为什么要在等待后检查WIFEXITED以杀死Linux系统调用中的子进程? [英] Why should we check WIFEXITED after wait in order to kill child processes in Linux system call?

问题描述

我在C语言中遇到了一些代码，我们在其中检查 wait 的返回值，如果这不是错误，则还要对 WIFEXITED 和 WIFEXITSTATUS .为什么这不是多余的?据我了解，如果发生错误，则 wait 返回-1，而如果 wait 子级正常终止，则 WIFEXITED 返回非零值.因此，如果该行中没有任何错误 if(wait(& status)< 0)，为什么在 WIFEXITED 检查期间出现任何错误?

I came across some code in C where we check the return value of wait and if it's not an error there's yet another check of WIFEXITED and WIFEXITSTATUS. Why isn't this redundant? As far as I understand wait returns -1 if an error occurred while WIFEXITED returns non-zero value if wait child terminated normally. So if there wasn't any error in this line if ( wait(&status) < 0 ) why would anything go wrong durng WIFEXITED check?

这是代码:

#include <stdio.h>
#include <signal.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>

#define CHILDREN_NUM 5

int main () {

    int i, status, pid, p;
    for(i = 0; (( pid = fork() ) < 0) && i < CHILDREN_NUM;i++)
        sleep(5);


    if ( pid == 0 )
    {
        printf(" Child %d : successfully created!\n",i);
        exit( 0 );  /* Son normally exits here! */
    }

    p = CHILDREN_NUM;
    /* The father waits for agents to return succesfully */
    while ( p >= 1 )
    {
        if ( wait(&status) < 0 ) {
            perror("Error");
            exit(1);
        }

        if ( ! (WIFEXITED(status) && (WEXITSTATUS(status) == 0)) )  /* kill all running agents */
        {
            fprintf( stderr,"Child failed. Killing all running children.\n");
           //some code to kill children here
            exit(1);
        }
        p--;
    }

    return(0);
}

推荐答案

wait 返回> = 0 会告诉您子进程已终止(并且调用 wait 没有失败)，但是它不会告诉您该进程是否成功终止(或是否已发出信号).

wait returning >= 0 tells you a child process has terminated (and that calling wait didn't fail), but it does not tell you whether that process terminated successfully or not (or if it was signalled).

但是，在这里，查看您的代码，很明显，程序确实在乎终止的子进程是否成功完成了

But, here, looking at your code, it's fairly obvious the program does care about whether the child process that terminated did so successfully or not:

fprintf(stderr，子代失败.杀死所有正在运行的子代.\ n");

因此，程序需要对由 wait 填充的 status 结构进行进一步的测试:

So, the program needs to do further tests on the status structure that was populated by wait:

• WIFEXITED(状态):该进程是否正常退出?(而不是发出信号).
• WEXITSTATUS(status)== 0 :进程是否以退出代码0(又称为成功")退出.有关更多信息，请参见:Linux返回的退出状态1的含义命令.

• WIFEXITED(status): did the process exit normally? (as opposed to being signalled).
• WEXITSTATUS(status) == 0: did the process exit with exit code 0 (aka "success"). For more information, see: Meaning of exit status 1 returned by linux command.

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