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__NAME__

stdin, stdout, stderr - standard I/O streams

__SYNOPSIS__
<verbatim>
#include <stdio.h>
extern FILE stdin;
extern FILE stdout;
extern FILE stderr;
</verbatim>

__DESCRIPTION__

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 mes-
sages. 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). 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


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, the child inherits all open
file descriptors, but all old streams have become inaccessi-
ble.


Since the symbols stdin, stdout, and stderr are specified to
be macros, assigning to them is non-portable. 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 nor- mal program
termination.

__SEE ALSO__

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

__CONSIDERATIONS__

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).

__CONFORMING TO__

The stdin, stdout, and stderr macros conform to
ANSI X3.159-1989 (`` ANSI C''), and this standard also stipulates that these three streams shall be open at program startup.
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Last edited on Friday, May 11, 2007 6:40:51 pm by CraigBox
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