使用“sigaction(2)"忽略“SIGCHLD"信号有什么用?

问题描述

事实证明，我们可以通过指定来防止出现僵尸进程(即，其父进程没有 wait() 等待它到 _exit())SIGCHLD 信号被其父级用 sigaction() 忽略.然而，似乎 SIGCHLD 在默认情况下被忽略.这是怎么回事?

It turns out that we can prevent appearing of a zombie process (i.e. the one whose parent doesn't wait() for it to _exit()) by specifying SIGCHLD signal to be ignored with sigaction() by its parent. However, it seems like SIGCHLD is ignored by default anyway. How come does this work?

int main (void) {
    struct sigaction sa;
    sa.sa_handler = SIG_IGN; //handle signal by ignoring
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    if (sigaction(SIGCHLD, &sa, 0) == -1) {
        perror(0);
        exit(1);
    }
    int pid = fork();
    if (pid == 0) { //child process
        _exit(0);
    }
    do_something(); //parent process
    return 0;
}

推荐答案

SIGCHLD 的默认行为是丢弃信号，但是子进程保持僵尸进程直到父进程调用 wait()(或变体)以获取其终止状态.

The default behavior of SIGCHLD is to discard the signal, but the child process is kept as a zombie until the parent calls wait() (or a variant) to get its termination status.

但是如果你显式地调用带有 SIG_IGN 的 sigaction()，这会导致它不会把孩子变成僵尸——当孩子退出时它会被收割立即地.请参阅https://stackoverflow.com/a/7171836/1491895

But if you explicitly call sigaction() with the disposition SIG_IGN, that causes it not to turn the child into a zombie -- when the child exits it is reaped immediately. See https://stackoverflow.com/a/7171836/1491895

获得此行为的 POSIX 方法是通过使用 handler = SIG_DFL 和包含 SA_NOCLDWAIT 的 flags 调用 sigaction.这是从 2.6 开始的 Linux.

The POSIX way to get this behavior is by calling sigaction with handler = SIG_DFL and flags containing SA_NOCLDWAIT. This is in Linux since 2.6.

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