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  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 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.

HISTORY

       C89, POSIX.1-2001.

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)