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STDIN(3) BSD Library Functions Manual STDIN(3)
NAME
stdin, stdout, stderr - standard I/O streams
SYNOPSIS
#include <stdio.h>
extern FILE *stdin;
extern FILE *stdout;
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 descrip-
tors, 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 possi-
ble 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 associ-
ated 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.
Linux 2.0 March 24, 1998 Linux 2.0
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