stdin(3) — Linux manual page


stdin(3)                Library Functions Manual                stdin(3)

NAME         top

       stdin, stdout, stderr - standard I/O streams

LIBRARY         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)) 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.

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) 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).

SEE ALSO         top

       csh(1), sh(1), open(2), fopen(3), stdio(3)

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                       stdin(3)

Pages that refer to this page: intro(1)FILE(3type)stdio(3)pam_exec(8)