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.\"
.IX Title "Socket::MsgHdr 3pm"
.TH Socket::MsgHdr 3pm 2024-03-07 "perl v5.38.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH NAME
Socket::MsgHdr \- sendmsg, recvmsg and ancillary data operations
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 2
\&  use Socket::MsgHdr;
\&  use Socket;
\&
\&  # sendto() behavior
\&  my $echo = sockaddr_in(7, inet_aton("10.20.30.40"));
\&  my $outMsg = new Socket::MsgHdr(buf  => "Testing echo service",
\&                                  name => $echo);
\&  sendmsg(OUT, $outMsg, 0) or die "sendmsg: $!\en";
\&
\&  # recvfrom() behavior, OO\-style
\&  my $msgHdr = new Socket::MsgHdr(buflen => 512)
\&
\&  $msgHdr\->buflen(8192);    # maybe 512 wasn\*(Aqt enough!
\&  $msgHdr\->namelen(256);    # only 16 bytes needed for IPv4
\&
\&  die "recvmsg: $!\en" unless defined recvmsg(IN, $msgHdr, 0);
\&
\&  my ($port, $iaddr) = sockaddr_in($msgHdr\->name());
\&  my $dotted = inet_ntoa($iaddr);
\&  print "$dotted:$port said: " . $msgHdr\->buf() . "\en";
\&
\&  # 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(\*(Aqi\*(Aq, $data);
\&  open(NewFH, \*(Aq<&=\*(Aq . $new_fileno);     # voila!
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
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.
.PP
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.
.SS FUNCTIONS
.IX Subsection "FUNCTIONS"
.IP "sendmsg SOCKET, MSGHDR" 4
.IX Item "sendmsg SOCKET, MSGHDR"
.PD 0
.IP "sendmsg SOCKET, MSGHDR, FLAGS" 4
.IX Item "sendmsg SOCKET, MSGHDR, FLAGS"
.PD
Send a message as described by \f(CW\*(C`Socket::MsgHdr\*(C'\fR MSGHDR over SOCKET,
optionally as specified by FLAGS (default 0).  MSGHDR should supply
at least a \fIbuf\fR member, and connectionless socket senders might
also supply a \fIname\fR member.  Ancillary data may be sent via
\&\fIcontrol\fR.
.Sp
Returns number of bytes sent, or undef on failure.
.IP "recvmsg SOCKET, MSGHDR" 4
.IX Item "recvmsg SOCKET, MSGHDR"
.PD 0
.IP "recvmsg SOCKET, MSGHDR, FLAGS" 4
.IX Item "recvmsg SOCKET, MSGHDR, FLAGS"
.PD
Receive a message as requested by \f(CW\*(C`Socket::MsgHdr\*(C'\fR MSGHDR from
SOCKET, optionally as specified by FLAGS (default 0).  The caller
requests \fIbuflen\fR bytes in MSGHDR, possibly also recording up to
\&\fInamelen\fR bytes of the sender's (packed) address and perhaps
\&\fIcontrollen\fR bytes of ancillary data.
.Sp
Returns number of bytes received, or undef on failure.  \fIbuflen\fR
et. al. are updated to reflect the actual lengths of received data.
.SS Socket::MsgHdr
.IX Subsection "Socket::MsgHdr"
.IP "new [PARAMETERS]" 4
.IX Item "new [PARAMETERS]"
Return a new Socket::MsgHdr object.  Optional PARAMETERS may specify method
names (\f(CW\*(C`buf\*(C'\fR, \f(CW\*(C`name\*(C'\fR, \f(CW\*(C`control\*(C'\fR, \f(CW\*(C`flags\*(C'\fR or their corresponding \fI...len\fR
methods where applicable) and values, sparing an explicit call to those
methods.
.IP "buf [SCALAR]" 4
.IX Item "buf [SCALAR]"
.PD 0
.IP "buflen LENGTH" 4
.IX Item "buflen LENGTH"
.PD
\&\f(CW\*(C`buf\*(C'\fR gets the current message buffer or sets it to SCALAR.  \f(CW\*(C`buflen\*(C'\fR
allocates LENGTH bytes for use in recvmsg.
.IP "name [SCALAR]" 4
.IX Item "name [SCALAR]"
.PD 0
.IP "namelen LENGTH" 4
.IX Item "namelen LENGTH"
.PD
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.
.IP "controllen LENGTH" 4
.IX Item "controllen LENGTH"
Prepare the ancillary data buffer to receive LENGTH bytes.  There is a
corresponding \f(CW\*(C`control\*(C'\fR method, but its use is discouraged \-\- you have to
"pack" in perlfunc the \f(CW\*(C`struct cmsghdr\*(C'\fR yourself.  Instead see cmsghdr below
for convenient access to the control member.
.IP "flags [FLAGS]" 4
.IX Item "flags [FLAGS]"
Get or set the Socket::MsgHdr flags, distinct from the sendmsg or
recvmsg flags.  Example:
.Sp
.Vb 5
\&  $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\en";
\&  }
.Ve
.IP cmsghdr 4
.IX Item "cmsghdr"
.PD 0
.IP "cmsghdr LEVEL, TYPE, DATA [ LEVEL, TYPE, DATA ... ]" 4
.IX Item "cmsghdr LEVEL, TYPE, DATA [ LEVEL, TYPE, DATA ... ]"
.PD
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.
.Sp
In any case, DATA is in a message-specific format which likely requires
special treatment (packing or unpacking).
.Sp
Examples:
.Sp
.Vb 6
\&   my @cmsg = $hdr\->cmsghdr();
\&   while (my ($level, $type, $data) = splice(@cmsg, 0, 3)) {
\&     warn "unknown cmsg LEVEL\en", next unless $level == IPPROTO_IPV6;
\&     warn "unknown cmsg TYPE\en", 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);
.Ve
.SS EXPORT
.IX Subsection "EXPORT"
\&\f(CW\*(C`Socket::MsgHdr\*(C'\fR exports sendmsg and recvmsg by default into the
caller's namespace, and in any case these methods into the IO::Socket
namespace.
.SH BUGS
.IX Header "BUGS"
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 \f(CW\*(C`struct msghdr\*(C'\fR definitions with
\&\f(CW\*(C`msg_accrights\*(C'\fR members (instead of \f(CW\*(C`msg_control\*(C'\fR) are not supported at all.
.PP
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.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBsendmsg\fR\|(2), \fBrecvmsg\fR\|(2), File::FDpasser, RFC 2292 <https://tools.ietf.org/html/rfc2292>
.SH AUTHOR
.IX Header "AUTHOR"
Michael J. Pomraning, co-maintained by Felipe Gasper
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
Copyright 2003, 2010 by Michael J. Pomraning
.PP
This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.