- fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. If there is no real file descriptor at the OS level, as can happen with filehandles connected to memory objects via
openwith a reference for the third argument, -1 is returned.
This is mainly useful for constructing bitmaps for
selectand low-level POSIX tty-handling operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor: