Programming Perl, 4th Edition

fileno

fileno FILEHANDLE

This function returns the file descriptor underlying a filehandle. If the filehandle is not open, fileno returns undef. If there is no real file descriptor at the OS level, as can happen with filehandles connected to memory objects via open with a reference for the third argument, −1 is returned.

A file descriptor is a small, nonnegative integer like 0 or 1, in contrast to filehandles like STDIN and STDOUT, which are symbols. Unfortunately, the operating system doesn’t know about your cool symbols. It only thinks of open files using these small file numbers, and although Perl will usually do the translations for you automatically, occasionally you have to know the actual file descriptor.

So, for example, the fileno function is useful for constructing bitmaps for select and for passing to certain obscure system calls if syscall(2) is implemented. It’s also useful for double checking that the open function gave you the file descriptor you wanted and for determining whether two filehandles use the same system file descriptor.

if (fileno(THIS) == fileno(THAT)) {
    say "THIS and THAT are dups";
}

If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name or a reference to something resembling a filehandle object.

Don’t count on the association of a Perl filehandle and a numeric file descriptor throughout the life of the program. If ...