c - sleep 指令不适用于信号处理程序

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void handler ( int sig, siginfo_t * info, void * se)
{

}

int setup_sig(){
    struct sigaction sa;
    sa.sa_sigaction = handler;
    sa.sa_flags = SA_SIGINFO;
    sigfillset(&sa.sa_mask);
    if(-1 == sigaction(SIGCHLD, &sa , NULL)){return 0;} return 1;
}

void main()
{
    printf("before\n");
    setup_sig();
    sleep(3);
    printf("after\n");
}

此代码需要注册才能发出信号SIGCHLD然后睡3秒，然后出去。

此代码跳过 sleep 指令，为什么？

Answer 1

Linux documentation for sleep 在底部列出以下属性:

┌──────────┬───────────────┬─────────────────────────────┐
│Interface │ Attribute     │ Value                       │
├──────────┼───────────────┼─────────────────────────────┤
│sleep()   │ Thread safety │ MT-Unsafe sig:SIGCHLD/linux │
└──────────┴───────────────┴─────────────────────────────┘

那sig:SIGCHLD/linux在这种情况下很重要。根据 here ，解释如下(我的重点):

sig: Functions marked with sig as a MT-Safety issue may temporarily install a signal handler for internal purposes, which may interfere with other uses of the signal, identified after a colon.

This safety problem can be worked around by ensuring that no other uses of the signal will take place for the duration of the call. Holding a non-recursive mutex while calling all functions that use the same temporary signal; blocking that signal before the call and resetting its handler afterwards is recommended.

可以使用 sigprocmask 来阻止信号。调用，例如:

// Need signal.h for this stuff.

sigset_t mask;

if ((sigemptyset(&mask) == -1) || (sigaddset(&mask, SIGCHLD) == -1)) {
    // handle failure here.
}

if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1) {
    // handle failure here.
}

sleep(3);

// Probably need to re-establish CHLD signal handler as well as unmasking.

if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1) {
    // handle failure here.
}