linux - 在 Linux 上显式调用 SIG_DFL/SIG_IGN 处理程序

Question

我已经阻塞，然后通过以下代码等待信号:

sigset_t set;
sigfillset(&set); // all signals
sigprocmask(SIG_SETMASK, &set, NULL); // block all signals
siginfo_t info;
int signum = sigwaitinfo(&set, &info); // wait for next signal
struct sigaction act;
sigaction(signum, NULL, &act); // get the current handler for the signal
act.sa_handler(signum); // invoke it

最后一行生成段错误，因为处理程序设置为 SIG_DFL(定义为 0)。如果设置为 SIG_DFL 或 SIG_IGN，我如何手动调用默认处理程序？另请注意，SIG_IGN 定义为 1。

Answer 1

如您所见，您无法调用 SIG_DFL 和 SIG_IGN 本身。但是，您可以或多或少地模仿他们的行为。

简单地说，模仿正常的信号处理是:

• 对于用户定义的 sa_handler 来说非常容易
• 对于 SIG_IGN 来说足够简单，但需要注意的是在 CHLD 的情况下您需要 waitpid()
• 对于 SIG_DFL 来说直截了当但令人不快，重新加注以让内核发挥其魔力。

这是否符合您的要求？

#include <signal.h>
#include <stdlib.h>

/* Manually dispose of a signal, mimicking the behavior of current
 * signal dispositions as best we can.  We won't cause EINTR, for
 * instance.
 *
 * FIXME:  save and restore errno around the SIG_DFL logic and
 *         SIG_IGN/CHLD logic.
 */
void dispatch_signal(const int signo) {
    int stop = 0;
    sigset_t oset;
    struct sigaction curact;

    sigaction(signo, NULL, &curact);

    /* SIG_IGN => noop or soak up child term/stop signals (for CHLD) */
    if (SIG_IGN == curact.sa_handler) {
        if (SIGCHLD == signo) {
          int status;
          while (waitpid(-1, &status, WNOHANG|WUNTRACED) > 0) {;}
        } 
        return;
    }

    /* user defined => invoke it */
    if (SIG_DFL != curact.sa_handler) {
        curact.sa_handler(signo);
        return;
    }

    /* SIG_DFL => let kernel handle it (mostly).
     *
     *  We handle noop signals ourselves -- "Ign" and "Cont", which we
     *  can never intercept while stopped.
     */
    if (SIGURG == signo || SIGWINCH == signo || SIGCONT == signo) return;

    /*  Unblock CONT if this is a "Stop" signal, so that we may later be
     *  woken up.
     */
    stop = (SIGTSTP == signo || SIGTTIN == signo || SIGTTOU == signo);
    if (stop) {
        sigset_t sig_cont;

        sigemptyset(&sig_cont);
        sigaddset(&sig_cont, SIGCONT);
        sigprocmask(SIG_UNBLOCK, &sig_cont, &oset);
    }

    /*  Re-raise, letting the kernel do the work:
     *     - Set exit codes and corefiles for "Term" and "Core"
     *     - Halt us and signal WUNTRACED'ing parents for "Stop"
     *     - Do the right thing if we forgot to handle any special
     *       signals or signals yet to be introduced
     */
    kill(getpid(), signo);

    /* Re-block CONT, if needed */
    if (stop) sigprocmask(SIG_SETMASK, &oset, NULL);
}

---

更新(回应OP的优秀问题)

1: does this slot in after the sigwaitinfo?

是的。像这样的东西:

... block signals ...
signo = sigwaitinfo(&set, &info);
dispatch_signal(signo);

2: Why not raise those signals handled by SIG_IGN, they'll be ignored anyway

在用户空间中执行 noop 比通过三个系统调用(重新引发、取消屏蔽、重新屏蔽)稍微高效一些。此外，当 SIG_IGNored 时，CHLD 具有特殊的语义。

3: Why treat SIGCHLD specially?

最初(检查答案编辑)我没有——在 SIG_IGN 案例中重新提出它，因为IGNored CHLD signals tell the kernel to automatically reap children .

但是，我更改了它，因为"natural" CHLD signals carry information aboutthe terminated process (至少 PID、状态和真实 UID)。用户生成的 CHLD 信号不具有相同的语义，并且在我的测试中，忽略它们不会导致 2.6 自动获取 SIGCHLD 排队的僵尸被“错过”了。所以，我自己做。

4: Why are "stop" related signals unblocking CONT. Will not invoking the default handler for CONT unstop the process?

如果我们停止(不执行)并且 CONT 被阻塞，我们将永远不会收到叫醒我们的信号!

5: Why not call raise instead of the kill line you've given?

个人喜好； raise() 也可以。