The standard streams are three predefined streams between the program and the host environment open and available for use by every C program when the main function of the program is invoked. The three standard streams are the standard input, standard output, and the standard error streams.

<stdio.h>

The <stdio.h> standard header provides definitions for three external variables. The three predefined streams are:

#include <stdio.h>

extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;

stdin

The stdin extern variable which is of type FILE * is the standard input stream which is usually the normal source of input for the program such as the keyboard. On some environments, the host environment can also redirect files and other types of streams to the standard input.

stdout

The stdout extern variable which is of type FILE * is the standard output stream which is used for normal output from the program. Typically this stream is connected to a terminal.

stderr

The 'stderr extern variable which is of type FILE * is the standard error stream. This stream is used for error messages and reporting diagnostics issued by the program.