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 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).

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                                         STDIN(3)