12-4

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <fcntl.h>

enum
{
    BASE = 10,
    NAMELEN = 20
};

int
main(int argc, char *argv[])
{
    if (argc < 2) {
        return 0;
    }
    int count = strtol(argv[1], NULL, BASE);
    int semid = semget(IPC_PRIVATE, count + 1, 0666|IPC_CREAT);
    int i;
    for (i = 0; i < count; i++) {
        if (!fork()) {
            semop(semid, (struct sembuf[]){{count, 1, 0}}, 1);
            int num;
            while (semop(semid, (struct sembuf[]){{i, -1, 0}}, 1) >= 0) {
                if (scanf("%d", &num) <= 0) {
                    semctl(semid, 0, IPC_RMID, 0);
                    break;
                }
                printf("%d %d\n", i, num);
                fflush(stdout);
                num = num >= 0 ? num % count : num % count + count;
                semop(semid, (struct sembuf[]){{num % count, 1, 0}}, 1);
            }
            _exit(0);
        }
    }
    semop(semid, (struct sembuf[]){{count, -count, 0}}, 1);
    semop(semid, (struct sembuf[]){{0, 1, 0}}, 1);
    while (wait(NULL) >= 0);
    return 0;
}