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 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

       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.

       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 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).

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

       This page is part of release 5.13 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)