void error(char *msg){
    fputs(msg, stderr);
}

int main(int argc, char **argv){
    if (argc < 1){
        error("Usage: ./my_program [option] [file]");
        return 1; // The OS/Shell will interpret this as a failure, sometimes saying "[program] returned with an exit status of 1"
    }
    // [do stuff]
    return 0; // The OS/Shell will interpret this as a success
}