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
     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.

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 buffer-
     ing 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, nor-
     mally 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 C89''), and this standard also stipulates that these three streams
     shall be open at program startup.

Linux 2.0							  March 24, 1998							 Linux 2.0