What does “#define _GNU_SOURCE” imply?

Question

Today I had to use the basename() function, and the man 3 basename (here) gave me some strange message:

Notes

Answer 1

For exact details on what are all enabled by _GNU_SOURCE, documentation can help.

From the GNU documentation:

Macro: _GNU_SOURCE

If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.

From the Linux man page on feature test macros:

_GNU_SOURCE

Defining this macro (with any value) implicitly defines _ATFILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the value 200809L (200112L in glibc versions before 2.10; 199506L in glibc versions before 2.5; 199309L in glibc ver‐ sions before 2.1) and _XOPEN_SOURCE with the value 700 (600 in glibc versions before 2.10; 500 in glibc versions before 2.2). In addition, various GNU-specific extensions are also exposed.

Since glibc 2.19, defining _GNU_SOURCE also has the effect of implicitly defining _DEFAULT_SOURCE. In glibc versions before 2.20, defining _GNU_SOURCE also had the effect of implicitly defining _BSD_SOURCE and _SVID_SOURCE.

Note: _GNU_SOURCE needs to be defined before including header files so that the respective headers enable the features. For example:

#define _GNU_SOURCE

#include 
#include 
...

_GNU_SOURCE can be also be enabled per compilation using -D flag:

$ gcc -D_GNU_SOURCE file.c

(-D is not specific to _GNU_SOURCE but any macro be defined this way).