Fwd: A small issue with _GNU_SOURCE

Fwd: A small issue with _GNU_SOURCE

[Jon TURNEY]

Consider the following:

$ cat test.c
#include <string.h>
#include <stdlib.h>

int main()
{
  long long i = random();
  return ffsll(i);
}

ffsll() is a GNU extension and should be prototyped when _GNU_SOURCE is 
defined.

random() is in SUSv2 and requires _XOPEN_SOURCE=500

$ gcc test.c -Wall -ansi -D_XOPEN_SOURCE=700
test.c: In function ‘main’:
test.c:8:2: warning: implicit declaration of function ‘ffsll’

This is correct

$ gcc test.c -Wall -ansi -D_XOPEN_SOURCE=700 -D_GNU_SOURCE
test.c: In function ‘main’:
test.c:8:2: warning: implicit declaration of function ‘ffsll’

This looks like a problem with newlib's sys/cdefs.h.  _XOPEN_SOURCE 
causes _POSIX_C_SOURCE to be defined, which prevents _GNU_SOURCE from 
being considered.

$ gcc test.c -Wall -ansi -D_GNU_SOURCE
test.c: In function ‘main’:
test.c:7:2: warning: implicit declaration of function ‘random’

_GNU_SOURCE is supposed to imply some value of _XOPEN_SOURCE, but only 
actually turns on __XSI_VISIBLE.

The patch I wrote for cygwin's stdlib.h [1] explicitly checks 
_XOPEN_SOURCE.  Should this be a check for XSI_VISIBLE? or should 
cdefs.h also define _XOPEN_SOURCE (and all the other feature test macros 
that _GNU_SOURCE is defined to be equivalent to)?

[1] https://cygwin.com/ml/cygwin-patches/2013-q4/msg00004.html

Re: A small issue with _GNU_SOURCE

[Corinna Vinschen]

[-- Attachment #1: Type: text/plain, Size: 1747 bytes --]

On Feb 17 18:24, Jon TURNEY wrote:
> 
> Consider the following:
> 
> $ cat test.c
> #include <string.h>
> #include <stdlib.h>
> 
> int main()
> {
>  long long i = random();
>  return ffsll(i);
> }
> 
> ffsll() is a GNU extension and should be prototyped when _GNU_SOURCE is
> defined.
> 
> random() is in SUSv2 and requires _XOPEN_SOURCE=500
> 
> $ gcc test.c -Wall -ansi -D_XOPEN_SOURCE=700
> test.c: In function ‘main’:
> test.c:8:2: warning: implicit declaration of function ‘ffsll’
> 
> This is correct
> 
> $ gcc test.c -Wall -ansi -D_XOPEN_SOURCE=700 -D_GNU_SOURCE
> test.c: In function ‘main’:
> test.c:8:2: warning: implicit declaration of function ‘ffsll’
> 
> This looks like a problem with newlib's sys/cdefs.h.  _XOPEN_SOURCE causes
> _POSIX_C_SOURCE to be defined, which prevents _GNU_SOURCE from being
> considered.
> 
> $ gcc test.c -Wall -ansi -D_GNU_SOURCE
> test.c: In function ‘main’:
> test.c:7:2: warning: implicit declaration of function ‘random’
> 
> _GNU_SOURCE is supposed to imply some value of _XOPEN_SOURCE, but only
> actually turns on __XSI_VISIBLE.
> 
> The patch I wrote for cygwin's stdlib.h [1] explicitly checks _XOPEN_SOURCE.
> Should this be a check for XSI_VISIBLE? or should cdefs.h also define
> _XOPEN_SOURCE (and all the other feature test macros that _GNU_SOURCE is
> defined to be equivalent to)?

The check should be one for XSI_VISIBLE.  I'm not sure about cdefs.h,
though.  Maybe you want to ask the question on the newlib ML?  I'll
look into it tomorrow.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

[-- Attachment #2: Type: application/pgp-signature, Size: 819 bytes --]