stdin, stdout, stderr - standard I/O streams
Standard C library (
libc,
-lc)
#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 C99 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)