浅谈Linux vfork与fork简单对比分析
<p>本文分享了linux vfork与fork简单对比分析，分享给大家，具体如下：</p>
<p>
<span><strong>fork相关问题：</strong></span></p>
<p>
<strong>一、fork基础了解</strong></p>
<p>
fork作用为创建一个子进程，在使用了fork命令后，内核会分配新的内存块和数据结构给子进程，并且将父进程的部分数据结构内容拷贝到子进程，最后再将子进程添加到系统进程列表中，添加完成后fork返回，开始调度。</p>
<p>
头文件：#include < unistd.h ></p>
<p>
函数原型：pid_t fork( )</p>
<p>
返回值：返回值大于0则当前进程为父进程，等于0代表为子进程，小于零代表创建子进程失败。</p>
<p>
通过一个例子来了解：</p>
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<code class="cpp preprocessor">#include <stdio.h></code>
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<code class="cpp spaces"> </code><code class="cpp preprocessor">#include <unistd.h></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">tmp = 5;</code>
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<code class="cpp spaces"> </code><code class="cpp plain">pid_t res = fork();</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(res < 0){</code>
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<code class="cpp spaces"> </code><code class="cpp comments">//fork失败</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"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code><code class="cpp keyword bold">else</code> <code class="cpp keyword bold">if</code><code class="cpp plain">(res == 0){</code>
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<code class="cpp spaces"> </code><code class="cpp comments">//该进程为子进程</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">"im child[%d],fasther is %d,tmp is %d.\n"</code><code class="cpp plain">,getpid(),getppid(),tmp++);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code><code class="cpp keyword bold">else</code><code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp comments">//该进程为父进程</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">"im father[%d],tmp is %d.\n"</code><code class="cpp plain">,getpid(),tmp++);</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">printf</code><code class="cpp plain">(</code><code class="cpp string">"tmp = %d\n"</code><code class="cpp plain">,tmp);</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>
运行结果：</p>
<p>
im father,tmp is 5.<br>
tmp = 6<br>
im child,fasther is 1,tmp is 5.<br>
tmp = 6</p>
<p>
<strong>相关问题小结：</strong></p>
<p>
通过结果很明显的能看出本次调用中，先执行父进程，对应pid为3128，在父进程中tmp++，所以输出为6；关键问题在于子进程，有两个关键点。</p>
<p>
①为什么结果中子进程父亲pid为1：通过输出我们能看出父进程先执行完成后才执行的子进程，也就是说当子进程执行时父进程已结束，此时该子进程相当于一个孤儿进程，被pid为1也就是init进程所管理，所以子进程的ppid为1；</p>
<p>
②为什么子进程最后输出tmp值还为6： fork进程采用的是写时拷贝，父子进程一开始共享一片内存区域，但是只有有一方要对数据进行修改，则再开辟一块空间，防止相互修改影响。所以在上述代码中，虽说是一个tmp，其实内存中各自保留了一份值。</p>
<p>
<span><strong>二、关于fork过程中写时拷贝：</strong></span></p>
<p>
<img title="浅谈Linux vfork与fork简单对比分析" alt="浅谈Linux vfork与fork简单对比分析" src="https://zhuji.jb51.net/uploads/img/202305/962e38b20bbdce7b0c4e1db8dc2f56b4.jpg"></p>
<p>
<img title="浅谈Linux vfork与fork简单对比分析" alt="浅谈Linux vfork与fork简单对比分析" src="https://zhuji.jb51.net/uploads/img/202305/d01c5e941caf68cf0139fca6f6a4a6a0.jpg"></p>
<p>
这下就不难看出，父子进程数据段和代码段开始时是共享一块对应的内存，当一方尝试写入时，便产生了写时拷贝。需要注意的是：fork之前，父进程独立执行，fork之后，父子两个执行流分别执行，至于谁先执行，由调度器决定。可通过下面例子很明显的看出是从fork之后才分别执行。</p>
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<code class="cpp preprocessor">#include <stdio.h></code>
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<code class="cpp spaces"> </code><code class="cpp preprocessor">#include <unistd.h></code>
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<code class="cpp spaces"> </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">main()</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">tmp = 5;</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">"there is fork before\n"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">pid_t res = fork();</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(res < 0){</code>
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<code class="cpp spaces"> </code><code class="cpp comments">//fork失败</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"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code><code class="cpp keyword bold">else</code> <code class="cpp keyword bold">if</code><code class="cpp plain">(res == 0){</code>
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<code class="cpp spaces"> </code><code class="cpp comments">//该进程为子进程</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">"im child[%d],tmp is %d.\n"</code><code class="cpp plain">,getpid(),tmp++);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code><code class="cpp keyword bold">else</code><code class="cpp plain">{</code>
