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           #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  sym-
           bols 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 masochis-
           tic  among  you:  POSIX.1,  section 8.2.3, describes in detail how this
           interaction is supposed to work.)  A general rule is that file descrip-
           tors  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.


           The stdin, stdout, and stderr macros conform to C89 and  this  standard
           also  stipulates  that  these  three  streams  shall be open at program


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

    Linux 2008-07-14 STDIN(3)


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