That's not what's going on.  Without -D_GNU_SOURCE you don't have a
prototype at all; GCC infers one.  Try using gcc -Wall to see what's
going on.

It'd be nice if the documentation said what #define you needed to use
this...

Hello Daniel,

thanks for your prompt reply.

You are right concerning the missing prototype. But there is still a
problem with the documentation. That's what I get:

Compiling example from " 25.3.13 Parsing of Suboptions Example" as is:
     ginster:/home/werner/csource# gcc -Wall -o testprg testprg.c
     testprg.c: In function `main':
     testprg.c:47: warning: implicit declaration of function `getsubopt'
Just as you told. No prototype.

Adding "#define _GNU_SOURCE" in the first line:
ginster:/home/werner/csource# gcc -Wall -o testprg testprg.c
     testprg.c: In function `main':
     testprg.c:48: warning: passing arg 2 of `getsubopt'
                            from incompatible pointer type
The declaration of parameter 2 does not match the declaration of
getsubopt().

Removing "const" from the declaration of "mount_opts[]":
     ginster:/home/werner/csource# gcc -Wall -o testprg testprg.c
     ginster:/home/werner/csource#
No more warnings.

So my conclusion:
- The prototype is:
     int getsubopt (char **optionp, char* const *tokens, char **valuep);
   "const char* const *tokens" for the second argument seems to be an
   error in the documentation.
- In the example:
     "const char *mount_opts[] ="
   should be replaced by
     "char *mount_opts[] ="

Greetings
Werner

Hello,

Bug #335171 in glibc-doc-reference reported by you has been fixed in the
Git repository and is awaiting an upload. You can see the commit
message below and you can check the diff of the fix at:

https://salsa.debian.org/glibc-team/glibc-doc-reference/-/commit/ee1fef187595e605412b5a52c43b8b3f2d78e225

(this message was generated automatically)
-- 
Greetings

https://bugs.debian.org/335171

We believe that the bug you reported is fixed in the latest version of
glibc-doc-reference, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 335171@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Aurelien Jarno <aurel32@debian.org> (supplier of updated glibc-doc-reference package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Wed, 29 Jul 2026 21:50:14 +0200
Source: glibc-doc-reference
Architecture: source
Version: 2.43-1
Distribution: unstable
Urgency: medium
Maintainer: GNU Libc Maintainers <debian-glibc@lists.debian.org>
Changed-By: Aurelien Jarno <aurel32@debian.org>
Closes: 335171 501435
Changes:
 glibc-doc-reference (2.43-1) unstable; urgency=medium
 .
   * New upstream version.  Closes: #335171, #501435.
   * debian/watch: update to v5 and use git instead of ftp.
   * debian/copyright: rewrite in Machine-readable format.
   * debian/rules: remove get-orig-source target and related code, now that the
     orig tarball can be generated with uscan.
   * debian/control: improve package description.
   * debian/control: drop Priority field, now the default.
   * debian/control: bump Standards-Version to 4.7.4 (no changes).
   * debian/salsa-ci.yml: add.
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