Linux的system()和popen()差异

Linux的system()和popen()差异

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1. system()和popen()简介

在linux中我们可以通过system()来执行一个shell命令，popen()也是执行shell命令并且通过管道和shell命令进行通信。 
system()、popen()给我们处理了fork、exec、waitpid等一系列的处理流程，让我们只需要关注最后的返回结果(函数的返回值)即可。

2. system()、popen()源码

首先我们来看一下这两个函数在源码(伪代码)上面的差异。

<code class="language-c++ hljs objectivec has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> system(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *command){    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">struct</span> sigaction sa_ignore, sa_intr, sa_quit;    sigset_t block_mask, orig_mask;    pid_t pid;    sigemptyset(&block_mask);    sigaddset(&block_mask, SIGCHLD);    sigprocmask(SIG_BLOCK, &block_mask, &orig_mask);        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//1. block SIGCHLD</span>    sa_ignore<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.sa_handler</span> = SIG_IGN;    sa_ignore<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.sa_flags</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;    sigemptyset(&sa_ignore<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.sa_mask</span>);    sigaction(SIGINT, &sa_ignore, &sa_intr);                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//2. ignore SIGINT signal</span>    sigaction(SIGQUIT, &sa_ignore, &sa_quit);                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//3. ignore SIGQUIT signal</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span>((pid = fork()))    {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>:            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>:            sigaction(SIGINT, &sa_intr, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NULL</span>);             sigaction(SIGQUIT, &sa_quit, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NULL</span>);             sigprocmask(SIG_SETMASK, &orig_mask, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NULL</span>);            execl(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/bin/sh"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sh"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"-c"</span>, command, (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *) <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);            exit(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">127</span>);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span>:            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span>(waitpid(pid, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NULL</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) == -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//4. wait child process exit</span>            {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(errno != EINTR)                {                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                }            }    }}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li></ul>

上面是一个不算完整的system函数源码，后面需要我们关注和popen差异的部分已经用数字标示出来了。

<code class="language-c++ hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">static pid_t    *childpid = <span class="hljs-constant" style="box-sizing: border-box;">NULL</span>;                          <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* ptr to array allocated at run-time */</span>  static int      maxfd;  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* from our open_max(), {Prog openmax} */</span>  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#define SHELL   "/bin/sh"  </span>FILE *  popen(const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *cmdstring, const <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *type)  {      int     i, pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>];      pid_t   pid;      FILE    *fp;              <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* only allow "r" or "w" */</span>      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((type[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>] != <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'r'</span> && type[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>] != <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'w'</span>) || type[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>] != <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {          errno = EINVAL;     <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* required by POSIX.2 */</span>          <span class="hljs-constant" style="box-sizing: border-box;">return</span>(<span class="hljs-constant" style="box-sizing: border-box;">NULL</span>);      }      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (childpid == <span class="hljs-constant" style="box-sizing: border-box;">NULL</span>) {     <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* first time through */</span>                  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* allocate zeroed out array for child pids */</span>          maxfd = open_max();          <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( (childpid = calloc(maxfd, sizeof(pid_t))) == <span class="hljs-constant" style="box-sizing: border-box;">NULL</span>)              <span class="hljs-constant" style="box-sizing: border-box;">return</span>(<span class="hljs-constant" style="box-sizing: border-box;">NULL</span>);      }      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (pipe(pfd) < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)          <span class="hljs-constant" style="box-sizing: border-box;">return</span>(<span class="hljs-constant" style="box-sizing: border-box;">NULL</span>);   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* errno set by pipe() */</span>      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( (pid = fork()) < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)          <span class="hljs-constant" style="box-sizing: border-box;">return</span>(<span class="hljs-constant" style="box-sizing: border-box;">NULL</span>);   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* errno set by fork() */</span>      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (pid == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {                            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* child */</span>          <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (*type == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'r'</span>) {              <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">close</span>(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]);              <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>] != STDOUT_FILENO) {                  dup2(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>], STDOUT_FILENO);                  <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">close</span>(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]);              }          } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {              <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">close</span>(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]);              <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>] != STDIN_FILENO) {                  dup2(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], STDIN_FILENO);                  <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">close</span>(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]);              }          }              <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* close all descriptors in childpid[] */</span>          <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < maxfd; i++)              <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (childpid[ i ] > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)                  <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">close</span>(i);          execl(SHELL, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"sh"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"-c"</span>, cmdstring, (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> *) <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);          <span class="hljs-title" style="box-sizing: border-box;">_exit</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">127</span>);      }                                  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* parent */</span>      <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (*type == <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'r'</span>) {          <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">close</span>(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]);          <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( (fp = fdopen(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>], type)) == <span class="hljs-constant" style="box-sizing: border-box;">NULL</span>)              <span class="hljs-constant" style="box-sizing: border-box;">return</span>(<span class="hljs-constant" style="box-sizing: border-box;">NULL</span>);      } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {          <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">close</span>(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]);          <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ( (fp = fdopen(pfd[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>], type)) == <span class="hljs-constant" style="box-sizing: border-box;">NULL</span>)              <span class="hljs-constant" style="box-sizing: border-box;">return</span>(<span class="hljs-constant" style="box-sizing: border-box;">NULL</span>);      }      childpid[fileno(fp)] = pid; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* remember child pid for this fd */</span>      <span class="hljs-constant" style="box-sizing: border-box;">return</span>(fp);  }  </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li></ul>

