信号灯的PV操作

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

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

int semid;

void sem_init();
void sem_p();
void sem_v();
void sem_delete();

int main(int argc, char *argv[])
{
    sem_init();
    char ch = 'A';

    if (argc > 1)
    {
        ch = 'B';
        sleep(2);
    }

    int i;
    for(i = 0; i < 10; i++)
    {
        sem_p();
        printf("%c", ch);
        fflush(stdout);
        sleep(1);
        printf("%c", ch);
        fflush(stdout);
        sleep(1);
        sem_v();
    }
    sem_delete();

    return 0;
}

void sem_init()
{
    semid = semget((key_t)123, 1, IPC_CREAT | 0666);
    if (-1 == semid)
    {
        perror("semget");
    exit(1);
    }
    union semun sem;
    sem.val = 1;
    int ret = semctl(semid, 0, SETVAL, sem);
    if (-1 == ret)
    {
        perror("semctl");
    exit(2);
    }
}

void sem_p()
{
    struct sembuf buffer;
    buffer.sem_num = 0;
    buffer.sem_op = -1;
    buffer.sem_flg = SEM_UNDO;
    int ret = semop(semid, &buffer, 1);
}

void sem_v()
{
    struct sembuf buffer;
    buffer.sem_num = 0;
    buffer.sem_op = 1;
    buffer.sem_flg = SEM_UNDO;
    int ret = semop(semid, &buffer, 1);
}

void sem_delete()
{
    union semun sem;
    int ret = semctl(semid, 0, IPC_RMID, sem);
    if (-1 == ret)
    {
        perror("semctl");
        exit(3);
    }
}