c - signalfd() 遗漏信号

Question

在我的程序中，我使用 signalfd 来处理信号并将其与 poll 结合用于异步 IO。下面是我的代码:

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/syscall.h>
#include <sys/signalfd.h>
#include <unistd.h>
#include <string.h>
#include <pthread.h>
#include <sys/types.h>
#include <poll.h>
#include <assert.h>
#include <errno.h>


volatile sig_atomic_t cont = 1;
volatile sig_atomic_t usrcnt = 0;
volatile sig_atomic_t susrcnt = 0;

volatile sig_atomic_t wsig = 0;
volatile sig_atomic_t wtid = 0;

int GetCurrentThreadId()
{
    return syscall(__NR_gettid);
}

void Segv1(int p1, siginfo_t * p2, void * p3)
{
    //printf("SIGSEGV signal on illegal memory access handled by thread: %d\n", GetCurrentThreadId());
    wtid = GetCurrentThreadId();
    wsig = SIGSEGV;
    _exit(SIGSEGV);
}

void Fpe1(int p1 , siginfo_t * p2, void * p3)
{
    //printf is only for test.
    //printf("FPE signal handled by thread: %d\n", GetCurrentThreadId());
    wtid = GetCurrentThreadId();
    wsig = SIGFPE;
    _exit(SIGFPE);
}

void User1(int p1 , siginfo_t * p2, void * p3)
{
    printf("User signal 1 handled by thread: %d\n", GetCurrentThreadId());
    ++susrcnt;
    wtid = GetCurrentThreadId();
    wsig = SIGUSR1;
}


void* ThreadFunc (void* d)
{

    //Let us use signalfd.
    int sfd;
    sigset_t mask;

    /* We will handle SIGTERM and SIGINT. */
    sigemptyset (&mask);
    sigaddset (&mask, SIGUSR1);

    /* Create a file descriptor from which we will read the signals. */
    sfd = signalfd (-1, &mask, 0);
    if (sfd < 0) {
        printf ("signalfd failed with %d\n", errno);
        return NULL;
    }

    pthread_sigmask(SIG_BLOCK, &mask, NULL);

  /* This is the main loop */
        struct pollfd pfd[1];
        int ret;
        ssize_t bytes;

        pfd[0].fd = sfd;
        pfd[0].events = POLLIN | POLLERR | POLLHUP;

        for (;;) {
                ret = poll(pfd, 1, -1);

                /* Bail on errors (for simplicity) */
                assert(ret > 0);
                assert(pfd[0].revents & POLLIN);

                /* We have a valid signal, read the info from the fd */
                struct signalfd_siginfo info;
                bytes = read(sfd, &info, sizeof(info));
                assert(bytes == sizeof(info));

                unsigned sig = info.ssi_signo;
                unsigned user = info.ssi_uid;

                if (sig == SIGUSR1) {
                    ++usrcnt;
                    printf ("Got SIGUSR1 by POLL in thread: %d: Handler count: %d,  %d\n", GetCurrentThreadId(), susrcnt, usrcnt);
                }
        }

    /* Close the file descriptor if we no longer need it. */
    close (sfd);

    return NULL;
}


int main()
{

    const int numthreads = 1;
    sigset_t sset;
    struct sigaction act;
    int sleepval = 15;
    int pid;
    int i;
        int * a = 0;
    //*a = 1;
    int c=0;
    //c = 0;
    int b;


    printf("My PID: %d\n", getpid());
    printf("SIGSEGV: %d\nSIGFPE: %d\nSIGUSR1: %d\n", SIGSEGV, SIGFPE, SIGUSR1);
    //Create a thread for signal
    memset(&act, 0, sizeof act);
    act.sa_sigaction = User1;
    act.sa_flags    = SA_SIGINFO;

    //Set Handler for SIGUSR1 signal.
    if(sigaction(SIGUSR1, &act, NULL)<0) {
        fprintf(stderr, "sigaction failed\n");
        return 1;
    }


    //Set handler for SIGSEGV signal.
    act.sa_sigaction    = Segv1;
    sigaction(SIGSEGV, &act, NULL);

    //Set handler for SIGFPE (floating point exception) signal.
    act.sa_sigaction    = Fpe1;
    sigaction(SIGFPE, &act, NULL);


    sigemptyset(&sset);
    sigaddset(&sset, SIGUSR1);

    sigprocmask(SIG_UNBLOCK, &sset, NULL);

    pthread_t tid[numthreads];
    for(i=0;i<numthreads;++i)
        pthread_create(&tid[i], NULL, ThreadFunc, NULL);

    //Block the signal for main thread so that other thread handles the the signal.
    pthread_sigmask(SIG_BLOCK, &sset, NULL);

    sleep(numthreads/2);

    //Raise user signal SIGUSR1.
    //raise(SIGUSR1);
    pid = fork();
    if(pid) {
        while(sleepval) {
            sleepval = sleep(sleepval);
            if(sleepval)
                switch(wsig) {
                    case SIGSEGV:
                        printf("[Main] Segmenation fault in thread: %d\n", wtid);
                        exit(1);
                        break;
                    case SIGFPE:
                        printf("[Main] Floating point exception in thread: %d\n", wtid);
                        exit(1);
                        break;
                    case SIGUSR1:
                        printf("[Main] User 1 signal in thread: %d\n", wtid);
                        break;
                    default:
                        printf("[Main] Unhandled signal: %d in thread: %d\n", wsig, wtid);
                        break;
                }
        }

    } else {
         sleep(1); //To avoid race between signal handler and signal fd.

        for(i=0;i<10;++i) {
            //If sleep is not used, signal SIGUSR1 will be handled one time in parent
            //as other signals will be ignored while SIGUSR1 is being handled.
            sleep(1);
            //Problem is here. When the sleep(1) is commented out, it missed the signals.
            kill(getppid(), SIGUSR1);
        }
        return 0;
    }

    return 0;
}

在程序中，进程生成一个线程，该线程创建 signalfd 并开始使用 poll。然后进程产生一个子进程，该子进程将 SIGUSR1 发送到父进程。当以 1s 的间隔发送信号时，它会处理所有信号。然而，当 sleep 被移除时，它错过了通知。

我想知道如果 signalfd 正在处理相同的信号，它是否也会丢弃信号通知。另外，信号处理程序和 signalfd 之间的优先顺序是什么？

Answer 1

如果一个进程有多个标准(即:非实时)信号挂起，操作系统可能会决定将多个相同类型的信号合并为一个。

来自 POSIX :

2.4.1 Signal Generation and Delivery

[...]

If a subsequent occurrence of a pending signal is generated, it is implementation-defined as to whether the signal is delivered or accepted more than once in circumstances other than those in which queuing is required.

默认情况下，标准信号不排队。使标准信号排队的唯一方法是使用 sigqueue() 发出它们.