今天看socket编程，遇到了fork函数，以前学过linux编程，对fork了解一点。可是都说它有两个返回值我就想不通了。
后来我又琢磨了会，查了查man手册，终于被我搞清楚了。fork既然是创建子进程的，那么说它返回两个值不如说是程序执行了两次：一次是父进程在执行，另一次是子进程在执行。
看看下面的代码：
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<span style="color: #339900;">#include <unistd.h> </span>
<span style="color: #339900;">#include <stdio.h> </span>
<span style="color: #0000ff;">int</span> main<span style="color: #008000;">(</span><span style="color: #008000;">)</span>
<span style="color: #008000;">{</span>
pid_t pid<span style="color: #008080;">;</span>
<span style="color: #0000ff;">if</span><span style="color: #008000;">(</span><span style="color: #008000;">(</span>pid <span style="color: #000080;">=</span> fork<span style="color: #008000;">(</span><span style="color: #008000;">)</span><span style="color: #008000;">)</span> <span style="color: #000080;"><</span> <span style="color: #0000dd;">0</span><span style="color: #008000;">)</span>
<span style="color: #008000;">{</span>
<span style="color: #0000dd;">printf</span><span style="color: #008000;">(</span><span style="color: #ff0000;">"fork error<span style="color: #000099; font-weight: bold;">n</span>"</span><span style="color: #008000;">)</span><span style="color: #008080;">;</span>
<span style="color: #0000ff;">return</span><span style="color: #008080;">;</span>
<span style="color: #008000;">}</span>
<span style="color: #0000ff;">else</span> <span style="color: #0000ff;">if</span> <span style="color: #008000;">(</span>pid <span style="color: #000080;">==</span> <span style="color: #0000dd;">0</span><span style="color: #008000;">)</span>
<span style="color: #0000dd;">printf</span><span style="color: #008000;">(</span><span style="color: #ff0000;">"output from child:%d<span style="color: #000099; font-weight: bold;">n</span>"</span>,getpid<span style="color: #008000;">(</span><span style="color: #008000;">)</span><span style="color: #008000;">)</span><span style="color: #008080;">;</span>
<span style="color: #0000ff;">else</span>
<span style="color: #0000dd;">printf</span><span style="color: #008000;">(</span><span style="color: #ff0000;">"output from parent:%d<span style="color: #000099; font-weight: bold;">n</span>"</span>,getpid<span style="color: #008000;">(</span><span style="color: #008000;">)</span><span style="color: #008000;">)</span><span style="color: #008080;">;</span>
<span style="color: #0000dd;">printf</span><span style="color: #008000;">(</span><span style="color: #ff0000;">"output %d<span style="color: #000099; font-weight: bold;">n</span>"</span>,pid<span style="color: #008000;">)</span><span style="color: #008080;">;</span>
<span style="color: #0000ff;">return</span> <span style="color: #0000dd;">0</span><span style="color: #008080;">;</span>
<span style="color: #008000;">}</span>
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编译执行，结果如下：
output from child:4694
output 0
output from parent:4693
output 4694
可以很明显的看出，这个程序执行了两次，第一次fork返回了0，相当于是pid=4694的子进程在执行，第二次返回值为4694，也就是返回了子进程的pid，相当于是父进程在执行。
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