📄 recvmsg.html

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>recvmsg</title></head><body bgcolor=white><center><font size=2>The Single UNIX &reg; Specification, Version 2<br>Copyright &copy; 1997 The Open Group</font></center><hr size=2 noshade><h4>NAME</h4><blockquote>recvmsg - receive a message from a socket</blockquote><h4>SYNOPSIS</h4><blockquote><pre><code>#include &lt;<a href="syssocket.h.html">sys/socket.h</a>&gt;ssize_t recvmsg(int <I>socket</I>, struct msghdr *<I>message</I>, int <I>flags</I>);</code></pre></blockquote><h4>DESCRIPTION</h4><blockquote>The<i>recvmsg()</i>function receives a message from a connection-mode or connectionless-modesocket.  It is normally used with connectionless-modesockets because it permitsthe application to retrieve the source address of received data.<p>The function takes the following arguments:<dl compact><dt><I>socket</I><dd>Specifies the socket file descriptor.<dt><I>message</I><dd>Points to a <B>msghdr</B> structure, containing both the buffer tostore the source address and the buffers for the incoming message.The length and format of the address depend on the address familyof the socket.  The <B>msg_flags</B> member is ignored on input, butmay contain meaningful values on output.<dt><I>flags</I><dd>Specifies the type of message reception.  Values of this argument are formedby logically OR'ing zero or more of the following values:<dl compact><dt>MSG_OOB<dd>Requests out-of-band data.The significance and semantics of out-of-band data are protocol-specific.<dt>MSG_PEEK<dd>Peeks at the incoming message.<dt>MSG_WAITALL<dd>Requests that the function block until the full amount of data requested canbe returned.  The function may return a smaller amount of data if a signal iscaught, if the connection is terminated,if MSG_PEEK was specified,or if an error is pending for the socket.</dl><p></dl><p>The<i>recvmsg()</i>function receives messages from unconnected or connected sockets and returnsthe length of the message.<p>The<i>recvmsg()</i>function returns the total length of the message.  For message-based socketssuch as SOCK_DGRAM and SOCK_SEQPACKET, the entire message must be read in asingle operation.  If a message is too long to fit in the supplied buffers,and MSG_PEEK is not set in the <I>flags</I> argument, the excess bytes arediscarded, and MSG_TRUNC is set in the <B>msg_flags</B> member of the<B>msghdr</B> structure.  For stream-based sockets such as SOCK_STREAM, messageboundaries are ignored.  In this case, data is returned to the user as soon asit becomes available, and no data is discarded.<p>If the MSG_WAITALL flag is not set, data will be returnedonly up to the end of the first message.<p>If no messages are available at the socket and O_NONBLOCK isnot set on the socket's file descriptor,<i><a href="recvfrom.html">recvfrom()</a></i>blocks until a message arrives.If no messages are available at the socket and O_NONBLOCK isset on the socket's file descriptor,<i><a href="recvfrom.html">recvfrom()</a></i>function fails and sets <I>errno</I> to [EAGAIN] or [EWOULDBLOCK].<p>In the <B>msghdr</B> structure, the <B>msg_name</B> and <B>msg_namelen</B>members specify the source address if the socket is unconnected.If the socket is connected, the <B>msg_name</B> and <B>msg_namelen</B> membersare ignored.  The <B>msg_name</B> member may be a null pointer if no names aredesired or required.  The <I>msg_iov</I>and <I>msg_iovlen</I>fields are used to specifywhere the received data will be stored. <I>msg_iov</I>pointsto an array of <B>iovec</B>structures; <I>msg_iovlen</I>must be setto the dimension of this array. In each <B>iovec</B>structure,the <I>iov_base</I>field specifies a storage area and the<I>iov_len</I>field gives its size in bytes. Each storage areaindicated by <I>msg_iov</I>is filled with received data in turn until all of the received data is stored or all of the areas have been filled.