第 22 章 信号：高级特性 (Signals: Advanced Features)

$ ulimit -c unlimited            # 允许任意大小 core
$ echo "core.%p" | sudo tee /proc/sys/kernel/core_pattern
# 之后所有 core 文件名形如 core.12345

static sigjmp_buf env;
static void handler(int sig) { siglongjmp(env, 1); }
int main(void) {
    signal(SIGSEGV, handler);
    volatile int *bad = NULL;
    if (sigsetjmp(env, 1) == 0) {
        *bad = 42;  /* 触发 SIGSEGV */
        printf("Should not reach here\n");
    } else {
        printf("Caught SIGSEGV via longjmp\n");
    }
}

// 摘自《The Linux Programming Interface》第 22 章（Listing 22-1）
sighandler_t signal(int sig, sighandler_t handler) {
    struct sigaction newDisp, prevDisp;
    newDisp.sa_handler = handler;
    sigemptyset(&newDisp.sa_mask);
#ifdef OLD_SIGNAL
    newDisp.sa_flags = SA_RESETHAND | SA_NODEFER;  /* 旧 SysV 语义 */
#else
    newDisp.sa_flags = SA_RESTART;                 /* 可靠信号语义 */
#endif
    if (sigaction(sig, &newDisp, &prevDisp) == -1)
        return SIG_ERR;
    return prevDisp.sa_handler;
}

// 发送：sigqueue(pid, sig, value)
union sigval { int sival_int; void *sival_ptr; };
int sigqueue(pid_t pid, int sig, const union sigval value);

// 接收：SA_SIGINFO handler
struct sigaction sa = {0};
sa.sa_sigaction = handler;         /* 不是 sa_handler */
sa.sa_flags = SA_SIGINFO | SA_RESTART;
sigaction(SIGRTMIN + 1, &sa, NULL);

// handler
static void handler(int sig, siginfo_t *info, void *ucontext) {
    printf("got signal %d, value = %d, from PID %d\n",
           sig, info->si_value.sival_int, info->si_pid);
}

// 摘自《The Linux Programming Interface》第 22 章（Listing 22-2）
union sigval sv;
for (j = 0; j < numSigs; j++) {
    sv.sival_int = sigData + j;
    sigqueue(getLong(argv[1], 0, "pid"), sig, sv);
}

// 摘自《The Linux Programming Interface》第 22 章（Listing 22-5 简化）
sigset_t origMask, blockMask;
sigfillset(&blockMask);
sigdelset(&blockMask, SIGINT);    /* 不阻塞 SIGINT/SIGTERM */
sigdelset(&blockMask, SIGTERM);

sigprocmask(SIG_BLOCK, &blockMask, &origMask);  /* 阻塞多数信号 */

while (!gotSigquit) {
    /* 临界区 */
    sigsuspend(&origMask);          /* 原子恢复原掩码 + 等待 */
    /* 此处是 handler 已返回 */
}

sigset_t allSigs;
sigfillset(&allSigs);
sigprocmask(SIG_BLOCK, &allSigs, NULL);  /* 阻塞所有信号 */

siginfo_t si;
for (;;) {
    int sig = sigwaitinfo(&allSigs, &si);
    if (sig == SIGINT || sig == SIGTERM) break;
    printf("got signal %d from PID %ld, value = %d\n",
           sig, (long) si.si_pid, si.si_value.sival_int);
}

#include <sys/signalfd.h>
sigset_t mask;
sigemptyset(&mask);
sigaddset(&mask, SIGINT);
sigaddset(&mask, SIGTERM);

/* 关键：必须先 sigprocmask 阻塞信号，否则信号仍按 handler 处理 */
sigprocmask(SIG_BLOCK, &mask, NULL);

int sfd = signalfd(-1, &mask, SFD_CLOEXEC);

/* sfd 可加入 epoll */
struct epoll_event ev = { .events = EPOLLIN, .data.fd = sfd };
epoll_ctl(epfd, EPOLL_CTL_ADD, sfd, &ev);

/* 事件循环 */
while (1) {
    int n = epoll_wait(epfd, events, MAX, -1);
    for (i = 0; i < n; i++) {
        if (events[i].data.fd == sfd) {
            struct signalfd_siginfo si;
            read(sfd, &si, sizeof(si));
            if (si.ssi_signo == SIGINT) goto cleanup;
        } else {
            /* 其他 I/O */
        }
    }
}