Firstly, getopt() is not defined by the C standards (it's not a concern of the language), but is defined by POSIX standards (it's a concern of the portability of the OS interfaces). And because <unistd.h> is not a standard-C header file (https://en.cppreference.com/w/c/header), just including it does not provide the function getopt() unless you specify which version of POSIX (or later) you require. This is described in the online manual for getopt() You have two simple choices: or