STDIN(3)                                            Linux 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 interface, 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 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(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 nor-
       mal program termination.

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

NOTES
       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, espe-
       cially 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).

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

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-07-14                                                   STDIN(3)