Linux下connect超时处理（总结）

利钊發表於 2023-10-24 00:00:00

<p>
<strong>1、前言</strong></p>
<p>
最近在写一个测试工具，要求快速的高效率的扫描出各个服务器开放了哪些端口。当时想了一下，ping只能检测ip，判断服务器的网络是连通的，而不能判断是否开放了端口。我们知道端口属于网络的传输层，因此需要用ip和端口来探测，这个时候就可以用connect来探测一下，针对TCP协议，connect函数要进行TCP三次握手，如果connect成功，则说明服务器开放了某个端口，如果connect失败，则说明服务器没有开放某个端口。而connect失败是通过超时来控制的，在规定的时间内，connect会发起多次连接，一直执行到超时，才返回错误。默认情况下，connect是阻塞的，而且默认的超时时间为75s，正常情况下，检测网络的连通性都是毫秒级，如果要判断10万台服务器的，用阻塞的默认的connect去做，效率非常低下。因此采用非阻塞的connect，而且需要自定义超时间（我自定义超时时间为5s）。</p>
<p>
<strong>2、非阻塞connect</strong></p>
<p>
对于阻塞式套接字，调用connect函数将激发TCP的三次握手过程，而且仅在连接建立成功或者出错时才返回；对于非阻塞式套接字，如果调用connect函数会之间返回-1（表示出错），且错误为EINPROGRESS，表示连接建立，建立启动但是尚未完成；如果返回0，则表示连接已经建立，这通常是在服务器和客户在同一台主机上时发生。</p>
<p>
select是一种IO多路复用机制，它允许进程指示内核等待多个事件的任何一个发生，并且在有一个或者多个事件发生或者经历一段指定的时间后才唤醒它。connect本身并不具有设置超时功能，如果想对套接字的IO操作设置超时，可使用select函数。</p>
<p>
对于select和非阻塞connect，注意两点：当连接成功建立时，描述符变成可写；当连接建立遇到错误时，描述符变为即可读，也可写，遇到这种情况，可调用getsockopt函数。</p>
<p>
<strong>3、实现步骤</strong></p>
<p>
（1）创建socket，并利用fcntl将其设置为非阻塞</p>
<p>
（2）调用connect函数，如果返回0，则连接建立；如果返回-1，检查errno ，如果值为 EINPROGRESS，则连接正在建立。</p>
<p>
（3）为了控制连接建立时间，将该socket描述符加入到select的可读可写集合中，采用select函数设定超时。</p>
<p>
（4）如果规定时间内成功建立，则描述符变为可写；否则，采用getsockopt函数捕获错误信息</p>
<p>
（5）恢复套接字的文件状态并返回。</p>
<p>
测试代码如下所示：</p>
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<code class="cpp preprocessor">#include <stdio.h></code>
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<code class="cpp preprocessor">#include <stdlib.h></code>
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<code class="cpp preprocessor">#include <string.h></code>
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<code class="cpp preprocessor">#include <unistd.h></code>
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<div class="line number5 index4 alt2">
<code class="cpp preprocessor">#include <sys/types.h>     /* See NOTES */</code>
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<div class="line number6 index5 alt1">
<code class="cpp preprocessor">#include <sys/socket.h></code>
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<code class="cpp preprocessor">#include <netinet/in.h></code>
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<code class="cpp preprocessor">#include <fcntl.h></code>
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<code class="cpp preprocessor">#include <errno.h></code>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">main(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">argc, </code><code class="cpp color1 bold">char</code> <code class="cpp plain">**argv)</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces">  </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(argc < 3) {</code>
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<code class="cpp spaces">    </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"please input ip and port, for example ./main 120.12.34.56 80.\n"</code><code class="cpp plain">);</code>
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<code class="cpp spaces">    </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
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<code class="cpp spaces">  </code><code class="cpp plain">}</code>
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<code class="cpp spaces">  </code>
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<code class="cpp spaces">  </code><code class="cpp color1 bold">char</code> <code class="cpp plain">*ipaddr = argv;</code>
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<code class="cpp spaces">  </code><code class="cpp plain">unsigned </code><code class="cpp color1 bold">int</code> <code class="cpp plain">port = </code><code class="cpp functions bold">atoi</code><code class="cpp plain">(argv);</code>
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<code class="cpp spaces">  </code><code class="cpp color1 bold">int</code> <code class="cpp plain">fd = 0;</code>
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<code class="cpp spaces">  </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sockaddr_in addr;</code>
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<code class="cpp spaces">  </code><code class="cpp plain">fd_set fdr, fdw;</code>
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<code class="cpp spaces">  </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">timeval timeout;</code>
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<code class="cpp spaces">  </code><code class="cpp color1 bold">int</code> <code class="cpp plain">err = 0;</code>
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<code class="cpp spaces">  </code><code class="cpp color1 bold">int</code> <code class="cpp plain">errlen = </code><code class="cpp keyword bold">sizeof</code><code class="cpp plain">(err);</code>
