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