system V semaphore实现应用层lock和unlock

以下代码段是在nvram读写时用的lock和unlock函数及实现，只要在get和set nvram之前和之后加上_nvram_lock()和_nvram_unlock(),就能实现当前进程读写nvram时不被其他进程打断。
参考的博文：http://blog.csdn.net/xiajun07061225/article/details/8475738
参考文件：Linux信号量学习.pdf

#include <sys/ipc.h>
#include <sys/sem.h>//包含信号量定义的头文件
#define NVRAM_SEM_KEY 0XAEEAEE

//联合类型semun定义
union semun{
    int val;
    struct semid_ds *buf;
    unsigned short *array;
};
static int sem_id;//信号量ID


//函数：设置信号量的值
static int set_semvalue(void)
{
    union semun sem_union;
    sem_union.val = 1;

    if(semctl(sem_id,0,SETVAL,sem_union))
        return 0;

    return 1;
}

//函数：删除信号量
static void del_semvalue(void)
{
    union semun sem_union;

    if(semctl(sem_id,0,IPC_RMID,sem_union))
        printf("NVRAMERRO:Failed to delete semaphore\n");
}

//函数：信号量P操作：对信号量进行减一操作
static int semaphore_p(void)
{
    struct sembuf sem_b;

    sem_b.sem_num = 0;//信号量编号
    sem_b.sem_op = -1;//P操作
    sem_b.sem_flg = SEM_UNDO;

    if(semop(sem_id,&sem_b,1) == -1)
    {
        printf("NVRAMERRO:semaphore_p failed\n");
        return 0;
    }

    return 1;
}

//函数：信号量V操作：对信号量进行加一操作
static int semaphore_v(void)
{
    struct sembuf sem_b;

    sem_b.sem_num = 0;//信号量编号
    sem_b.sem_op = 1;//V操作
    sem_b.sem_flg = SEM_UNDO;

    if(semop(sem_id,&sem_b,1) == -1)
    {
        printf("NVRAMERRO:semaphore_v failed\n");
        return 0;
    }

    return 1;

}


static int _lock()
{
    //取得一个已有信号量的键
    sem_id = semget((key_t)NVRAM_SEM_KEY,0,0666 );
    if((sem_id<0)&&(errno==ENOENT))
    {
        printf("NVRAM:MUST creat semaphore\n");
        //创建一个新的信号量或者是取得一个已有信号量的键
        sem_id = semget((key_t)NVRAM_SEM_KEY,1,0666 | IPC_CREAT);
        if(sem_id < 0)
        {
            printf("NVRAMERRO:failed to creat semaphore\n");
            return 0;
        }
        if(!set_semvalue())
        {
            printf("NVRAMERRO:failed to initialize semaphore\n");
            return 0;
        }
    }
    else if(sem_id<0)
    {
        printf("NVRAMERRO:failed to get semaphore\n");
        return 0;
    }
    if(!semaphore_p())
    {
        printf("NVRAMERRO:failed to lock\n");
        return 0;
    }

    return 1;
}

static int _unlock()
{
    if(!semaphore_v())
    {
        printf("NVRAMERRO:failed to unlock\n");
        return 0;
    }
    return 1;
}


static int _nvram_lock()
{
    int i=0;

    while (i++ < MAX_LOCK_WAIT) {
        if(_lock())
            return 1;
        else
            usleep(500000);
    }
    return 0;
}
/*nvram file unlock*/
static int _nvram_unlock()
{
    int i=0;

    while (i++ < MAX_LOCK_WAIT) {
        if(_unlock())
            return 1;
        else
            usleep(500000);
    }
    return 0;
}