#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <signal.h>

enum { INTEGER_BASE = 10 };

volatile sig_atomic_t flag = 0;
pid_t pid1, pid2;

void
handler(int s)
{
    flag = 1;
}

void
work(int serial, FILE *p1, FILE *p2, int maximum)
{
    pid_t pid;

    int val;
    while (1) {
        sigset_t s2;
        sigemptyset(&s2);
        while (!flag) {
            sigsuspend(&s2);
        }
        flag = 0;
        //fscanf(p1, "%d", &pid);
        if (getpid() == pid1) {
            pid = pid2;
        } else {
            pid = pid1;
        }
        if (pid == -1) {
            break;
        }
        fscanf(p1, "%d", &val);

        if (val >= maximum) {
            fprintf(p2, "%d\n", -1);
            kill(pid, SIGUSR1);
            break;
        }
        printf("%d %d\n", serial, val);
        fflush(stdout);
        //fprintf(p2, "%d\n", getpid());
        //fflush(p2);
        fprintf(p2, "%d\n", val+1);
        fflush(p2);
        kill(pid, SIGUSR1);

    }
    fclose(p1);
    fclose(p2);
    _exit(0);
}

void
execute(int ser, FILE *f1_r, FILE *f1_w, int maximum)
{
    pid_t pid = fork();
    if (pid == -1) {
        _exit(1);
    } else if (!pid) {
        if (ser == 1) {
            pid1 = getpid();
        } else {
            pid2 = getpid();
        }
        work(ser, f1_r, f1_w, maximum);
        _exit(0);
    }
    return;
}

int
main(int argc, char *argv[])
{
    FILE *f1_r, *f1_w;
    int p1[2], maximum, val = 1;
    pipe(p1);
    f1_w = fdopen(p1[1], "w");
    f1_r = fdopen(p1[0], "r");
    maximum = strtol(argv[1], NULL, INTEGER_BASE);

    sigset_t s1;
    sigemptyset(&s1);
    sigaddset(&s1, SIGUSR1);
    sigprocmask(SIG_BLOCK, &s1, NULL);
    sigaction(SIGUSR1, &(struct sigaction){ .sa_handler = handler, .sa_flags = SA_RESTART }, NULL);

    execute(1, f1_r, f1_w, maximum);
    execute(2, f1_r, f1_w, maximum);

    //fprintf(f1_w, "%d\n", pid2);
    //fflush(f1_w);
    fprintf(f1_w, "%d\n", val);
    fflush(f1_w);
    kill(pid1, SIGUSR1);

    fclose(f1_w);
    fclose(f1_r);
    waitpid(pid1, NULL, 0);
    waitpid(pid2, NULL, 0);
    printf("Done\n");
    fflush(stdout);
}

//код частично позаимствован из конспектов