The standard output stream
#include <stdio.h> FILE * stdout;
This global variable defines the standard output stream. It's set to the console by default, but you can redirect it by calling freopen().
STDOUT_FILENO, which is defined in <unistd.h>, defines the file descriptor that corresponds to stdout.
fputchar(), printf(), putchar(), putchar_unlocked(), puts(), putwchar(), stderr, stdin, vprintf(), vwprintf(), wprintf()