explanation of D_GNU_SOURCE Why to use it and when?

Question

I know it is a Macro we are passing to a function. How do you explain what is the use of this macro, and in which scenario i have to use this ?.

Answer 1

_GNU_SOURCE enables GNU extensions to the C and OS standards supported by the GNU C library, such as asprintf. Define it when you're using such non-standard functions and macros.

Answer 2

From glibc manual:

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.

http://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html