📄 select.htm
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<html><head><meta http-equiv="Content-Language" content="en-us"><meta http-equiv="Content-Type" content="text/html; charset=windows-1252"><title>UDT Reference</title></head><body><table border="1" cellpadding="0" cellspacing="0" style="border-collapse: collapse" width="100%" id="table2" bgcolor="#99CCFF" bordercolor="#99CCFF"> <tr> <td width="100%"><font face="Verdana" size="2"><i> UDT Reference: UDT Functions</i></font></td> </tr></table><h1 style="margin-bottom: 0"><font face="Verdana" size="4">select</font></h1><p><font face="Verdana" size="2">The <b>select</b> method queries the status of a group of UDT sockets.</font></p><table border="1" cellpadding="0" cellspacing="0" style="border-collapse: collapse" width="100%" id="table3" bgcolor="#C0C0C0" bordercolor="#C0C0C0"> <tr> <td width="100%"> <p style="margin-top: 0; margin-bottom: 0"> <font face="Courier New" size="1">int select(<br> int <font color="#FFFFFF"> <a class="synParam" onclick="showTip(this)">nfds</a></font>,<br> UDSET *<font color="#FFFFFF"><a class="synParam" onclick="showTip(this)">readfds</a></font>,</font></p> <p style="margin-top: 0; margin-bottom: 0"> <font face="Courier New" size="1"> UDSET *<font color="#FFFFFF"><a class="synParam" onclick="showTip(this)">writefds</a></font>,<br> UDSET *<font color="#FFFFFF"><a class="synParam" onclick="showTip(this)">exceptfds</a></font>,</font></p> <p style="margin-top: 0; margin-bottom: 0"> <font face="Courier New" size="1"> const struct timeval *<font color="#FFFFFF"><a class="synParam" onclick="showTip(this)">timeout</a> </font> <br> );</font></p> </td> </tr></table><h4><font face="Verdana" size="2">Parameters</font></h4><dl> <dt><font face="Verdana"><i><font size="2">nfds</font></i><font size="2"> </font></font> </dt> <dd><font face="Verdana" size="2">[in] Ignored. For compatibility purpose only. </font></dd> <dt><font face="Verdana"><i><font size="2">readfds</font></i><font size="2"> </font></font> </dt> <dd><font face="Verdana" size="2">[in, out] Optional pointer to a set of sockets to be checked for readability. </font></dd> <dt><font face="Verdana"><i><font size="2">writefds</font></i><font size="2"> </font></font> </dt> <dd><font face="Verdana" size="2">[in, out] Optional pointer to a set of sockets to be checked for writability </font></dd> <dt><font face="Verdana"><i><font size="2">exceptfds</font></i><font size="2"> </font></font> </dt> <dd><font face="Verdana" size="2">[in, out] Ignored. For compatibility purpose only. </font></dd> <dt><font face="Verdana"><i><font size="2">timeout</font></i><font size="2"> </font></font> </dt> <dd><font face="Verdana" size="2">[in] Maximum time for select to wait, provided in the form of a timeval structure. Set the timeout parameter to NULL for blocking operation. </font> </dd></dl><h4><font face="Verdana" size="2">Return Values</font></h4><p><font face="Verdana" size="2">If any of the read/write/exception queries is positive, select returns the number of UDT sockets that are ready; If no socket is ready before timeout, 0 is returned; If there is any error, UDT::ERROR is returned and the specific error information can be retrieved by<a href="error.htm">getlasterror</a>.