3.2.38 fileno

fileno 

FILEHANDLE

This function returns the file descriptor for a filehandle. (A file descriptor is a small integer, unlike the filehandle, which is a symbol.) It returns undef if the handle is not open. It's 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 - see the example under fcntl .

If FILEHANDLE is an expression, its value is taken to represent a filehandle, either indirectly by name, or directly as a reference to a filehandle object.

A caution: don't count on the association of a Perl filehandle and a numeric file descriptor throughout the life of the program. If a file has been closed and reopened, the file descriptor may change. Filehandles STDIN , STDOUT , and STDERR start with file descriptors of 0, 1, and 2 (the UNIX standard convention), but even they can change if you start closing and opening them with wild abandon. But you can't get into trouble with 0, 1, and 2 as long as you always reopen immediately after closing, since the basic rule on UNIX systems is to pick the lowest available descriptor, and that'll be the one you just closed.