What does "#define _GNU_SOURCE" imply?
c, gnu, posix
Solution
Defining `_GNU_SOURCE` has nothing to do with license and everything to do with writing (non-)portable code. If you define `_GNU_SOURCE`, you will get:
- access to lots of nonstandard GNU/Linux extension functions
- access to traditional functions which were omitted from the POSIX standard (often for good reason, such as being replaced with better alternatives, or being tied to particular legacy implementations)
- access to low-level functions that cannot be portable, but that you sometimes need for implementing system utilities like `mount`, `ifconfig`, etc.
- broken behavior for lots of POSIX-specified functions, where the GNU folks disagreed with the standards committee on how the functions should behave and decided to do their own thing.
As long as you're aware of these things, it should not be a problem to define `_GNU_SOURCE`, but you should avoid defining it and instead define `_POSIX_C_SOURCE=200809L` or `_XOPEN_SOURCE=700` when possible to ensure that your programs are portable.
In particular, the things from `_GNU_SOURCE` that you should never use are #2 and #4 above.
Problem
Today I had to use the `basename()` function, and the `man 3 basename` (here) gave me some strange message: Notes There are two different versions of basename() - the POSIX version described above, and the GNU version, which one gets after `#define _GNU_SOURCE` `#include <string.h>` I'm wondering what this `#define _GNU_SOURCE` means: is it tainting the code I write with a GNU-related license? Or is it simply used to tell the compiler something like "Well, I know, this set of functions is not POSIX, thus not portable, but I'd like to use it anyway". If so, why not give people different headers, instead of having to define some obscure macro to get one function implementation or the other? Something also bugs me: how does the compiler know which function implementation to link with the executable? Does it use this `#define` as well? Anybody have some pointers to give me?