blocks|key|4391857|text|您的程序具有未定义的行为。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4391858|如果用户不传递任何参数，则永远不会进入循环，而您将进入|4391859|char+com[60];
//+...
system(com);|code-block|syntax|javascript|4391860|此时com的内容未初始化，因此对system的调用具有未定义的行为。|offset|length|style|CODE|4391861|如果用户传递参数，则执行此操作|4391862|strcat(com,+argv[i]);|4391863|此时com的内容未初始化，因此对strcat的调用具有未定义的行为。|4391864|4391865|要解决此问题，请在循环之前将com设置为有效字符串：|4391866|char+com[60];
com[0]+=+'\0';|4391867|entityMap^0|0|0|0|2|3|G|6|0|0|0|2|3|G|6|0|0|E|3|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|12|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|13|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|14|8|@$K|15|L|16|M|N]|$K|17|L|18|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|19|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|1A|8|@]|9|@]|A|$G|H]]|$1|S|3|T|5|6|7|1B|8|@$K|1C|L|1D|M|N]|$K|1E|L|1F|M|N]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|V|3|W|5|6|7|1H|8|@$K|1I|L|1J|M|N]]|9|@]|A|$]]|$1|X|3|Y|5|F|7|1K|8|@]|9|@]|A|$G|H]]|$1|Z|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|10|$]]

Your program has undefined behavior.

If the user passes no arguments, the loop is never entered and you do

<pre><code>char com[60];
// ...
system(com);
</code></pre>

The contents of <code>com</code> are uninitialized at this point, so the call to <code>system</code> has undefined behavior.

If the user passes arguments, you do

<pre><code>strcat(com, argv[i]);
</code></pre>

The contents of <code>com</code> are uninitialized at this point, so the call to <code>strcat</code> has undefined behavior.

<hr>

To fix this, make <code>com</code> a valid string before the loop:

<pre><code>char com[60];
com[0] = '\0';
</code></pre>

blocks|key|3735246|text|这有时会使缓冲区溢出(非常糟糕)，有时会出现引用错误，没有参数的情况下，缓冲区将无法初始化(=>未定义行为)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3735247|当shell运行你的程序时，它会去掉字符串的引号并插入$-variables。要健壮地完成此转发，除了检查缓冲区溢出之外，还需要重新报价。对于一个短小的程序来说，这可能有点乏味。|offset|length|style|CODE|3735248|更简单的方法是直接使用argv数组执行posix_spawn或fork/exec：|3735249|int+status;
pid_t+pid;

if+(0>(pid=fork()))+return+-1;
if(0==pid){
++++execvp(argv[1],argv%2B1);
++++_exit(127);
}
while(0>(waitpid(pid,&status,0)))
++++if+(EINTR==errno)+continue;+else+abort();+/*shouldn't+happen*/|code-block|syntax|javascript|3735250|现在，真正的system()还会忽略父进程中的SIGINT/SIGQUIT，并在获取状态之前阻止父进程中的SIGCHLD。posix_spawnp也是首选。|3735251|要复制这一点，您可以从musl+libc的system实现开始，然后修改它以创建一个|3735252|int+my_system(char+const+*file,+char+*argv[i]);|3735253|这将跳过shell+(以及创建具有正确引号和缓冲区溢出检查的字符串的需要)：|3735254|#include+<unistd.h>
#include+<stdlib.h>
#include+<signal.h>
#include+<sys/wait.h>
#include+<spawn.h>
#include+<errno.h>
#include+<pthread.h>

//extern+char+**__environ;
extern+char+**environ;

int+my_system(char+const+*file,+char+*argv[])
{
++++pid_t+pid;
++++sigset_t+old,+reset;
++++struct+sigaction+sa+=+{+.sa_handler+=+SIG_IGN+},+oldint,+oldquit;
++++int+status+=+-1,+ret;
++++posix_spawnattr_t+attr;

++++//pthread_testcancel();

++++//if+(!cmd)+return+1;

++++sigaction(SIGINT,+&sa,+&oldint);
++++sigaction(SIGQUIT,+&sa,+&oldquit);
++++sigaddset(&sa.sa_mask,+SIGCHLD);
++++sigprocmask(SIG_BLOCK,+&sa.sa_mask,+&old);

++++sigemptyset(&reset);
++++if+(oldint.sa_handler+!=+SIG_IGN)+sigaddset(&reset,+SIGINT);
++++if+(oldquit.sa_handler+!=+SIG_IGN)+sigaddset(&reset,+SIGQUIT);
++++posix_spawnattr_init(&attr);
++++posix_spawnattr_setsigmask(&attr,+&old);
++++posix_spawnattr_setsigdefault(&attr,+&reset);
++++posix_spawnattr_setflags(&attr,+POSIX_SPAWN_SETSIGDEF%7CPOSIX_SPAWN_SETSIGMASK);

#if+0
++++ret+=+posix_spawn(&pid,+"/bin/sh",+0,+&attr,
++++++++(char+*[]){"sh",+"-c",+(char+*)cmd,+0},+environ);
#else
++++ret+=+posix_spawnp(&pid,+file,+0,+&attr,+argv,+environ);
#endif

++++posix_spawnattr_destroy(&attr);

++++if+(!ret)+while+(waitpid(pid,+&status,+0)<0+&&+errno+==+EINTR);
++++sigaction(SIGINT,+&oldint,+NULL);
++++sigaction(SIGQUIT,+&oldquit,+NULL);
++++sigprocmask(SIG_SETMASK,+&old,+NULL);

