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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2004 IEEE and The Open Group, All Rights Reserved --><title>recvfrom</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="recvfrom"></a> <a name="tag_03_601"></a><!-- recvfrom --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2004 Edition<br>Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_601_01"></a>NAME</h4><blockquote>recvfrom - receive a message from a socket</blockquote><h4><a name="tag_03_601_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><p><code><tt>#include <<a href="../basedefs/sys/socket.h.html">sys/socket.h</a>><br><br> ssize_t recvfrom(int</tt> <i>socket</i><tt>, void *restrict</tt> <i>buffer</i><tt>, size_t</tt> <i>length</i><tt>,<br> int</tt> <i>flags</i><tt>, struct sockaddr *restrict</tt> <i>address</i><tt>,<br> socklen_t *restrict</tt> <i>address_len</i><tt>);<br></tt></code></p></blockquote><h4><a name="tag_03_601_03"></a>DESCRIPTION</h4><blockquote><p>The <i>recvfrom</i>() function shall receive a message from a connection-mode or connectionless-mode socket. It is normally usedwith connectionless-mode sockets because it permits the application to retrieve the source address of received data.</p><p>The <i>recvfrom</i>() function takes the following arguments:</p><dl compact><dt><i>socket</i></dt><dd>Specifies the socket file descriptor.</dd><dt><i>buffer</i></dt><dd>Points to the buffer where the message should be stored.</dd><dt><i>length</i></dt><dd>Specifies the length in bytes of the buffer pointed to by the <i>buffer</i> argument.</dd><dt><i>flags</i></dt><dd>Specifies the type of message reception. Values of this argument are formed by logically OR'ing zero or more of the followingvalues: <dl compact><dt>MSG_PEEK</dt><dd>Peeks at an incoming message. The data is treated as unread and the next <i>recvfrom</i>() or similar function shall stillreturn this data.</dd><dt>MSG_OOB</dt><dd>Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific.</dd><dt>MSG_WAITALL</dt><dd>On SOCK_STREAM sockets this requests that the function block until the full amount of data can be returned. The function mayreturn the smaller amount of data if the socket is a message-based socket, if a signal is caught, if the connection is terminated,if MSG_PEEK was specified, or if an error is pending for the socket.</dd></dl></dd><dt><i>address</i></dt><dd>A null pointer, or points to a <b>sockaddr</b> structure in which the sending address is to be stored. The length and format ofthe address depend on the address family of the socket.</dd><dt><i>address_len</i></dt><dd>Specifies the length of the <b>sockaddr</b> structure pointed to by the <i>address</i> argument.</dd></dl><p>The <i>recvfrom</i>() function shall return the length of the message written to the buffer pointed to by the <i>buffer</i>argument. For message-based sockets, such as <sup>[<a href="javascript:open_code('RS')">RS</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> SOCK_RAW, <img src="../images/opt-end.gif" alt="[Option End]"border="0"> SOCK_DGRAM, and SOCK_SEQPACKET, the entire message shall be read in a single operation. If a message is too long to fitin the supplied buffer, and MSG_PEEK is not set in the <i>flags</i> argument, the excess bytes shall be discarded. For stream-basedsockets, such as SOCK_STREAM, message boundaries shall be ignored. In this case, data shall be returned to the user as soon as itbecomes available, and no data shall be discarded.</p><p>If the MSG_WAITALL flag is not set, data shall be returned only up to the end of the first message.</p><p>Not all protocols provide the source address for messages. If the <i>address</i> argument is not a null pointer and the protocolprovides the source address of messages, the source address of the received message shall be stored in the <b>sockaddr</b>structure pointed to by the <i>address</i> argument, and the length of this address shall be stored in the object pointed to by the<i>address_len</i> argument.</p><p>If the actual length of the address is greater than the length of the supplied <b>sockaddr</b> structure, the stored addressshall be truncated.</p><p>If the <i>address</i> argument is not a null pointer and the protocol does not provide the source address of messages, the valuestored in the object pointed to by <i>address</i> is unspecified.</p><p>If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, <i>recvfrom</i>() shallblock until a message arrives. If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor,<i>recvfrom</i>() shall fail and set <i>errno</i> to [EAGAIN] or [EWOULDBLOCK].</p></blockquote><h4><a name="tag_03_601_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, <i>recvfrom</i>() shall return the length of the message in bytes. If no messages are available tobe received and the peer has performed an orderly shutdown, <i>recvfrom</i>() shall return 0. Otherwise, the function shall return-1 and set <i>errno</i> to indicate the error.</p></blockquote><h4><a name="tag_03_601_05"></a>ERRORS</h4><blockquote><p>The <i>recvfrom</i>() function shall fail if:</p><dl compact><dt>[EAGAIN] or [EWOULDBLOCK]</dt><dd>The socket's file descriptor is marked O_NONBLOCK and no data is waiting to be received; or MSG_OOB is set and no out-of-band datais available and either the socket's file descriptor is marked O_NONBLOCK or the socket does not support blocking to awaitout-of-band data.</dd><dt>[EBADF]</dt><dd>The <i>socket</i> argument is not a valid file descriptor.</dd><dt>[ECONNRESET]</dt><dd>A connection was forcibly closed by a peer.</dd><dt>[EINTR]</dt><dd>A signal interrupted <i>recvfrom</i>() before any data was available.</dd><dt>[EINVAL]</dt><dd>The MSG_OOB flag is set and no out-of-band data is available.</dd><dt>[ENOTCONN]</dt><dd>A receive is attempted on a connection-mode socket that is not connected.</dd><dt>[ENOTSOCK]</dt><dd>The <i>socket</i> argument does not refer to a socket.</dd><dt>[EOPNOTSUPP]</dt><dd>The specified flags are not supported for this socket type.</dd><dt>[ETIMEDOUT]</dt><dd>The connection timed out during connection establishment, or due to a transmission timeout on active connection.</dd></dl><p>The <i>recvfrom</i>() function may fail if:</p><dl compact><dt>[EIO]</dt><dd>An I/O error occurred while reading from or writing to the file system.</dd><dt>[ENOBUFS]</dt><dd>Insufficient resources were available in the system to perform the operation.</dd><dt>[ENOMEM]</dt><dd>Insufficient memory was available to fulfill the request.</dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_601_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_601_07"></a>APPLICATION USAGE</h4><blockquote><p>The <a href="../functions/select.html"><i>select</i>()</a> and <a href="../functions/poll.html"><i>poll</i>()</a> functions canbe used to determine when data is available to be received.</p></blockquote><h4><a name="tag_03_601_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_601_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_601_10"></a>SEE ALSO</h4><blockquote><p><a href="poll.html"><i>poll</i>()</a>, <a href="read.html"><i>read</i>()</a>, <a href="recv.html"><i>recv</i>()</a>, <a href="recvmsg.html"><i>recvmsg</i>()</a>, <a href="select.html"><i>select</i>()</a>, <a href="send.html"><i>send</i>()</a>, <a href="sendmsg.html"><i>sendmsg</i>()</a>, <a href="sendto.html"><i>sendto</i>()</a>, <a href="shutdown.html"><i>shutdown</i>()</a>,<a href="socket.html"><i>socket</i>()</a>, <a href="write.html"><i>write</i>()</a>, the Base Definitions volume ofIEEE Std 1003.1-2001, <a href="../basedefs/sys/socket.h.html"><i><sys/socket.h></i></a></p></blockquote><h4><a name="tag_03_601_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 6. Derived from the XNS, Issue 5.2 specification.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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