使用waitpid和WNOHANG区分进程状态

Question

使用waitpid和WNOHANG区分进程状态

3

在构建一个Shell程序时，我遇到了识别进程状态的问题。我所面临的问题描述如下：我有一个子进程列表，我试图使用waitpid和WNOHANG来确定它们的状态。我希望区分三种状态：终止、运行和暂停。（如下代码所定义）我希望将进程状态更改为上述三种之一，但现在这个函数会将正在运行的进程状态更改为终止，并且该函数也无法识别已暂停的进程。我想知道我做错了什么，应该如何编写updateProcessList函数来实现它？

#define TERMINATED  -1
#define RUNNING 1
#define SUSPENDED 0

typedef struct process{
    cmdLine* cmd;                     /* the parsed command line*/
    pid_t pid;                        /* the process id that is running the command*/
    int status;                       /* status of the process: RUNNING/SUSPENDED/TERMINATED */
    struct process *next;             /* next process in chain */
} process;

void updateProcessList(process **process_list) {
    process *p = *process_list;
    int code = 0, status = 0,pidd = 0;
    while (p) {
        pidd = p->pid;
        code = waitpid(pidd, &status, WNOHANG);
        if (code == -1) {            /* child terminated*/
            p->status = TERMINATED;
        } else if(WIFEXITED(status)){
            p->status = TERMINATED;
        }else if(WIFSTOPPED(status)){
            p->status = SUSPENDED;
        }
        p = p->next;
    }
}

- Avi Ferdman

1

"我想知道我做错了什么。请问您遇到了哪些具体问题？" - kaylum

1

什么没做好？当您运行程序时会发生什么？您原本希望发生什么？ - Nate Eldredge

我编辑了帖子，使其更具体。 - Avi Ferdman

你是说即使进程仍在运行，其状态也会变为“TERMINATED”吗？而你是如何停止进程以测试“SUSPENDED”状态的？ - kaylum

1

请提供确切的测试用例 - 运行了哪个测试，期望的确切结果是什么，实际的确切结果是什么。 - kaylum

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1个回答

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可以查看英文原文，
原文链接

- Marco Bonelli · Accepted Answer

来自man 2 waitpid:

RETURN VALUE

    waitpid():  on  success, returns the process ID of the child whose state has changed;
    if WNOHANG was specified and one or more child(ren) specified by pid exist, but  have
    not yet changed state, then 0 is returned.  On error, -1 is returned.

你应该检查返回值是否为0，并修复其余的检查。

code = waitpid(ppid, &status, WNOHANG | WUNTRACED | WCONTINUED);

if (code == -1) {
    // Handle error somehow... 
    // This doesn't necessarily mean that the child was terminated!
    // See manual page section "ERRORS".

    if (errno == ECHILD) {
        // Child was already terminated by something else.
        p->status = TERMINATED;
    } else {
        perror("waitpid failed");
    }
} else if (code == 0) {
    // Child still in previous state.
    // Do nothing.
} else if (WIFEXITED(status)) {
    // Child exited.
    p->status = TERMINATED;
} else if (WIFSIGNALED(status)) {
    // Child killed by a signal.
    p->status = TERMINATED;
} else if (WIFSTOPPED(status)) {
    // Child stopped.
    p->status = SUSPENDED;
} else if (WIFCONTINUED(status)) {
    // This branch seems unnecessary, you should already know this
    // since you are the one that should kill(pid, SIGCONT) to make the
    // children continue.
    p->status = RUNNING; 
} else {
    // This should never happen!
    abort();
}

还请注意：

我在标记中添加了 WUNTRACED 和 WCONTINUED：只有当您使用 ptrace() 追踪子进程或使用 WUNTRACED 标记时，WIFSTOPPED() 才可能发生；而且只有在使用了 WCONTINUED 之后，WIFCONTINUED() 才会发生。
code 和 ppid 变量应该是 pid_t，而不是 int（ppid 变量似乎也没必要）。

无论如何，考虑为 SIGCHLD 添加信号处理程序，并在那里更新子进程状态。每个终止/停止/恢复的子进程都会向您的程序发送一个 SIGCHLD 信号。这样做更简单，而且速度更快（不需要在每个单独的子进程上持续调用 waitpid()）。