++++if+(ret)+errno+=+ret;
++++return+status;
}|3735255|现在有了my_system，您就不需要报价了，只需用以下命令调用它|3735256|my_system(argv[1],+argv%2B1);|3735257|entityMap^0|0|R|B|0|0|0|6|8|N|E|1H|7|1P|C|0|L|6|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|15|8|@$D|16|E|17|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|18|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|19|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1A|8|@$D|1B|E|1C|F|G]|$D|1D|E|1E|F|G]|$D|1F|E|1G|F|G]|$D|1H|E|1I|F|G]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1J|8|@$D|1K|E|1L|F|G]]|9|@]|A|$]]|$1|S|3|T|5|L|7|1M|8|@]|9|@]|A|$M|N]]|$1|U|3|V|5|6|7|1N|8|@]|9|@]|A|$]]|$1|W|3|X|5|L|7|1O|8|@]|9|@]|A|$M|N]]|$1|Y|3|Z|5|6|7|1P|8|@]|9|@]|A|$]]|$1|10|3|11|5|L|7|1Q|8|@]|9|@]|A|$M|N]]|$1|12|3|-4|5|6|7|1R|8|@]|9|@]|A|$]]]|13|$]]

That'll sometimes overflow the buffer (very bad), sometimes it'll get the quoting wrong, and with no arguments, the buffer will be uninitialized (=> undefined behavior).

When a shell runs your program, it'll de-quote strings and interpolate <code>$-variables</code>. To do this forwarding robustly, you'd need to requote, in addition to checking against buffer overflow. That would be a bit tedious for in a short program. 

The easier thing would be to simply posix_spawn or fork/exec with the argv array directly:

<pre><code>int status;
pid_t pid;

if (0&gt;(pid=fork())) return -1;
if(0==pid){
 execvp(argv[1],argv+1);
 _exit(127);
}
while(0&gt;(waitpid(pid,&amp;status,0)))
 if (EINTR==errno) continue; else abort(); /*shouldn't happen*/
</code></pre>

Now, a real <code>system()</code> would additionally ignore <code>SIGINT/SIGQUIT</code> in the parent and block <code>SIGCHLD</code> in the parent before the status is reaped. <code>posix_spawnp</code> would also be preferred.

To copy that right, you can start off with the musl libc's implementation of <code>system</code>, and modify it to create a 

<pre><code>int my_system(char const *file, char *argv[i]);
</code></pre>

that'll skip the shell (and with it, the need to create a string with correct quoting and buffer overflow checking):

<pre><code>#include &lt;unistd.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;signal.h&gt;
#include &lt;sys/wait.h&gt;
#include &lt;spawn.h&gt;
#include &lt;errno.h&gt;
#include &lt;pthread.h&gt;

//extern char **__environ;
extern char **environ;

int my_system(char const *file, char *argv[])
{
 pid_t pid;
 sigset_t old, reset;
 struct sigaction sa = { .sa_handler = SIG_IGN }, oldint, oldquit;
 int status = -1, ret;
 posix_spawnattr_t attr;

 //pthread_testcancel();

 //if (!cmd) return 1;

 sigaction(SIGINT, &amp;sa, &amp;oldint);
 sigaction(SIGQUIT, &amp;sa, &amp;oldquit);
 sigaddset(&amp;sa.sa_mask, SIGCHLD);
 sigprocmask(SIG_BLOCK, &amp;sa.sa_mask, &amp;old);

 sigemptyset(&amp;reset);
 if (oldint.sa_handler != SIG_IGN) sigaddset(&amp;reset, SIGINT);
 if (oldquit.sa_handler != SIG_IGN) sigaddset(&amp;reset, SIGQUIT);
 posix_spawnattr_init(&amp;attr);
 posix_spawnattr_setsigmask(&amp;attr, &amp;old);
 posix_spawnattr_setsigdefault(&amp;attr, &amp;reset);
 posix_spawnattr_setflags(&amp;attr, POSIX_SPAWN_SETSIGDEF|POSIX_SPAWN_SETSIGMASK);

#if 0
 ret = posix_spawn(&amp;pid, "/bin/sh", 0, &amp;attr,
 (char *[]){"sh", "-c", (char *)cmd, 0}, environ);
#else
 ret = posix_spawnp(&amp;pid, file, 0, &amp;attr, argv, environ);
#endif

 posix_spawnattr_destroy(&amp;attr);

 if (!ret) while (waitpid(pid, &amp;status, 0)&lt;0 &amp;&amp; errno == EINTR);
 sigaction(SIGINT, &amp;oldint, NULL);
 sigaction(SIGQUIT, &amp;oldquit, NULL);
 sigprocmask(SIG_SETMASK, &amp;old, NULL);

 if (ret) errno = ret;
 return status;
}
</code></pre>

Now with my_system, you have no need for quoting and you can simply call it with

<pre><code>my_system(argv[1], argv+1);
</code></pre>

I am trying to make a program in Linux in which the command to be executed is passed as a command line argument. I have made a code but it runs file sometimes and sometimes it does not work and i don't know what the problem is.

Here is my code:

<pre><code>#include&lt;stdio.h&gt;
#include&lt;string.h&gt;
#include&lt;unistd.h&gt;

void main(int argc, char* argv[]){

 char com[60];
 int i;

 for(i = 1 ; i &lt; argc ; i++){

 strcat(com, argv[i]);
 strcat(com, " ");
 }

 system(com);
}
</code></pre>

using system() with command line arguments in C

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我正在尝试用Linux编写一个程序，其中要执行的命令是作为命令行参数传递的。我写了一段代码，但它有时运行文件，有时不工作，我不知道问题是什么。下面是我的代码：#include<stdio.h>#include<string.h>#include<unistd.h>void main(int argc, char* ar...

问在C++中使用带命令行参数的system()
EN

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问在C++中使用带命令行参数的system()EN