stdin(3) - Linux manual page (original) (raw)
stdin(3) Library Functions Manual stdin(3)
NAME top
stdin, stdout, stderr - standard I/O streamsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <stdio.h>**
**extern FILE ***_stdin_**;**
**extern FILE ***_stdout_**;**
**extern FILE ***_stderr_**;**DESCRIPTION top
Under normal circumstances every UNIX program has three streams
opened for it when it starts up, one for input, one for output,
and one for printing diagnostic or error messages. These are
typically attached to the user's terminal (see [tty(4)](../man4/tty.4.html)) but might
instead refer to files or other devices, depending on what the
parent process chose to set up. (See also the "Redirection"
section of **sh**(1).)
The input stream is referred to as "standard input"; the output
stream is referred to as "standard output"; and the error stream
is referred to as "standard error". These terms are abbreviated
to form the symbols used to refer to these files, namely _stdin_,
_stdout_, and _stderr_.
Each of these symbols is a [stdio(3)](../man3/stdio.3.html) macro of type pointer to _FILE_,
and can be used with functions like [fprintf(3)](../man3/fprintf.3.html) or [fread(3)](../man3/fread.3.html).
Since _FILE_s are a buffering wrapper around UNIX file descriptors,
the same underlying files may also be accessed using the raw UNIX
file interface, that is, the functions like [read(2)](../man2/read.2.html) and [lseek(2)](../man2/lseek.2.html).
On program startup, the integer file descriptors associated with
the streams _stdin_, _stdout_, and _stderr_ are 0, 1, and 2,
respectively. The preprocessor symbols **STDIN_FILENO**,
**STDOUT_FILENO**, and **STDERR_FILENO** are defined with these values in
_<unistd.h>_. (Applying [freopen(3)](../man3/freopen.3.html) to one of these streams can
change the file descriptor number associated with the stream.)
Note that mixing use of _FILE_s and raw file descriptors can produce
unexpected results and should generally be avoided. (For the
masochistic among you: POSIX.1, section 8.2.3, describes in detail
how this interaction is supposed to work.) A general rule is that
file descriptors are handled in the kernel, while stdio is just a
library. This means for example, that after an [exec(3)](../man3/exec.3.html), the child
inherits all open file descriptors, but all old streams have
become inaccessible.
Since the symbols _stdin_, _stdout_, and _stderr_ are specified to be
macros, assigning to them is nonportable. The standard streams
can be made to refer to different files with help of the library
function [freopen(3)](../man3/freopen.3.html), specially introduced to make it possible to
reassign _stdin_, _stdout_, and _stderr_. The standard streams are
closed by a call to [exit(3)](../man3/exit.3.html) and by normal program termination.STANDARDS top
C11, POSIX.1-2008.
The standards also stipulate that these three streams shall be
open at program startup.HISTORY top
C89, POSIX.1-2001.NOTES top
The stream _stderr_ is unbuffered. The stream _stdout_ is line-
buffered when it points to a terminal. Partial lines will not
appear until [fflush(3)](../man3/fflush.3.html) or [exit(3)](../man3/exit.3.html) is called, or a newline is
printed. This can produce unexpected results, especially with
debugging output. The buffering mode of the standard streams (or
any other stream) can be changed using the [setbuf(3)](../man3/setbuf.3.html) or [setvbuf(3)](../man3/setvbuf.3.html)
call. Note that in case _stdin_ is associated with a terminal,
there may also be input buffering in the terminal driver, entirely
unrelated to stdio buffering. (Indeed, normally terminal input is
line buffered in the kernel.) This kernel input handling can be
modified using calls like [tcsetattr(3)](../man3/tcsetattr.3.html); see also [stty(1)](../man1/stty.1.html), and
[termios(3)](../man3/termios.3.html).SEE ALSO top
**csh**(1), **sh**(1), [open(2)](../man2/open.2.html), [fopen(3)](../man3/fopen.3.html), [stdio(3)](../man3/stdio.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-12-24 stdin(3)
Pages that refer to this page:intro(1), FILE(3type), stdio(3), pam_exec(8)