When the main function of your program is invoked, it already
has three predefined streams open and available for use. These
represent the “standard” input and output channels that have
been established for the process. These streams are declared in
the header file stdio.h. — Variable: FILE * stdin The standard
input stream, which is the normal source of input for the —
Variable: FILE * stdout The standard output stream, which is
used for normal output from — Variable: FILE * stderr The
standard error stream, which is used for error messages and
diagnostics issued by the program. In the GNU system, you can
specify what files or processes correspond to these streams
using the pipe and redirection facilities provided by the shell.
(The primitives shells use to implement these facilities are
described in File System Interface.) Most other operating
systems provide similar mechanisms, but the details of how to
use them can vary. In the GNU C library, stdin, stdout, and
stderr are normal variables which you can se