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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 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).  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

     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.