Substitutes the filenamed by a file specification for the open
    file addressed by a file pointer. The latter file is closed.

    Format

      #include  <stdio.h>

      FILE *freopen  (const char *file_spec, const char *a_mode, FILE
                     *file_ptr, . . . );

1 - Arguments

 file_spec

    A pointer to a string that contains a valid OpenVMS or UNIX
    style file specification. After the function call, the given
    file pointer is associated with this file.

 a_mode

    The access mode indicator. See the fopen function for a
    description.

 file_ptr

    A file pointer.

  . . .

    Optional file attribute arguments. The file attribute arguments
    are the same as those used in the creat function.

2 - Description

    The freopen function is typically used to associate one of the
    predefined names stdin, stdout, or stderr with a file.

3 - Return Values

    file_ptr           The file pointer, if freopen is successful.
    NULL               Indicates an error.