📄 recvfrom.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>recvfrom</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>recvfrom - 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 recvfrom(int <I>socket</I>, void *<I>buffer</I>, size_t <I>length</I>, int <I>flags</I>,             struct sockaddr *<I>address</I>, socklen_t *<I>address_len</I>);</code></pre></blockquote><h4>DESCRIPTION</h4><blockquote>The<i>recvfrom()</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>buffer</I><dd>Points to the buffer where the message should be stored.<dt><I>length</I><dd>Specifies the length in bytes of the buffer pointed to by the<I>buffer</I> argument.<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_PEEK<dd>Peeks at an incoming message.  The data is treated as unread and the next<i>recvfrom()</i>or similar function will still return this data.<dt>MSG_OOB<dd>Requests out-of-band data.The significance and semantics of out-of-band data are protocol-specific.<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><dt><I>address</I><dd>A null pointer, or points to a <B>sockaddr</B> structure in which the sendingaddress is to be stored.  The length and format of the address depend on theaddress family of the socket.<p><dt><I>address_len</I><dd>Specifies the length of the <B>sockaddr</B> structure pointed to by the<I>address</I> argument.<p></dl><p>The<i>recvfrom()</i>function returns the length of the message written to the buffer pointed to bythe <I>buffer</I> argument.  For message-based sockets such as SOCK_DGRAM andSOCK_SEQPACKET, the entire message must be read in a single operation.  If amessage is too long to fit in the supplied buffer, and MSG_PEEK is not set inthe <I>flags</I> argument, the excess bytes are discarded.  For stream-basedsockets such as SOCK_STREAM, message boundaries are ignored.  In this case,data is returned to the user as soon as it becomes available, and no data isdiscarded.<p>If the MSG_WAITALL flag is not set, data will be returnedonly up to the end of the first message.<p>Not all protocols provide the source address for messages.  If the<I>address</I> argument is not a null pointer and the protocol provides thesource address of messages, the source address of the received message isstored in the <B>sockaddr</B> structure pointed to by the <I>address</I>argument, and the length of this address is stored in the object pointed to bythe <I>address_len</I> argument.<p>If the actual length of the address is greater than the length of thesupplied <B>sockaddr</B> structure, the stored address will betruncated.<p>If the <I>address</I> argument is not a null pointer and the protocol does notprovide the source address of messages, the the value stored in the objectpointed to by <I>address</I> is unspecified.<p>If no messages are available at the socket and O_NONBLOCK isnot set on the socket's file descriptor,<i>recvfrom()</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>recvfrom()</i>fails and sets <I>errno</I> to [EAGAIN] or [EWOULDBLOCK].</blockquote><h4>RETURN VALUE</h4><blockquote>Upon successful completion,<i>recvfrom()</i>returns the length of the message in bytes.  If no messages are availableto be received and the peer has performed an orderly shutdown,<i>recvfrom()</i>returns 0.  Otherwise the function returns -1 and sets<I>errno</I> to indicate the error.</blockquote><h4>ERRORS</h4><blockquote>The<i>recvfrom()</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>The <I>socket</I> argument is not a valid file descriptor.<dt>[ECONNRESET]<dd>A connection was forcibly closed by a peer.<dt>[EFAULT]<dd>The <I>buffer</I>,<I>address</I>or <I>address_len</I>parameter can not be accessed or written.<dt>[EINTR]<dd>A signal interrupted<i>recvfrom()</i>before any data was available.<dt>[EINVAL]<dd>The MSG_OOB flag is set and no out-of-band data is available.<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 dueto a transmission timeout on active connection.</dl><p>The<i>recvfrom()</i>function may fail if:<dl compact><dt>[EIO]<dd>An I/O 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.<br></blockquote><h4>SEE ALSO</h4><blockquote><i><a href="poll.html">poll()</a></i>,<i><a href="read.html">read()</a></i>,<i><a href="recv.html">recv()</a></i>,<i><a href="recvmsg.html">recvmsg()</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="write.html">write()</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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