The standard input stream
#include <stdio.h> FILE * stdin;
This global variable defines the standard input stream. It's set to the console by default, but you can redirect it by calling freopen().
STDIN_FILENO, which is defined in <unistd.h>, defines the file descriptor that corresponds to stdin.
fgetchar(), getchar(), getchar_unlocked(), gets(), getwchar(), scanf(), stderr, stdout, vscanf(), vwscanf()