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  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,  std-
       out, 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  tcse-
       tattr(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 4.15 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/.

Linux                                                               2017-09-15                                                            STDIN(3)