ gcc -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=199506L -std=gnu99 -D_GNU_SOURCE
x.c
x.c: In function ‘main’:
x.c:4:5: warning: implicit declaration of function ‘bsd_signal’; did you mean
‘ssignal’? [-Wimplicit-function-declaration]
     bsd_signal(0, 0);
     ^~~~~~~~~~
     ssignal
```

As seen above, defining `_GNU_SOURCE` makes bsd_signal unavailable. Is this
expected?

Compiling using gcc 7.2.0/glibc 2.26 (Ubuntu 17.10).

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