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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).

       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 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.


       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 C89 and this standard
       also stipulates that these three  streams  shall  be  open  at  program


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


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