Linux 僵尸进程产生原因及解决方法
<p>linux 允许进程查询内核以获得其父进程的 pid，或者其任何子进程的执行状态。例如，进程可以创建一个子进程来执行特定的任务，然后调用诸如 wait() 这样的一些库函数检查子进程是否终止。如果子进程已经终止，那么，它的终止代号将告诉父进程这个任务是否已成功地完成。</p>
<p>
为了遵循这些设计原则，不允许 linux 内核在进程一终止后就丢弃包含在进程描述符字段中的数据。只有父进程发出了与被终止的进程相关的 wait() 类系统调用之后，才允许这样做。这就是引入僵死状态的原因：尽管从技术上来说进程已死，但必须保存它的描述符，直到父进程得到通知。</p>
<p>
如果一个进程已经终止，但是它的父进程尚未调用 wait() 或 waitpid() 对它进行清理，这时的进程状态称为僵死状态，处于僵死状态的进程称为僵尸进程(zombie process)。任何进程在刚终止时都是僵尸进程，正常情况下，僵尸进程都立刻被父进程清理了。</p>
<p>
<strong>僵尸进程是如何产生的</strong></p>
<p>
在unix 系统中，一个进程结束了，但是他的父进程没有等待（调用wait / waitpid）他，那么他将变成一个僵尸进程。通过ps命令查看其带有defunct的标志。僵尸进程是一个早已死亡的进程，但在进程表 （processs table）中仍占了一个位置（slot）。</p>
<p>
但是如果该进程的父进程已经先结束了，那么该进程就不会变成僵尸进程。因为每个进程结束的时候，系统都会扫描当前系统中所运行的所有进程，看看有没有哪个 进程是刚刚结束的这个进程的子进程，如果是的话，就由init进程来接管他，成为他的父进程，从而保证每个进程都会有一个父进程。而init进程会自动 wait其子进程，因此被init接管的所有进程都不会变成僵尸进程。</p>
<p>
为了观察到僵尸进程，我们自己写一个不正常的程序，父进程 fork 出子进程，子进程终止，而父进程既不终止也不调用 wait 清理子进程：</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 plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">i = 100;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">pid_t pid=fork();</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(pid < 0)</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">perror</code><code class="cpp plain">(</code><code class="cpp string">"fork failed."</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">exit</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 keyword bold">if</code><code class="cpp plain">(pid > 0)</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">"this is the parent process. my pid is %d.\n"</code><code class="cpp plain">, getpid());</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">for</code><code class="cpp plain">(; i > 0; i--)</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">sleep(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">}</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">else</code> <code class="cpp keyword bold">if</code><code class="cpp plain">(pid == 0)</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 keyword bold">return</code> <code class="cpp plain">0;</code>
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<code class="cpp plain">}</code>
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<p>
把上面的代码保存到文件 zomprocdemo.c 文件中，并执行下面的命令编译：</p>
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<code class="cpp plain">$ gcc zomprocdemo.c -o zomprocdemo</code>
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然后运行编译出来的 zomprocdemo 程序：</p>
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<code class="cpp plain">$ ./zomprocdemo</code>
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<img title="Linux 僵尸进程产生原因及解决方法" alt="Linux 僵尸进程产生原因及解决方法" src="https://zhuji.jb51.net/uploads/img/202305/2f4e96ba24e072d14bd23426cff95994.jpg"></p>
<p>
此时子进程已经退出，但是父进程没有退出也没有通过 wait() 调用处理子进程。我们使用 ps 命令查看进程的状态：</p>
<p>
<img title="Linux 僵尸进程产生原因及解决方法" alt="Linux 僵尸进程产生原因及解决方法" src="https://zhuji.jb51.net/uploads/img/202305/7df9ab86130db6a887afc62451030d91.jpg"></p>
<p>
上图红框中的大写字母 "z" 说明 pid 为 112712 的进程此时处于僵死的状态。</p>
<p>
让我们接着往下看！在结束 sleep 后父进程退出。当父进程退出后，子进程会变成孤儿进程，此时它会被一个管理进程收养。在不同的系统中，这个管理进程不太一样，早期一般是 init 进程，ubuntu 上是 upstart，还有近来的 systemd。但是它们都完成相同的任务，就是 wiat() 这些孤儿进程，并最终释放它们占用的系统进程表中的资源。这样，这些已经僵死的孤儿进程就彻底的被清除了。</p>
<p>
<strong>僵尸进程的危害</strong></p>
<p>
在进程退出的时候，内核释放该进程所有的资源，包括打开的文件，占用的内存等。但是仍然为其保留一定的信息(包括进程号 pid，退出状态 the termination status of the process，运行时间 the amount of cpu time taken by the process 等)。直到父进程通过 wait / waitpid 来取时才释放。</p>
<p>
如果进程不调用 wait / waitpid 的话， 那么保留的那段信息就不会释放，其进程号就会一直被占用，但是系统所能使用的进程号是有限的，如果大量的产生僵死进程，将因为没有可用的进程号而导致系统不能产生新的进程。</p>
<p>
<strong>如何处理僵尸进程</strong></p>
<p>
僵尸进程的产生是因为父进程没有 wait() 子进程。所以如果我们自己写程序的话一定要在父进程中通过 wait() 来避免僵尸进程的产生。</p>
<p>
当系统中出现了僵尸进程时，我们是无法通过 kill 命令把它清除掉的。但是我们可以杀死它的父进程，让它变成孤儿进程，并进一步被系统中管理孤儿进程的进程收养并清理。</p>
<p>
下面的 demo 中，父进程通过 wait() 等待子进程结束：</p>
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<code class="cpp preprocessor">#include <sys/types.h></code>
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<code class="cpp preprocessor">#include <sys/wait.h></code>
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<code class="cpp preprocessor">#include <unistd.h></code>
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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 color1 bold">int</code> <code class="cpp plain">main(</code><code class="cpp keyword bold">void</code><code class="cpp plain">)</code>
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<code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp plain">pid_t pid;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">pid = fork();</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(pid < 0)</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 functions bold">perror</code><code class="cpp plain">(</code><code class="cpp string">"fork failed"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">exit</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 keyword bold">if</code> <code class="cpp plain">(pid == 0) {</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">i;</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">for</code> <code class="cpp plain">(i = 3; i > 0; i--)</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 comments">// exit with code 3 for test.</code>
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<code class="cpp spaces"> </code><code class="cpp functions bold">exit</code><code class="cpp plain">(3);</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 keyword bold">else</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 color1 bold">int</code> <code class="cpp plain">stat_val;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">wait(&stat_val);</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code> <code class="cpp plain">(wifexited(stat_val))</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">} </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0;</code>
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<p>
demo 中父进程不仅等待子进程结束，还通过 wexitstatus 宏取到了子进程的 exit code。</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/sparkdev/p/8275221.html</p>
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