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<code class="cpp spaces">  </code><code class="cpp plain">fd = socket(AF_INET,SOCK_STREAM,0);</code>
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<code class="cpp spaces">  </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(fd < 0) {</code>
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<code class="cpp spaces">    </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"create socket failed,error:%s.\n"</code><code class="cpp plain">, </code><code class="cpp functions bold">strerror</code><code class="cpp plain">(</code><code class="cpp functions bold">errno</code><code class="cpp plain">));</code>
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<code class="cpp spaces">    </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
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<code class="cpp spaces">  </code><code class="cpp plain">}</code>
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<code class="cpp spaces">  </code><code class="cpp plain">bzero(&addr, </code><code class="cpp keyword bold">sizeof</code><code class="cpp plain">(addr));</code>
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<code class="cpp spaces">  </code><code class="cpp plain">addr.sin_family = AF_INET;</code>
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<code class="cpp spaces">  </code><code class="cpp plain">addr.sin_port = htons(port);</code>
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<code class="cpp spaces">  </code><code class="cpp plain">inet_pton(AF_INET, ipaddr, &addr.sin_addr);</code>
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<code class="cpp spaces">  </code><code class="cpp comments">/*设置套接字为非阻塞*/</code>
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<code class="cpp spaces">  </code><code class="cpp color1 bold">int</code> <code class="cpp plain">flags = fcntl(fd, F_GETFL, 0);</code>
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<code class="cpp spaces">  </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(flags < 0) {</code>
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<code class="cpp spaces">    </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"Get flags error:%s\n"</code><code class="cpp plain">, </code><code class="cpp functions bold">strerror</code><code class="cpp plain">(</code><code class="cpp functions bold">errno</code><code class="cpp plain">));</code>
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<code class="cpp spaces">    </code><code class="cpp plain">close(fd);</code>
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<code class="cpp spaces">    </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
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<code class="cpp spaces">  </code><code class="cpp plain">}</code>
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<code class="cpp spaces">  </code><code class="cpp plain">flags |= O_NONBLOCK;</code>
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<code class="cpp spaces">  </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(fcntl(fd, F_SETFL, flags) < 0) {</code>
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<div class="line number49 index48 alt2">
<code class="cpp spaces">    </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"Set flags error:%s\n"</code><code class="cpp plain">, </code><code class="cpp functions bold">strerror</code><code class="cpp plain">(</code><code class="cpp functions bold">errno</code><code class="cpp plain">));</code>
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<code class="cpp spaces">    </code><code class="cpp plain">close(fd);</code>
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<code class="cpp spaces">    </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
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<div class="line number52 index51 alt1">
<code class="cpp spaces">  </code><code class="cpp plain">}</code>
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<code class="cpp spaces">  </code><code class="cpp comments">/*阻塞情况下linux系统默认超时时间为75s*/</code>
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<code class="cpp spaces">  </code><code class="cpp color1 bold">int</code> <code class="cpp plain">rc = connect(fd, (</code><code class="cpp keyword bold">struct</code> <code class="cpp plain">sockaddr*)&addr, </code><code class="cpp keyword bold">sizeof</code><code class="cpp plain">(addr));</code>
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<code class="cpp spaces">  </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(rc != 0) {</code>
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<code class="cpp spaces">    </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(</code><code class="cpp functions bold">errno</code> <code class="cpp plain">== EINPROGRESS) {</code>
