STDIN(3)                   Linux Programmer's Manual                  STDIN(3)

       stdin, stdout, stderr - standard I/O streams

       #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  sym-
       bols 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 in-
       terface, 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 un-
       expected results and should generally be avoided.  (For the masochistic
       among  you: POSIX.1, section 8.2.3, describes in detail how this inter-
       action 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.

       The stdin, stdout, and stderr macros conform to C89 and  this  standard
       also  stipulates  that  these  three  streams  shall be open at program

       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 pro-
       duce 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 as-
       sociated with a terminal, there may also be input buffering in the ter-
       minal driver, entirely unrelated to stdio buffering.  (Indeed, normally
       terminal input is line buffered in the kernel.)  This kernel input han-
       dling  can be modified using calls like tcsetattr(3); see also stty(1),
       and termios(3).

       csh(1), sh(1), open(2), fopen(3), stdio(3)

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Linux                             2017-09-15                          STDIN(3)
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