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Socket::MsgHdr(3) User Contributed Perl Documentation Socket::MsgHdr(3)


Socket::MsgHdr - sendmsg, recvmsg and ancillary data operations


  use Socket::MsgHdr;
  use Socket;
  # sendto() behavior
  my $echo = sockaddr_in(7, inet_aton(""));
  my $outMsg = new Socket::MsgHdr(buf  => "Testing echo service",
                                  name => $echo);
  sendmsg(OUT, $outMsg, 0) or die "sendmsg: $!\n";
  # recvfrom() behavior, OO-style
  my $msgHdr = new Socket::MsgHdr(buflen => 512)
  $msgHdr->buflen(8192);    # maybe 512 wasn't enough!
  $msgHdr->namelen(256);    # only 16 bytes needed for IPv4
  die "recvmsg: $!\n" unless defined recvmsg(IN, $msgHdr, 0);
  my ($port, $iaddr) = sockaddr_in($msgHdr->name());
  my $dotted = inet_ntoa($iaddr);
  print "$dotted:$port said: " . $msgHdr->buf() . "\n";
  # Pack ancillary data for sending
  $outHdr->cmsghdr(SOL_SOCKET,                # cmsg_level
                   SCM_RIGHTS,                # cmsg_type
                   pack("i", fileno(STDIN))); # cmsg_data
  sendmsg(OUT, $outHdr);
  # Unpack the same
  my $inHdr = Socket::MsgHdr->new(buflen => 8192, controllen => 256);
  recvmsg(IN, $inHdr, $flags);
  my ($level, $type, $data) = $inHdr->cmsghdr();
  my $new_fileno = unpack('i', $data);
  open(NewFH, '<&=' . $new_fileno);     # voila!


Socket::MsgHdr provides advanced socket messaging operations via sendmsg and recvmsg. Like their C counterparts, these functions accept few parameters, instead stuffing a lot of information into a complex structure.

This structure describes the message sent or received (buf), the peer on the other end of the socket (name), and ancillary or so-called control information (cmsghdr). This ancillary data may be used for file descriptor passing, IPv6 operations, and a host of implemenation-specific extensions.


Send a message as described by "Socket::MsgHdr" MSGHDR over SOCKET, optionally as specified by FLAGS (default 0). MSGHDR should supply at least a buf member, and connectionless socket senders might also supply a name member. Ancillary data may be sent via control.

Returns number of bytes sent, or undef on failure.

Receive a message as requested by "Socket::MsgHdr" MSGHDR from SOCKET, optionally as specified by FLAGS (default 0). The caller requests buflen bytes in MSGHDR, possibly also recording up to namelen bytes of the sender's (packed) address and perhaps controllen bytes of ancillary data.

Returns number of bytes received, or undef on failure. buflen et. al. are updated to reflect the actual lengths of received data.


Return a new Socket::MsgHdr object. Optional PARAMETERS may specify method names ("buf", "name", "control", "flags" or their corresponding ...len methods where applicable) and values, sparing an explicit call to those methods.
"buf" gets the current message buffer or sets it to SCALAR. "buflen" allocates LENGTH bytes for use in recvmsg.
Get or set the socket name (address) buffer, an attribute analogous to the optional TO and FROM parameters of "send" in perlfunc and "recv" in perlfunc. Note that socket names are packed structures.
Prepare the ancillary data buffer to receive LENGTH bytes. There is a corresponding "control" method, but its use is discouraged -- you have to "pack" in perlfunc the "struct cmsghdr" yourself. Instead see cmsghdr below for convenient access to the control member.
Get or set the Socket::MsgHdr flags, distinct from the sendmsg or recvmsg flags. Example:

  $hdr = new Socket::MsgHdr (buflen => 512, controllen => 3);
  recvmsg(IN, $hdr);
  if ($hdr->flags & MSG_CTRUNC) {   # &Socket::MSG_CTRUNC
    warn "Yikes!  Ancillary data was truncated\n";
Without arguments, this method returns a list of "LEVEL, TYPE, DATA, ...", or an empty list if there is no ancillary data. With arguments, this method copies and flattens its parameters into the internal control buffer.

In any case, DATA is in a message-specific format which likely requires special treatment (packing or unpacking).


   my @cmsg = $hdr->cmsghdr();
   while (my ($level, $type, $data) = splice(@cmsg, 0, 3)) {
     warn "unknown cmsg LEVEL\n", next unless $level == IPPROTO_IPV6;
     warn "unknown cmsg TYPE\n", next unless $type == IPV6_PKTINFO;
   my $data = pack("i" x @filehandles, map {fileno $_} @filehandles);
   my $hdr->cmsghdr(SOL_SOCKET, SCM_RIGHTS, $data);
   sendmsg(S, $hdr);


"Socket::MsgHdr" exports sendmsg and recvmsg by default into the caller's namespace, and in any case these methods into the IO::Socket namespace.


The underlying XS presently makes use of RFC 2292 CMSG_* manipulation macros, which may not be available on all systems supporting sendmsg/recvmsg as known to 4.3BSD Reno/POSIX.1g. Older "struct msghdr" definitions with "msg_accrights" members (instead of "msg_control") are not supported at all.

There is no Socket::CMsgHdr, which may be a good thing. Examples are meager, see the t/ directory for send(to) and recv(from) emulations in terms of this module.


sendmsg(2), recvmsg(2), File::FDpasser, RFC 2292 <>


Michael J. Pomraning, co-maintained by Felipe Gasper


Copyright 2003, 2010 by Michael J. Pomraning

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

2018-12-06 perl v5.40.0