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 er‐
       ror".  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 de‐
       scriptor 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)

Linux                                                                          2017-09-15                                                                       STDIN(3)