上面是popen的源码。

3. 执行流程

从上面的源码可以看到system和popen都是执行了类似的运行流程，大致是fork->execl->return。但是我们看到system在执行期间调用进程会一直等待shell命令执行完成(waitpid等待子进程结束)才返回，但是popen无须等待shell命令执行完成就返回了。我们可以理解system为串行执行，在执行期间调用进程放弃了”控制权”，popen为并行执行。 
popen中的子进程没人给它”收尸”了啊？是的，如果你没有在调用popen后调用pclose那么这个子进程就可能变成”僵尸”。 
上面我们没有给出pclose的源码，其实我们根据system的源码差不多可以猜测出pclose的源码就是system中第4部分的内容。

4. 信号处理

我们看到system中对SIGCHLD、SIGINT、SIGQUIT都做了处理，但是在popen中没有对信号做任何的处理。 
SIGCHLD是子进程退出的时候发给父进程的一个信号，system()中为什么要屏蔽SIGCHLD信号可以参考：system函数的总结、waitpid(or wait)和SIGCHILD的关系，总结一句就是为了system()调用能够及时的退出并且能够正确的获取子进程的退出状态(成功回收子进程)。 
popen没有屏蔽SIGCHLD，主要的原因就是popen是”并行”的。如果我们在调用popen的时候屏蔽了SIGCHLD，那么如果在调用popen和pclose之间调用进程又创建了其它的子进程并且调用进程注册了SIGCHLD信号处理句柄来处理子进程的回收工作(waitpid)那么这个回收工作会一直阻塞到pclose调用。这也意味着如果调用进程在pclose之前执行了一个wait()操作的话就可能获取到popen创建的子进程的状态，这样在调用pclose的时候就会回收(waitpid)子进程失败，返回-1，同时设置errno为ECHLD，标示pclose无法获取子进程状态。 
system()中屏蔽SIGINT、SIGQUIT的原因可以继续参考上面提到的system函数的总结，popen()函数中没有屏蔽SIGINT、SIGQUIT的原因也还是因为popen是”并行的”，不能影响其它”并行”进程。

4. 功能

从上面的章节我们基本已经把这两个函数剖析的差不多了，这两个的功能上面的差异也比较明显了，system就是执行shell命令最后返回是否执行成功，popen执行命令并且通过管道和shell命令进行通信。

NOTE

在特权(setuid、setgid)进程中千万注意不要使用system和popen。