subject_38672.htm

一些关于vc的问答

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序号：38672 发表者：xiongli 发表日期：2003-05-05 11:55:55
<br>主题：我的WSARecvFrom怎么老是错阿!
<br>内容：int WSARecvFrom(<BR>&nbsp;&nbsp;SOCKET s,<BR>&nbsp;&nbsp;LPWSABUF lpBuffers,<BR>&nbsp;&nbsp;DWORD dwBufferCount,<BR>&nbsp;&nbsp;LPDWORD lpNumberOfBytesRecvd,<BR>&nbsp;&nbsp;LPDWORD lpFlags,<BR>&nbsp;&nbsp;struct sockaddr FAR *lpFrom,<BR>&nbsp;&nbsp;LPINT lpFromlen,<BR>&nbsp;&nbsp;LPWSAOVERLAPPED lpOverlapped,<BR>&nbsp;&nbsp;LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine<BR>);<BR><BR><BR>recvfrom可以一次接收一个数据包,如果队列里面没有数据包,他就会挂起<BR><BR>WSARecvFrom可以把数据包放到一个WSABUF数组里面.但是我有几点不明白的地方:<BR>1,书上上面说WSARecvFrom可以实现scatter-gather I/O,我想也就是调用一次就把尽量把对列里面的数据包放到WSABUF数组里面.既然是放到WSABUF数组里面,那么是一个WSABUF数组的原素对应一个数据包呢还是连贯地存储到WSABUF数组里面?<BR><BR>2,如果调用一次就把尽量把对列里面的数据包放到WSABUF数组里面,当不同的client给我发过来数据包,那么一个lpFrom如何能够辨别它们呢?<BR><BR>所以我猜想应该是没调用一次WSARecvFrom就把某一个client发过来的数据包尽可能地填充满lpBuffers,而且是一个数据包对应一个lpBuffers的原素.<BR>如果有多个client发数据包过来,那么就需要调用WSARecvFrom多次.<BR><BR>然后我想写程序来证实这个东西,我这样调用:<BR><BR><BR>WSADATA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;wsd;<BR>&nbsp;&nbsp;&nbsp;&nbsp;SOCKET&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; s;<BR>&nbsp;&nbsp;&nbsp;&nbsp;char&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*recvbuf = NULL;<BR>&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ret,<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; i;<BR>&nbsp;&nbsp;&nbsp;&nbsp;DWORD&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dwSenderSize,count;<BR>&nbsp;&nbsp;&nbsp;&nbsp;SOCKADDR_IN&nbsp;&nbsp;&nbsp;&nbsp;sender,<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; local;<BR>&nbsp;&nbsp;&nbsp;&nbsp;WSABUF&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; mybuf[1];<BR>&nbsp;&nbsp;&nbsp;&nbsp;char&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; str[100];<BR>&nbsp;&nbsp;&nbsp;&nbsp;mybuf[0].buf=str;<BR>&nbsp;&nbsp;&nbsp;&nbsp;mybuf[0].len=100;<BR><BR>//....<BR>//初始化socket,bind完成后<BR>ret = WSARecvFrom(s,mybuf, 1,&amp;count,0,(SOCKADDR *)&amp;sender, &amp;dwSenderSize,0,0);<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; if (ret == SOCKET_ERROR)<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;printf("recvfrom() failed; %d\n", WSAGetLastError());<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;break;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR><BR>结果 WSARecvFrom老是出错,错误是WSAEFAULT<BR>msdn的解释是:<BR>The lpBuffers, lpFlags, lpFrom, lpNumberOfBytesRecvd, lpFromlen, lpOverlapped, or lpCompletionRoutine parameter is not totally contained in a valid part of the user address space: the lpFrom buffer was too small to accommodate the peer address.<BR><BR>检查来检查去都不知道为什么<BR>:(
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<font color=red>答案被接受</font><br>回复者：Neil Gan 回复日期：2003-05-05 12:28:55
<br>内容：I never used WSARecvFrom. I only use recvfrom.<BR><BR>1.lpFrom is a buffer, so it can contain multiple addresses.<BR>2.Init dwSenderSize: dwSenderSize=1
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回复者：xiongli 回复日期：2003-05-05 12:35:03
<br>内容：不对阿<BR>我用dwSenderSize=1还是不正确(原来是dwSenderSize = sizeof(sender);)<BR><BR>如果用<BR>dwSenderSize = sizeof(sender);<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ret = recvfrom(s, recvbuf, dwLength, 0, <BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(SOCKADDR *)&amp;sender, &amp;dwSenderSize);<BR>就是正确的<BR><BR>这是msdn对recvfrom里面的fromlen的解释,他应该是sizeof(sender)<BR>fromlen <BR>[in, out] Optional pointer to the size of the from buffer. <BR><BR>这是msdn对WSARecvFrom的解释,看得出两个是一样的!<BR>lpFromlen <BR>[in, out] Pointer to the size of the from buffer, required only if lpFrom is specified. <BR>
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回复者：Neil Gan 回复日期：2003-05-05 12:35:26
<br>内容：You are very quick. Have you trid my answer? I am not sure of it.<BR><BR>And, because lpFrom is a buffer, I guess that every package will be put in a recv buf, and its sender address will be put in lpFrom. Please tell me whether it is correct.<BR>
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回复者：xiongli 回复日期：2003-05-05 12:35:38
<br>内容：我现在是一看到WSA开头的就头痛.......
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