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 it define some additional things which must be defined depending on other defines?

Answer 1

The features.h header file provides various macro definitions that indicate standard conformance to other header files, i.e. which features (hence the name) should be turned on or off depending on which standard the user wishes to use.

Most C/C++ compilers have command line options to handle standards conformance. Let's take GCC as an example: when you pass the -std=gnu9x option, you ask for the GNU dialect of the C99 standard. The features.h header makes sure that all other headers that include it will turn exactly those features on or off that are needed to support this particular dialect. This is achieved by #define -ing or #undef - ing some "intermediate" macros.

As a bonus, features.h also provides the glibc version information macros as well, and various other bits & bobs.

Answer 2

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 <stdio.h>
#include <features.h>

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.