What is the purpose of features.h header?

Question

What is the purpose of the features.h header? Why and when can it be used in my code?

Does it define source features supported by the system? Or does i

Answer 1

I have grepped POSIX 7 as explained at: https://unix.stackexchange.com/questions/340285/install-the-latest-posix-man-pages/483198#483198 and there are no hits for features.h, so it must be a glibc extension only.

In glibc 2.28, it is present at include/features.h.

One of the interesting things that it defines are version macros:

#include 
#include 

int main(void) {
  printf("__GLIBC__       %u\n", __GLIBC__);
  printf("__GLIBC_MINOR__ %u\n", __GLIBC_MINOR__);
  return 0;
}

Ubuntu 16.04, which has glibc 2.23, this outputs:

__GLIBC__       2
__GLIBC_MINOR__ 23

See also: Check glibc version for a particular gcc compiler

Also, this header seems to get included in most / all glibc headers, which might allow you to check if glibc is being used: How to tell if glibc is used but TODO I couldn't find a documentation for that.