Returns the file descriptor associated with the specified file
    pointer.

    Format

      #include  <stdio.h>

      int fileno  (FILE *file_ptr);

1  –  Argument

 file_ptr

    A file pointer.

2  –  Description

    If you are using version 5.2 or lower of the C compiler, undefine
    the fileno macro:

    #if defined(fileno)
    #undef fileno
    #endif

3  –  Return Values

    x                  Integer file descriptor.
    -1                 Indicates an error.