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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 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-07-14 STDIN(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface