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 set just like any others. For example,
to redirect the standard output to a file, you could do:
Note however, that in other systems stdin, stdout, and
stderr are macros that you cannot assign to in the normal way.
But you can use freopen to get the effect of closing one and
reopening it. See Opening Streams.
The three streams stdin, stdout, and stderr are not
unoriented at program start (see Streams and I18N).
Published under the terms of the GNU General Public License