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.