A small issue with _GNU_SOURCE
Corinna Vinschen
corinna-cygwin@cygwin.com
Tue Feb 17 21:43:00 GMT 2015
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
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