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<div class="line number58 index57 alt1">
<code class="cpp spaces">      </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"Doing connection.\n"</code><code class="cpp plain">);</code>
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<div class="line number59 index58 alt2">
<code class="cpp spaces">      </code><code class="cpp comments">/*正在处理连接*/</code>
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<div class="line number60 index59 alt1">
<code class="cpp spaces">      </code><code class="cpp plain">FD_ZERO(&fdr);</code>
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<div class="line number61 index60 alt2">
<code class="cpp spaces">      </code><code class="cpp plain">FD_ZERO(&fdw);</code>
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<div class="line number62 index61 alt1">
<code class="cpp spaces">      </code><code class="cpp plain">FD_SET(fd, &fdr);</code>
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<div class="line number63 index62 alt2">
<code class="cpp spaces">      </code><code class="cpp plain">FD_SET(fd, &fdw);</code>
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<div class="line number64 index63 alt1">
<code class="cpp spaces">      </code><code class="cpp plain">timeout.tv_sec = 10;</code>
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<div class="line number65 index64 alt2">
<code class="cpp spaces">      </code><code class="cpp plain">timeout.tv_usec = 0;</code>
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<div class="line number66 index65 alt1">
<code class="cpp spaces">      </code><code class="cpp plain">rc = select(fd + 1, &fdr, &fdw, NULL, &timeout);</code>
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<div class="line number67 index66 alt2">
<code class="cpp spaces">      </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"rc is: %d\n"</code><code class="cpp plain">, rc);</code>
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<code class="cpp spaces">      </code><code class="cpp comments">/*select调用失败*/</code>
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<div class="line number69 index68 alt2">
<code class="cpp spaces">      </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(rc < 0) {</code>
</div>
<div class="line number70 index69 alt1">
<code class="cpp spaces">        </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"connect error:%s\n"</code><code class="cpp plain">, </code><code class="cpp functions bold">strerror</code><code class="cpp plain">(</code><code class="cpp functions bold">errno</code><code class="cpp plain">));</code>
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<div class="line number71 index70 alt2">
<code class="cpp spaces">        </code><code class="cpp plain">close(fd);</code>
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<div class="line number72 index71 alt1">
<code class="cpp spaces">        </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
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<div class="line number73 index72 alt2">
<code class="cpp spaces">      </code><code class="cpp plain">}</code>
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<div class="line number74 index73 alt1">
<code class="cpp spaces">      </code>
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<div class="line number75 index74 alt2">
<code class="cpp spaces">      </code><code class="cpp comments">/*连接超时*/</code>
</div>
<div class="line number76 index75 alt1">
<code class="cpp spaces">      </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(rc == 0) {</code>
</div>
<div class="line number77 index76 alt2">
<code class="cpp spaces">        </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"Connect timeout.\n"</code><code class="cpp plain">);</code>
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<div class="line number78 index77 alt1">
<code class="cpp spaces">        </code><code class="cpp plain">close(fd);</code>
</div>
<div class="line number79 index78 alt2">
<code class="cpp spaces">        </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
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<div class="line number80 index79 alt1">
<code class="cpp spaces">      </code><code class="cpp plain">}</code>
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<div class="line number81 index80 alt2">
<code class="cpp spaces">      </code><code class="cpp comments">/* 当连接成功建立时，描述符变成可写,rc=1*/</code>
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<div class="line number82 index81 alt1">
<code class="cpp spaces">      </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(rc == 1 && FD_ISSET(fd, &fdw)) {</code>
</div>
