c - What does -D_XOPEN_SOURCE do/mean?

Question

I recently encountered some code that gcc would not compile without this arg. I checked the gcc man page, but did not find this specific option. I did find XOPEN_SOURCE, but there was little explanation of what it does.

Can someone please elaborate? I know -D_XOPEN_SOURCE can be set to different values, such 400, 600, but what do those do?

Answer 1

When you do

#define _XOPEN_SOURCE <some number>

or

cc -D_XOPEN_SOURCE=<some number>

it tells your compiler to include definitions for some extra functions that are defined in the X/Open and POSIX standards.

This will give you some extra functionality that exists on most recent UNIX/BSD/Linux systems, but probably doesn't exist on other systems such as Windows.

The numbers refer to different versions of the standard.

• 500 - X/Open 5, incorporating POSIX 1995
• 600 - X/Open 6, incorporating POSIX 2004
• 700 - X/Open 7, incorporating POSIX 2008

You can tell which one you need (if any) by looking at the man page for each function you call.

For example, man strdup says:

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       strdup(): _SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE >= 500
       strndup(), strdupa(), strndupa(): _GNU_SOURCE

Which means that you should put one of these:

#define _SVID_SOURCE
#define _BSD_SOURCE
#define _XOPEN_SOURCE 500
#define _XOPEN_SOURCE 600
#define _XOPEN_SOURCE 700

at the top of your source file before doing any #includes if you want to use strdup.

Or you could put

#define _GNU_SOURCE

there instead, which enables all functionality, with the downside that it might not compile on Solaris, FreeBSD, Mac OS X, etc.

It's a good idea to check each man page before doing a #include, #define, or using a new function, because sometimes their behavior changes depending on what options and #defines you have, for example with basename(3).

See also:

• Linux: gcc with -std=c99 complains about not knowing struct timespec
• glibc feature test macros
• The Compilation Environment - Open Group Base Specification issue 6 (a.k.a. X/Open 6)
• POSIX - Wikipedia
• Single UNIX Specification - Wikipedia