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

#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
    /* union semun is defined by including <sys/sem.h> */
#else
    /* according to X/OPEN we have to define it ourselves */
    union semun {
        int val;                    /* value for SETVAL */
        struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
        unsigned short int *array;  /* array for GETALL, SETALL */
        struct seminfo *__buf;      /* buffer for IPC_INFO */
    };
#endif

/* After the #includes, the function prototypes and the global variable, we come to the

 main function. There the semaphore is created with a call to semget, which returns the

 semaphore ID. If the program is the first to be called (i.e. it‘s called with a parameter

 and argc > 1), a call is made to set_semvalue to initialize the semaphore and op_char is

 set to X. */



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

static int set_semvalue(void);
static void del_semvalue(void);
static int semaphore_p(void);
static int semaphore_v(void);
static int sem_id;
int main(int argc, char *argv[])

{    int i;
    int pause_time;
    char op_char = ‘O‘;
    srand((unsigned int)getpid());
    sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT);
    if (argc > 1) {
        if (!set_semvalue()) {
            fprintf(stderr, "Failed to initialize semaphore\n");
            exit(EXIT_FAILURE);
        }
        op_char = ‘X‘;
        sleep(2);
    }



/* Then we have a loop which enters and leaves the critical section ten times.
 There, we first make a call to semaphore_p which sets the semaphore to wait, as
 this program is about to enter the critical section. */
    for(i = 0; i < 10; i++) {        
        if (!semaphore_p()) exit(EXIT_FAILURE);
        printf("%c", op_char);fflush(stdout);
        pause_time = rand() % 3;
        sleep(pause_time);
        printf("%c", op_char);fflush(stdout);
/* After the critical section, we call semaphore_v, setting the semaphore available,
 before going through the for loop again after a random wait. After the loop, the call
 to del_semvalue is made to clean up the code. */
        if (!semaphore_v()) exit(EXIT_FAILURE);     

        pause_time = rand() % 2;
        sleep(pause_time);
    }    
    printf("\n%d - finished\n", getpid());
    if (argc > 1) {    
        sleep(10);
        del_semvalue();
    }      

    exit(EXIT_SUCCESS);
}

/* The function set_semvalue initializes the semaphore using the SETVAL command in a
 semctl call. We need to do this before we can use the semaphore. */
static int set_semvalue(void)
{
    union semun sem_union;
    sem_union.val = 1;
    if (semctl(sem_id, 0, SETVAL, sem_union) == -1) return(0);
    return(1);
}
/* The del_semvalue function has almost the same form, except the call to semctl uses
 the command IPC_RMID to remove the semaphore‘s ID. */
static void del_semvalue(void){
    union semun sem_union;
    if (semctl(sem_id, 0, IPC_RMID, sem_union) == -1)
        fprintf(stderr, "Failed to delete semaphore\n");
}
/* semaphore_p changes the semaphore by -1 (waiting). */
static int semaphore_p(void){
    struct sembuf sem_b;
    sem_b.sem_num = 0;
    sem_b.sem_op = -1; /* P() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_p failed\n");
        return(0);
    }
    return(1);
}

/* semaphore_v is similar except for setting the sem_op part of the sembuf structure to 1,
 so that the semaphore becomes available. */
static int semaphore_v(void)
{
    struct sembuf sem_b;
    sem_b.sem_num = 0;
    sem_b.sem_op = 1; /* V() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_v failed\n");
        return(0);
    }
    return(1);
}