Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


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
     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 masochis-
     tic  among  you:  POSIX.1,  section 8.2.3, describes in detail how this
     interaction is supposed to work.)  A general rule is that file descrip-
     tors  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
     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).


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


     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

Linux 2017-09-15 STDIN(3)

/data/webs/external/dokuwiki/data/pages/man/stdin.txt · Last modified: 2019/05/17 09:47 by

Was this page helpful?-10+1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki