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.
C11, POSIX.1-2008.
The standards also stipulate that these three streams shall be open at program startup.
C89, POSIX.1-2001.
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).