_GNU_SOURCE and __USE_GNU

Question

I want to use CPU_SET, which is a glibc linux-specific macro that should be defined in sched.h The manpage clearly states that _GNU_SOURCE

Answer 1

_GNU_SOURCE is the only one you should ever define yourself. __USE_GNU is defined internally through a mechanism in features.h (which is included by all other glibc headers) when _GNU_SOURCE is defined, and possibly under other conditions. Defining or undefining __USE_GNU yourself will badly break the glibc headers.

Answer 2

you have to define_GNU_SOURCE before anything else. This snippet works here:

#define _GNU_SOURCE
#include <sched.h>


int main()
{
    cpu_set_t set;
    CPU_SET(0, &set);
    return 0;
}