[Bug 216168] New: updwtmp(3) doesn't mention need for _GNU_SOURCE for updwtmpx

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=216168

            Bug ID: 216168
           Summary: updwtmp(3) doesn't mention need for _GNU_SOURCE for
                    updwtmpx
           Product: Documentation
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: man-pages
          Assignee: documentation_man-pages@kernel-bugs.osdl.org
          Reporter: sam@gentoo.org
        Regression: No

I think utmpxh's updwtmpx might need _GNU_SOURCE to be defined to be usable?

Noticed when investigating a warning when building gdm 42.0:
```
../gdm-42.0/daemon/gdm-session-record.c:200:9: error: implicit declaration of
function ‘updwtmpx’; did you mean ‘updwtmp’?
[-Werror=implicit-function-declaration]
updwtmpx (GDM_NEW_SESSION_RECORDS_FILE, &session_record);
```

... but:
```
#if defined(HAVE_UTMPX_H)
#include <utmpx.h>
#endif

#if defined(HAVE_UTMP_H)
#include <utmp.h>
#endif
```

And config.h, set by Meson, contains HAVE_UTMPX_H, HAVE_UTMP_H, HAVE_UPDWTMP,
HAVE_UPDWTMPX.

From looking at glibc-2.35's /usr/include/utmpx.h, I think it might need
_GNU_SOURCE? It's guarded by __USE_GNU within glibc headers:
```
[..]
#ifdef __USE_GNU
/* Change name of the utmpx file to be examined.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern int utmpxname (const char *__file);
[...]
/* Append entry UTMP to the wtmpx-like file WTMPX_FILE.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern void updwtmpx (const char *__wtmpx_file,
                      const struct utmpx *__utmpx);
[...]
```

Aside: a friend points out that NetBSD needs NETBSD_SOURCE defined for it too:
https://github.com/NetBSD/src/blob/6c9d506c6146a69f3807ce59b4c063792ef32829/include/utmpx.h#L143.

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