<div class="line number83 index82 alt2">
<code class="cpp spaces">        </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"Connect success\n"</code><code class="cpp plain">);</code>
</div>
<div class="line number84 index83 alt1">
<code class="cpp spaces">        </code><code class="cpp plain">close(fd);</code>
</div>
<div class="line number85 index84 alt2">
<code class="cpp spaces">        </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0;</code>
</div>
<div class="line number86 index85 alt1">
<code class="cpp spaces">      </code><code class="cpp plain">}</code>
</div>
<div class="line number87 index86 alt2">
<code class="cpp spaces">      </code><code class="cpp comments">/* 当连接建立遇到错误时，描述符变为即可读，也可写，rc=2 遇到这种情况，可调用getsockopt函数*/</code>
</div>
<div class="line number88 index87 alt1">
<code class="cpp spaces">      </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(rc == 2) {</code>
</div>
<div class="line number89 index88 alt2">
<code class="cpp spaces">        </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &errlen) == -1) {</code>
</div>
<div class="line number90 index89 alt1">
<code class="cpp spaces">          </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"getsockopt(SO_ERROR): %s"</code><code class="cpp plain">, </code><code class="cpp functions bold">strerror</code><code class="cpp plain">(</code><code class="cpp functions bold">errno</code><code class="cpp plain">));</code>
</div>
<div class="line number91 index90 alt2">
<code class="cpp spaces">          </code><code class="cpp plain">close(fd);</code>
</div>
<div class="line number92 index91 alt1">
<code class="cpp spaces">          </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
</div>
<div class="line number93 index92 alt2">
</div>
<div class="line number94 index93 alt1">
<code class="cpp spaces">        </code><code class="cpp plain">}</code>
</div>
<div class="line number95 index94 alt2">
</div>
<div class="line number96 index95 alt1">
<code class="cpp spaces">        </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(err) {</code>
</div>
<div class="line number97 index96 alt2">
<code class="cpp spaces">          </code><code class="cpp functions bold">errno</code> <code class="cpp plain">= err;</code>
</div>
<div class="line number98 index97 alt1">
<code class="cpp spaces">          </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"connect error:%s\n"</code><code class="cpp plain">, </code><code class="cpp functions bold">strerror</code><code class="cpp plain">(</code><code class="cpp functions bold">errno</code><code class="cpp plain">));</code>
</div>
<div class="line number99 index98 alt2">
<code class="cpp spaces">          </code><code class="cpp plain">close(fd);</code>
</div>
<div class="line number100 index99 alt1">
<code class="cpp spaces">          </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
</div>
<div class="line number101 index100 alt2">
</div>
<div class="line number102 index101 alt1">
<code class="cpp spaces">        </code><code class="cpp plain">}</code>
</div>
<div class="line number103 index102 alt2">
<code class="cpp spaces">      </code><code class="cpp plain">}</code>
</div>
<div class="line number104 index103 alt1">
</div>
<div class="line number105 index104 alt2">
<code class="cpp spaces">    </code><code class="cpp plain">} </code>
</div>
<div class="line number106 index105 alt1">
<code class="cpp spaces">    </code><code class="cpp functions bold">fprintf</code><code class="cpp plain">(stderr, </code><code class="cpp string">"connect failed, error:%s.\n"</code><code class="cpp plain">, </code><code class="cpp functions bold">strerror</code><code class="cpp plain">(</code><code class="cpp functions bold">errno</code><code class="cpp plain">));</code>
</div>
<div class="line number107 index106 alt2">
<code class="cpp spaces">    </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1;</code>
</div>
<div class="line number108 index107 alt1">
<code class="cpp spaces">  </code><code class="cpp plain">} </code>
</div>
<div class="line number109 index108 alt2">
<code class="cpp spaces">  </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0;</code>
</div>
<div class="line number110 index109 alt1">
<code class="cpp plain">}</code>
</div>
</div>
</td>
</tr></tbody></table>
</div>
</div>
<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
</div>
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</div>
<p>
<img title="Linux下connect超时处理（总结）" alt="Linux下connect超时处理（总结）" src="https://zhuji.jb51.net/uploads/img/202305/80a4a8ebdc10cbafa1c6ed4a819b0a45.jpg"></p>
<p>
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。</p>
<p>
原文链接：http://www.cnblogs.com/Anker/p/6413642.html</p>

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Linux下connect超时处理（总结）