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STDIN(3) Linux Programmer's Manual STDIN(3)
stdin, stdout, stderr - standard I/O streams
#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.
The stdin, stdout, and stderr macros conform to C89 and this standard
also stipulates that these three streams shall be open at program
startup.
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 release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 STDIN(3)
Pages that refer to this page: intro(1), stdio(3), pam_exec(8)
Copyright and license for this manual page
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