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.\" ========================================================================
.\"
.IX Title "Socket::MsgHdr 3pm"
.TH Socket::MsgHdr 3pm "2022-10-19" "perl v5.36.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.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 "\s-1FUNCTIONS\s0"
.IX Subsection "FUNCTIONS"
.IP "sendmsg \s-1SOCKET, MSGHDR\s0" 4
.IX Item "sendmsg SOCKET, MSGHDR"
.PD 0
.IP "sendmsg \s-1SOCKET, MSGHDR, FLAGS\s0" 4
.IX Item "sendmsg SOCKET, MSGHDR, FLAGS"
.PD
Send a message as described by \f(CW\*(C`Socket::MsgHdr\*(C'\fR \s-1MSGHDR\s0 over \s-1SOCKET,\s0
optionally as specified by \s-1FLAGS\s0 (default 0).  \s-1MSGHDR\s0 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 \s-1SOCKET, MSGHDR\s0" 4
.IX Item "recvmsg SOCKET, MSGHDR"
.PD 0
.IP "recvmsg \s-1SOCKET, MSGHDR, FLAGS\s0" 4
.IX Item "recvmsg SOCKET, MSGHDR, FLAGS"
.PD
Receive a message as requested by \f(CW\*(C`Socket::MsgHdr\*(C'\fR \s-1MSGHDR\s0 from
\&\s-1SOCKET,\s0 optionally as specified by \s-1FLAGS\s0 (default 0).  The caller
requests \fIbuflen\fR bytes in \s-1MSGHDR,\s0 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 [\s-1PARAMETERS\s0]" 4
.IX Item "new [PARAMETERS]"
Return a new Socket::MsgHdr object.  Optional \s-1PARAMETERS\s0 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 [\s-1SCALAR\s0]" 4
.IX Item "buf [SCALAR]"
.PD 0
.IP "buflen \s-1LENGTH\s0" 4
.IX Item "buflen LENGTH"
.PD
\&\f(CW\*(C`buf\*(C'\fR gets the current message buffer or sets it to \s-1SCALAR.\s0  \f(CW\*(C`buflen\*(C'\fR
allocates \s-1LENGTH\s0 bytes for use in recvmsg.
.IP "name [\s-1SCALAR\s0]" 4
.IX Item "name [SCALAR]"
.PD 0
.IP "namelen \s-1LENGTH\s0" 4
.IX Item "namelen LENGTH"
.PD
Get or set the socket name (address) buffer, an attribute analogous to the
optional \s-1TO\s0 and \s-1FROM\s0 parameters of \*(L"send\*(R" in perlfunc and \*(L"recv\*(R" in perlfunc.
Note that socket names are packed structures.
.IP "controllen \s-1LENGTH\s0" 4
.IX Item "controllen LENGTH"
Prepare the ancillary data buffer to receive \s-1LENGTH\s0 bytes.  There is a
corresponding \f(CW\*(C`control\*(C'\fR method, but its use is discouraged \*(-- you have to
\&\*(L"pack\*(R" in perlfunc the \f(CW\*(C`struct cmsghdr\*(C'\fR yourself.  Instead see cmsghdr below
for convenient access to the control member.
.IP "flags [\s-1FLAGS\s0]" 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 \s-1LEVEL, TYPE, DATA\s0 [ \s-1LEVEL, TYPE, DATA ...\s0 ]" 4
.IX Item "cmsghdr LEVEL, TYPE, DATA [ LEVEL, TYPE, DATA ... ]"
.PD
Without arguments, this method returns a list of \*(L"\s-1LEVEL, TYPE, DATA, ...\*(R",\s0 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, \s-1DATA\s0 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 "\s-1EXPORT\s0"
.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 \s-1XS\s0 presently makes use of \s-1RFC 2292\s0 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, \s-1RFC 2292\s0 <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.