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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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stdin(3) Library Functions Manual stdin(3)
stdin, stdout, stderr - standard I/O streams
Standard C library (libc, -lc)
#include <stdio.h>
extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;
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)) 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) macro of type pointer to FILE,
and can be used with functions like fprintf(3) or fread(3).
Since FILEs 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) and lseek(2).
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) to one of these streams can
change the file descriptor number associated with the stream.)
Note that mixing use of FILEs 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), 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), specially introduced to make it possible to
reassign stdin, stdout, and stderr. The standard streams are
closed by a call to exit(3) and by normal program termination.
C11, POSIX.1-2008.
The standards also stipulate that these three streams shall be
open at program startup.
C89, POSIX.1-2001.
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) or exit(3) 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) or setvbuf(3)
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); see also stty(1), and
termios(3).
csh(1), sh(1), open(2), fopen(3), stdio(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-06 stdin(3)
Pages that refer to this page: intro(1), FILE(3type), stdio(3), pam_exec(8)
Copyright and license for this manual page
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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