问关于sigprocmask掩码()、SIG_BLOCK和SIG_SETMASK

EN

#include <signal.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>

static void sig_quit(int signo) {
    printf("caught SIGQUIT\n");
    if (signal(SIGQUIT, SIG_DFL) == SIG_ERR)
        printf("can't reset SIGQUIT\n");
}

int main() {
    sigset_t newmask, oldmask, pendmask;

    if (signal(SIGQUIT, sig_quit) == SIG_ERR)
        printf("can't catch SIGQUIT\n");

    /* Block SIGQUIT and save current signal mask. */
    sigemptyset(&newmask);
    sigaddset(&newmask, SIGQUIT);

    if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)
       printf("SIG_BLOCK error");
    sleep(5);   /* SIGQUIT here will remain pending */
    if (sigpending(&pendmask) < 0)
       printf("sigpending error");
    if (sigismember(&pendmask, SIGQUIT))
       printf("\nSIGQUIT pending\n");

    /* Restore signal mask which unblocks SIGQUIT. */
    if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0)
       printf("SIG_SETMASK error");
    printf("SIGQUIT unblocked\n");
    sleep(5);   /* SIGQUIT here will terminate with core file */

    return 0;
}