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Manual Reference Pages  -  IO::FDPASS (3)

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IO::FDPass - pass a file descriptor over a socket



   use IO::FDPass;

   IO::FDPass::send fileno $socket, fileno $fh_to_pass
      or die "send failed: $!";

   my $fd = IO::FDPass::recv fileno $socket;
   $fd >= 0 or die "recv failed: $!";


This small low-level module only has one purpose: pass a file descriptor to another process, using a (streaming) unix domain socket (on POSIX systems) or any (streaming) socket (on WIN32 systems). The ability to pass file descriptors on windows is currently the unique selling point of this module. Have I mentioned that it is really small, too?


$bool = IO::FDPass::send $socket_fd, $fd_to_pass Sends the file descriptor given by $fd_to_pass over the socket $socket_fd. Return true if it worked, false otherwise.

Note that both parameters must be file descriptors, not handles.

When used on non-blocking sockets, this function might fail with $! set to EAGAIN or equivalent, in which case you are free to try. It should succeed if called on a socket that indicates writability (e.g. via select).

Example: pass a file handle over an open socket.

   IO::FDPass::send fileno $socket, fileno $fh
      or die "unable to pass file handle: $!";

$fd = IO::FDPass::recv $socket_fd Receive a file descriptor from the socket and return it if successful. On errors, return -1.

Note that both $socket_fd and the returned file descriptor are, in fact, file descriptors, not handles.

When used on non-blocking sockets, this function might fail with $! set to EAGAIN or equivalent, in which case you are free to try again. It should succeed if called on a socket that indicates readability (e.g. via select).

Example: receive a file descriptor from a blocking socket and convert it to a file handle.

  my $fd = IO::FDPass::recv fileno $socket;
  $fd >= 0 or die "unable to receive file handle: $!";
  open my $fh, "+<&=$fd"
     or die "unable to convert file descriptor to handle: $!";


This module has been tested on GNU/Linux x86 and amd64, NetBSD 6, OS X 10.5, Windows 2000 ActivePerl 5.10, Solaris 10, OpenBSD 4.4, 4.5, 4.8 and 5.0, DragonFly BSD, FreeBSD 7, 8 and 9, Windows 7 + ActivePerl 5.16.3 32 and 64 bit and Strawberry Perl 5.16.3 32 and 64 bit, and found to work, although ActivePerl 32 bit needed a newer MinGW version (that supports XP and higher).

However, windows doesn’t support asynchronous file descriptor passing, so the source process must still be around when the destination process wants to receive the file handle. Also, if the target process fails to fetch the handle for any reason (crashes, fails to call recv etc.), the handle will leak, so never do that.

Also, on windows, the receiving process must have the PROCESS_DUP_HANDLE access right on the sender process for this module to work.

Cygwin is not supported at the moment, as file descriptor passing in cygwin is not supported, and cannot be rolled on your own as cygwin has no (working) method of opening a handle as fd. That is, it has one, but that one isn’t exposed to programs, and only used for stdin/out/err. Sigh.


At the time of this writing, the author of this module was aware of two other file descriptor passing modules on CPAN: File::FDPasser and AnyEvent::FDPasser.

The former hasn’t seen any release for over a decade, isn’t 64 bit clean and it’s author didn’t respond to my mail with the fix, so doesn’t work on many 64 bit machines. It does, however, support a number of pre-standard unices, basically everything of relevance at the time it was written.

The latter seems to have similar support for antique unices, and doesn’t seem to suffer from 64 bit bugs, but inexplicably has a large perl part, doesn’t support mixing data and file descriptors, and requires AnyEvent. Presumably that makes it much more user friendly than this module (skimming the manpage shows that a lot of thought has gone into it, and you are well advised to read it and maybe use it before trying a low-level module such as this one). In fact, the manpage discusses even more file descriptor passing modules on CPAN.

Neither seems to support native win32 perls.


 Marc Lehmann <>

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