Why am I getting "error: implicit declaration of function ‘fileno’" when I try to compile on Linux but not on a Mac

Question

As the title states, I am getting the following error: "implicit declaration of function ‘fileno’" when I try to compile on Linux but not on a Mac. I know I can fix it with the simple line int fileno(FILE *stream);, but I want to know why this happening.

Answer 1

You should not declare functions provided by a system library. You should properly include an appropriate header.

Since fileno is not a standard C function, it is not normally declared by in <stdio.h> . You have to define a suitable macro in order to enable the declaration.

man fileno

Requirements for glibc (see feature_test_macros(7)):

fileno(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

Defining any of the the three macros before inclusion of should do the trick.