进程间信号量同步问题

#在学习linux信号量时，想用来做两个进程之间的同步，但出现的问题想不明白，某个进程等不到信号量，代码如下：

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/sem.h>
#include <string.h>
#include <sys/shm.h>

#define REPEART_NUM 10

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

int sem_p_op(int sem_id)
{
struct sembuf sem;
sem.sem_num = 0;
sem.sem_op = -1;
sem.sem_flg = SEM_UNDO;

if(semop(sem_id,&sem,1) < 0)
{
	printf("sem_p error\n");
	return -1;
}

return 1;

}

int sem_v_op(int sem_id)
{
struct sembuf sem;
sem.sem_num = 0;
sem.sem_op = 1;
sem.sem_flg = SEM_UNDO;

if(semop(sem_id,&sem,1) < 0)
{
	printf("s