</font></p> <table border="1" cellpadding="0" cellspacing="0" style="border-collapse: collapse" id="table4" bordercolordark="#C0C0C0" bordercolorlight="#C0C0C0" width="100%"> <tr> <th align="center" bgcolor="#C0C0C0" width="133"> <font face="Verdana" size="2">Error Code</font></th> <th align="center" bgcolor="#C0C0C0" width="812"><b> <font face="Verdana" size="2">Comment</font></b></th> </tr> <tr> <td width="133"> <p style="margin-top: 1; margin-bottom: 1"><font face="Verdana" size="1"> 5003</font></td> <td width="812"> <p style="margin-top: 1; margin-bottom: 1"><font face="Verdana" size="1"> all the three UDT socket descriptor sets are NULL, or at least one of them contains an invalid socket descriptor.</font></td> </tr> </table><H4><font face="Verdana" size="2">Description</font></H4><p><font size="2" face="Verdana">The UDSET is a structure to store the UDT socket descriptors. It should be processed with the following macros.</font></p><dl> <dt><font face="Verdana"><font size="2"><b>U</b></font><b><font size="2">D_CLR(</font></b><i><font size="2">u</font></i><font size="2"><b>, *</b><i>set</i><b>)</b></font></font></dt> <dd><font face="Verdana" size="2">Removes the descriptor s from set.</font></dd> <dt><font face="Verdana"><font size="2"><b>U</b></font><b><font size="2">D_ISSET(</font></b><font size="2"><i>u</i><b>, *</b><i>set</i><b>)</b></font></font></dt> <dd><font face="Verdana" size="2">Nonzero if s is a member of the set. Otherwise, zero.</font></dd> <dt><font face="Verdana"><font size="2"><b>U</b></font><b><font size="2">D_SET(</font></b><i><font size="2">u</font></i><font size="2"><b>, *</b><i>set</i><b>)</b></font></font></dt> <dd><font face="Verdana" size="2">Adds descriptor s to set.</font></dd> <dt><font face="Verdana"><font size="2"><b>U</b></font><b><font size="2">D_ZERO(*</font></b><font size="2"><i>set</i><b>)</b></font></font></dt> <dd><font face="Verdana" size="2">Initializes the set to the NULL set.</font></dd></dl><p><font size="2" face="Verdana">The UDT descriptors sets originally contain the sockets whose status are to be queried. When select returns successfully (returned value is greater than 0), the descriptors sets only contain the sockets that are ready. UD_ISSET is used to query which one is ready.</font></p><p><font face="Verdana" size="2"><i>readfds </i>detects if any socket in this sets is available for reading (<a href="recv.htm">recv</a>), for accepting a new connection (<a href="accept.htm">accept</a>), or the connection is broken. <i>writefds</i> detects if any socket in this sets has available buffer for sending (<a href="send.htm">send</a>). <i>exceptfds</i> is currently not used.</font></p><H4><font face="Verdana" size="2">Example</font></H4><p><font face="Verdana" size="2">The following codes show how to send a whole file to the remote node using UDT:</font></p><table border="1" cellpadding="0" cellspacing="0" style="border-collapse: collapse" width="100%" id="table5" bgcolor="#C0C0C0" bordercolor="#C0C0C0"> <tr> <td width="100%"> <p style="margin-top: 0; margin-bottom: 0"> <font face="Courier New" size="1">UDTSOCKET u;</font></p> <p style="margin-top: 0; margin-bottom: 0"> <font face="Courier New" size="1">...</font></p> <p style="margin-top: 0; margin-bottom: 0"> </p> <p style="margin-top: 0; margin-bottom: 0"> <font face="Courier New" size="1">timeval tv;<br> UDSET readfds;<br> <br> tv.tv_sec = 1;<br> tv.tv_usec = 0;<br> <br> UD_ZERO(&readfds);<br> UD_SET(u, &readfds);<br> <br> int res = select(0, &readfds, NULL, NULL, &tv);<br> <br> if ((res != UDT_ERROR) && (UD_ISSET(u, &readfds)))<br> // read data from u.<br> else<br> // timeout or error</font></td> </tr></table><H4><font face="Verdana" size="2">See Also</font></H4><P><font face="Verdana" size="2"><b><a href="send.htm">send</a>, <a href="recv.htm">recv</a></b></font></P></body></html>⌨️ 快捷键说明
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