NAME

stderr stdin stdout — standard I/O streams

SYNOPSIS

#include <stdio.h>

extern FILE *stderr, *stdin, *stdout;

DESCRIPTION

[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

A file with associated buffering is called a stream and is declared to be a pointer to a defined type FILE. The fopen(3) function shall create certain descriptive data for a stream and return a pointer to designate the stream in all further transactions. Normally, there are three open streams with constant pointers declared in the <stdio.h> header and associated with the standard open files.

At program start-up, three streams shall be predefined and already open: stdin (standard input, for conventional input) for reading, stdout (standard output, for conventional output) for writing, and stderr (standard error, for diagnostic output) for writing. When opened, stderr shall not be fully buffered; stdin and stdout shall be fully buffered if and only if [CX] the file descriptor associated with the stream is determined not to be associated with an interactive device.

[CX] The following symbolic values in <unistd.h> define the file descriptors that shall be associated with the C-language stdin, stdout, and stderr when the application is started:

STDIN_FILENO: Standard input value, stdin . Its value is 0.
STDOUT_FILENO: Standard output value, stdout . Its value is 1.
STDERR_FILENO: Standard error value, stderr . Its value is 2.

These file descriptors are often all associated with a single open file description which has access mode O_RDWR (e.g., in the case of a terminal device for a login shell). However, the stderr, stdin, and stdout streams need not be opened for both reading and writing at program start-up in this case.