STDIN(3)                   UNIX Programmer's 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 inter-
     face, 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, STD-
     OUT_FILENO, and STDERR_FILENO are defined with these values in
     <unistd.h>.

     Note that mixing use of FILEs and raw file descriptors can produce unex-
     pected results and should generally be avoided.  (For the masochistic
     among you: POSIX.1, section 8.2.3, describes in detail how this interac-
     tion 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 ex-
     ample, 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 nor-
     mal 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 termi-
     nal, 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 us-
     ing calls like tcsetattr(3);  see also stty(1),  and termios(3).

STANDARDS
     The stdin, stdout, and stderr macros conform to ANSI C3.159-1989 (``ANSI
     C''), and this standard also stipulates that these three streams shall be
     open at program startup.

 Linux 2.0                      March 24, 1998                               1