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<code class="cpp spaces"> </code><code class="cpp comments">//该进程为父进程</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">"im father[%d],tmp is %d.\n"</code><code class="cpp plain">,getpid(),tmp++);</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">printf</code><code class="cpp plain">(</code><code class="cpp string">"tmp = %d\n"</code><code class="cpp plain">,tmp);</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>
输出结果：</p>
<p>
there is fork before</p>
<p>
im father,tmp is 5.</p>
<p>
tmp = 6</p>
<p>
im child,tmp is 5.</p>
<p>
tmp = 6</p>
<p>
<span><strong>三、fork调用失败的原因：</strong></span></p>
<p>
①系统中已经存在太多进程，无法再创建新的进程。可通过ulimit -a命令查看当前所有的资源限制。</p>
<p>
②内存不足，由于开辟每个新的进程都要分配一个pcb，并为新进程分配资源，内存都不足也就别提还想着再创建进程了。</p>
<p>
<span><strong>vfork相关问题：</strong></span></p>
<p>
<strong>一、vfork基础了解</strong></p>
<p>
<1>vfork创建新进程的主要目的在于用exec函数执行另外的程序，实际上，在没调用exec或_exit之前子进程与父进程共享数据段。在vfork调用中，子进程先运行，父进程挂起，直到子进程调用exec或_exit，在这以后，父子进程的执行顺序不再有限制。</p>
<p>
头文件：#include < unistd.h ></p>
<p>
函数原型：pid_t vfork( )</p>
<p>
返回值：返回值大于0则当前进程为父进程，等于0代表为子进程，小于零代表创建子进程失败。</p>
<p>
通过一个例子来了解：</p>
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<code class="cpp preprocessor">#include <stdio.h></code>
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<code class="cpp spaces"> </code><code class="cpp preprocessor">#include <unistd.h></code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">tmp = 3;</code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">main()</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">pid_t res = vfork();</code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(res < 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">"vfork"</code><code class="cpp plain">);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">_exit();</code>
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<code class="cpp spaces"> </code><code class="cpp plain">tmp = 10;</code>
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<code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"child res = %d\n"</code><code class="cpp plain">,tmp);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">_exit(0);</code>
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<code class="cpp spaces"> </code><code class="cpp plain">}</code><code class="cpp keyword bold">else</code><code class="cpp plain">{</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">"father res = %d\n"</code><code class="cpp plain">,tmp);</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>
输出结果：</p>
<p>
child res = 10</p>
<p>
father res = 10</p>
<p>
结果分析：正如上面所说的，子进程直接公用父进程的页表，改变子进程的数据也会影响到父进程。</p>
<p>
<img title="浅谈Linux vfork与fork简单对比分析" alt="浅谈Linux vfork与fork简单对比分析" src="https://zhuji.jb51.net/uploads/img/202305/f3befb38f385542b410b8004a3bab1d9.jpg"></p>
<p>
<2>vfork用处：</p>
<p>
vfork()跟fork()类似，都是创建一个子进程，这两个函数的的返回值也具有相同的含义。但是vfork()创建的子进程基本上只能做一件事，那就是立即调用_exit()函数或者exec函数族成员，调用任何其它函数（包括exit()）、修改任何数据（除了保存vfork()返回值的那个变量）、执行任何其它语句（包括return）都是不应该的。更需要注意的是：调用vfork()之后，父进程会一直阻塞，直到子进程调用_exit()终止，或者调用exec函数族成员。</p>
<p>
<3>为什么只能用_exit退出：</p>
<p>
exit()是对_exit()的封装，它自己在调用_exit()前会做很多清理工作，其中包括刷新并关闭当前进程使用的流缓冲（比如stdio.h里面的printf等），由于vfork()的子进程完全共享了父进程地址空间，子进程里面的流也是共享的父进程的流，所以子进程里面是不能做这些事的。直接return就更不行了，子进程return以后，会从当前函数的外部调用点后面继续执行，这后面子进程可能将会执行很多语句，结果就没法预料了。在man手册中也强调了这一点，必须使用_exit退出。</p>
<p>
<span><strong>fork与vfork的区别 </strong></span></p>
<p>
1.vfork保证子进程先运行，在它调用exec或exit之后父进程才可能被调度运行。如果在调用这两个函数之前子进程依赖于父进程的进一步动作，则会导致死锁。</p>
<p>
2.fork要拷贝父进程的进程环境；而vfork则不需要完全拷贝父进程的进程环境，在子进程没有调用exec和exit之前，子进程与父进程共享进程环境，相当于线程的概念，此时父进程阻塞等待。</p>
<p>
<strong>为什么会有vfork呢?</strong></p>
<p>
因为以前的fork当它创建一个子进程时，将会创建一个新的地址空间，并且拷贝父进程的资源，然后将会有两种行为：</p>
<p>
1.执行从父进程那里拷贝过来的代码段</p>
<p>
2.调用一个exec执行一个新的代码段</p>
<p>
当进程调用exec函数时，一个新程序替换了当前进程的正文，数据，堆和栈段。这样，前面的拷贝工作就是白费力气了，这种情况下，聪明的人就想出了vfork。vfork并不复制父进程的进程环境，子进程在父进程的地址空间中运行，所以子进程不能进行写操作，并且在儿子“霸占”着老子的房子时候，要委屈老子一下了，让他在外面歇着（阻塞），一旦儿子执行了exec或者exit后，相当于儿子买了自己的房子了，这时候就相当于分家了。</p>
<p>
因此，如果创建子进程是为了调用exec执行一个新的程序的时候，就应该使用vfork</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://blog.csdn.net/sssssuuuuu666/article/details/78708577</p>
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