<p>On successful completion, the <B>msg_flags</B> member of the messageheader is the bitwise-inclusive OR of all of the following flags that indicateconditions detected for the received message:.<dl compact><dt>MSG_EOR<dd>End of record was received (if supported by the protocol).<dt>MSG_OOB<dd>Out-of-band data was received.<dt>MSG_TRUNC<dd>Normal data was truncated.<dt>MSG_CTRUNC<dd>Control data was truncated.</dl></blockquote><h4>RETURN VALUE</h4><blockquote>Upon successful completion,<i>recvmsg()</i>returns the length of the message in bytes.  If no messages are available tobe received and the peer has performed an orderly shutdown,<i>recvmsg()</i>returns 0.Otherwise, -1 is returned and <I>errno</I> is set to indicate theerror.</blockquote><h4>ERRORS</h4><blockquote>The<i>recvmsg()</i>function will fail if:<dl compact><dt>[EAGAIN] or [EWOULDBLOCK]<dd><br>The socket's file descriptor is marked O_NONBLOCK and no data is waiting tobe received; or MSG_OOB is set and no out-of-band data is available and eitherthe socket's file descriptor is marked O_NONBLOCK or the socket does notsupport blocking to await out-of-band data.<dt>[EBADF]<dd><index term="EBADF, in "></index>The <I>socket</I> argument is not a valid open file descriptor.<dt>[ECONNRESET]<dd>A connection was forcibly closed by a peer.<dt>[EFAULT]<dd>The <I>message</I>parameter,or storage pointed to by the <I>msg_name</I>,<I>msg_control</I>or <I>msg_iov</I>fields of the <I>message</I>parameter, or storage pointed to by the<B>iovec</B>structures pointed to by the <I>msg_iov</I>field can not be accessed or written.<dt>[EINTR]<dd>This function was interrupted by a signal before any data was available.<dt>[EINVAL]<dd>The sum of the <B>iov_len</B> values overflows an <B>ssize_t</B>.or the MSG_OOB flag is set and no out-of-band data is available.<dt>[EMSGSIZE]<dd>The<B>msg_iovlen</B>member of the<B>msghdr</B>structure pointed to by<I>message</I>is less than or equal to 0, or is greater than {IOV_MAX}.<dt>[ENOTCONN]<dd>A receive is attempted on a connection-mode socket that is not connected.<dt>[ENOTSOCK]<dd>The <I>socket</I> argument does not refer to a socket.<dt>[EOPNOTSUPP]<dd>The specified flags are not supported for this socket type.<dt>[ETIMEDOUT]<dd>The connection timed out during connection establishment, or due to atransmission timeout on active connection.</dl><p>The<i>recvmsg()</i>function may fail if:<dl compact><dt>[EIO]<dd>An I error occurred while reading from or writing to the file system.<dt>[ENOBUFS]<dd>Insufficient resources were available in the system to perform the operation.<dt>[ENOMEM]<dd>Insufficient memory was available to fulfill the request.<dt>[ENOSR]<dd>There were insufficient STREAMS resources available for the operation tocomplete.</dl></blockquote><h4>APPLICATION USAGE</h4><blockquote>The<i><a href="select.html">select()</a></i>and<i><a href="poll.html">poll()</a></i>functions can be used to determine when data is available to be received.</blockquote><h4>SEE ALSO</h4><blockquote><i><a href="poll.html">poll()</a></i>,<i><a href="recv.html">recv()</a></i>,<i><a href="recvfrom.html">recvfrom()</a></i>,<i><a href="select.html">select()</a></i>,<i><a href="send.html">send()</a></i>,<i><a href="sendmsg.html">sendmsg()</a></i>,<i><a href="sendto.html">sendto()</a></i>,<i><a href="shutdown.html">shutdown()</a></i>,<i><a href="socket.html">socket()</a></i>,<i><a href="syssocket.h.html">&lt;sys/socket.h&gt;</a></i>.</blockquote><hr size=2 noshade><center><font size=2>UNIX &reg; is a registered Trademark of The Open Group.<br>Copyright &copy; 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>
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