Standard streams

Standard input Standard input is the input stream into which text or other data can be entered into a program. Certain programs will, by default, use the standard input stream as a data source if not given a file to use as input.

Standard output Standard output is the output stream into which data is written from a program. Data written to standard output is usually written to the screen unless redirected. It is intended to be the used as the default output stream.

Standard error Standard error is the output stream used by programs for outputting error messages[?] or other diagnostics[?]. It is a separate stream from standard output and can be redirected separately from it.

Relevance to the C programming language The C standard library[?] header file <stdio.h> defines three variables of type FILE * which programs can use to access the standard streams. They are named 'stdin', 'stdout' and 'stderr' respectively.

When running programs from a terminal, standard input is taken from the keyboard, while standard output and error will appear on the screen.