# stdin

## NAME



           stdin, stdout, stderr - standard I/O streams
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## 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 _FILE_ s 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 _FILE_ s 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.
    


## STANDARDS



           C11, POSIX.1-2008.
    
           The standards also stipulate 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).
    


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