13-3-ipc_private.c

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

union semun
{
    int val;
    struct semid_ds* buf;
    unsigned short int* array;
    struct seminfo* __buf;
};

//op为-1时执行P操作， op为1时执行V操作
void pv(int sem_id, int op)
{
    struct sembuf sem_b;
    sem_b.sem_num = 0;
    sem_b.sem_op  = op;
    sem_b.sem_flg = SEM_UNDO;
    semop(sem_id, &sem_b, 1);
};

int main(int argc, char *argv[])
{
    int sem_id = semget(IPC_PRIVATE, 1, 0666);

    union semun sem_un;
    sem_un.val  = 1;
    semctl(sem_id, 0, SETVAL, sem_un);

    pid_t id = fork();
    if (id < 0) 
    {
        return 1;
    }
    else if (id == 0) 
    {
        printf("child try to get binary sem\n");
        //在父、子进程间共享IPC_PRIVATE信号量的关键就在于二者都可以操作该信号量的标识符
        //sem_id
        pv(sem_id, -1);
        printf("child get the sem and would release it after 5 seconds\n");
        sleep(5);
        pv(sem_id, 1);
        exit(0);
    }
    else
    {
        printf("parent try to get binary sem\n");
        pv(sem_id, -1);
        printf("parent get the same and would release it after 5 seconds\n");
        sleep(5);
        pv(sem_id, 1);
    }

    waitpid(id, NULL, 0);
    semctl(sem_id, 0, IPC_RMID, sem_un);  //删除信号量
    return 0;
}