stdout(3) -- Linux man page
NAME
stdin
stdout
stderr
- standard I/O streams
SYNOPSIS
Fd #include <stdio.h>
Fd extern FILE *stdin;
Fd extern FILE *stdout;
Fd extern FILE *stderr;
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 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).
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>.
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 inaccessible.
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 normal 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
St -ansiC ,
and this standard also stipulates that these three
streams shall be open at program startup.
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