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     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

     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).  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>.

     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, 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
     non-portable.  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.


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


     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).


     The stdin, stdout, and stderr macros conform to ANSI X3.159-1989 (“ANSI C89”), and this
     standard also stipulates that these three streams shall be open at program startup.