int semget(key_t key,int num_sems,int sem_flags);

int semop(int sem_id,struct sembuf *sem_opa,size_t num_sem_ops);

struct sembuf{  short sem_num;//除非使用一组信号量，否则它为0  short sem_op;//信号量在一次操作中需要改变的数据，通常是两个数，一个是-1，即P（等待）操作，  //一个是+1，即V（发送信号）操作。  short sem_flg;//通常为SEM_UNDO,使操作系统跟踪信号，  //并在进程没有释放该信号量而终止时，操作系统释放信号量
}; 

int semctl(int sem_id,int sem_num,int command,[union semun sem_union]);

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

#include<stdio.h>
#include<stdlib.h>
#include<sys/sem.h>
union semun
{int val;struct semid_ds *buf;unsigned short *array;
};
int sem_id;
int set_semvalue()
{union semun sem_union;    sem_union.val = 1;if(semctl(sem_id,0,SETVAL,sem_union)==-1)return 0;return 1;
}
int semaphore_p()
{struct sembuf sem_b;sem_b.sem_num = 0;sem_b.sem_op = -1;sem_b.sem_flg = SEM_UNDO;if(semop(sem_id,&sem_b,1)==-1){fprintf(stderr,"semaphore_p failed\n");return 0;}return 1;
}
int semaphore_v()
{struct sembuf sem_b;sem_b.sem_num = 0;sem_b.sem_op = 1;sem_b.sem_flg = SEM_UNDO;if(semop(sem_id,&sem_b,1)==-1){fprintf(stderr,"semaphore_v failed\n");return 0;}return 1;
}
void del_semvalue()
{//删除信号量
    union semun sem_union;if(semctl(sem_id,0,IPC_RMID,sem_union)==-1)fprintf(stderr,"Failed to delete semaphore\n");
}
int main(int argc,char *argv[])
{char message = 'x';//创建信号量sem_id = semget((key_t)1234,1,0666|IPC_CREAT);if(argc>1){//初始化信号量if(!set_semvalue()){fprintf(stderr,"init failed\n");exit(EXIT_FAILURE);}//参数的第一个字符赋给messagemessage = argv[1][0];}int i=0;for(i=0;i<5;i++){//等待信号量if(!semaphore_p())exit(EXIT_FAILURE);printf("%c",message);fflush(stdout);sleep(1);//发送信号量if(!semaphore_v())exit(EXIT_FAILURE);sleep(1);}printf("\n%d-finished\n",getpid());if(argc>1){//退出前删除信号量
        del_semvalue();}exit(EXIT_SUCCESS);
}