Re: Ensuring posix-ness

[Bruno Haible]

Hi Reuben,

> I find that some APIs on GNU systems need
> extra treatment (i.e. GNU is not POSIX-1.2001 by default). For
> example, for crypt(3) I need to define _XOPEN_SOURCE

For crypt() to be declared in <crypt.h>, you need nothing. But for it to
be declared in <unistd.h>, you need _XOPEN_SOURCE or _GNU_SOURCE.
_GNU_SOURCE means "make all extensions in the header files visible".

$ cat foo.c
#define _GNU_SOURCE 1
#include <unistd.h>
$ gcc -E foo.c | grep crypt
extern char *crypt (__const char *__key, __const char *__salt)
extern void encrypt (char *__block, int __edflag) __attribute__ ((__nothrow__)) 
__attribute__ ((__nonnull__ (1)));

> I can't find any gnulib extensions that bear on this: extensions is
> already running because of something else

The 'extensions' module sets _GNU_SOURCE, so it is this module which you want
and need.

Note that <config.h> must be the first header file to be included in every
compilation unit. If you don't include it, _GNU_SOURCE will not be defined,
and if you include it too late, <features.h> has already made the decision
whether to defined __USE_XOPEN or not.

> The crypt example is particularly annoying, as without _XOPEN_SOURCE
> defined it's not defined, even though POSIX_VERSION is >= 200112L, and
> it's in that version of POSIX (of course, that's a glibc bug

$ cat foo.c
#define _POSIX_C_SOURCE 200112L
#include <unistd.h>
$ gcc -E foo.c | grep crypt

Indeed crypt() is on POSIX:2001
<http://pubs.opengroup.org/onlinepubs/009604499/functions/crypt.html>.
But it is marked as [XSI]. I don't know whether that implies that glibc is
right in hiding the declaration, or it should enable it?
 
> Have I missed anything, or is this a gap in gnulib's coverage?

Certainly the fact that crypt() is not defined by default could be mentioned
in doc/posix-functions/crypt.texi.

Bruno
-- 
In memoriam Joseph Lee Heywood <http://en.wikipedia.org/wiki/Joseph_Lee_Heywood>