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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
       normal 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, 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).

SEE ALSO

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

COLOPHON

       This  page  is  part of release 4.16 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.