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

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

[bugzilla-daemon]

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

[bugzilla-daemon]

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

--- Comment #1 from Sam James (sam@gentoo.org) ---
FWIW, Gnulib indeed (as the man page hints) that it is a glibc extension:
https://www.gnu.org/software/gnulib/manual/gnulib.html#updwtmpx.

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

[bugzilla-daemon]

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

--- Comment #2 from Sam James (sam@gentoo.org) ---
Sent patch to ML: https://marc.info/?l=linux-man&m=165603977710424&w=2.

Suppose the commit message should be more detailed but I'll wait for someone to
tell me if this is even right first :)

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

[bugzilla-daemon]

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

Alejandro Colomar (man-pages) (alx.manpages@gmail.com) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |alx.manpages@gmail.com
         Resolution|---                         |DOCUMENTED

--- Comment #3 from Alejandro Colomar (man-pages) (alx.manpages@gmail.com) ---
Hi Sam,

Sorry for the delay; I read your posts, but didn't have much time to reply. 
Yes, normally I'd ask for a more detailed commit message, but since it is very
obvious to me that it's correct from this bugzilla report (and also from the
glibc code), and there's a link to this issue in the commit, I think it's OK as
is the patch.

Thanks!

Alex

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