sigtest.c

#include <errno.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

int main_parent(pid_t child_pid) {
    sleep(1);

    struct timespec tsig;
    clock_gettime(CLOCK_MONOTONIC, &tsig);
    union sigval sv;
    sv.sival_int = 0;
    int r = sigqueue(child_pid, SIGUSR1, sv);
    if (r == -1) {
        perror("sigqueue(2");
        exit(1);
    }
    printf("sent signal at: %ld %ld\n", tsig.tv_sec, tsig.tv_nsec);
    return 0;
}

void handler(int signo, siginfo_t *info, void *ctx) {
    struct timespec tsig;
    clock_gettime(CLOCK_MONOTONIC, &tsig);

    printf("received signal at: %ld %ld\n", tsig.tv_sec, tsig.tv_nsec);
    exit(0);
}

int main_child(void) {
    struct sigaction act = { 0 };
    act.sa_flags = SA_SIGINFO;
    act.sa_sigaction = &handler;
    int r = sigaction(SIGUSR1, &act, NULL);
    if (r == -1) {
        perror("sigaction(2)");
        exit(1);
    }

    for (;;) {
        // try having or not having sched_yield in here; results are similar.
        // sched_yield();
    }
    return 0;
}

int main(int argc, char **argv) {
    pid_t pid = fork();
    if (pid == -1) {
        perror("fork(2)");
        exit(1);
    } else if (pid == 0) {
        exit(main_child());
    } else {
        int r = main_parent(pid);
        int status;
        pid_t ret = waitpid(pid, &status, 0);
        if (ret == -1) {
            perror("waitpid(2)");
            exit(1);
        }
        exit(r || status || 0);
    }
}