进程间通信实例4> 信号量的用法

基本函数：

 

 

 


//semaphore function


int create_sem(key_t key, int members)
{
    int cntr;
    union semun semopts;
    int semid;
    
    jprintf("Attempting to create new semaphore set with %d members\n",members);
    if((semid = semget(key, members, IPC_CREAT|IPC_EXCL|0666)) == -1)
    {
        jprintf("Semaphore set already exists!\n");
        return -1;
    }
    semopts.val = 1;    //最大的资源个数，这里就是互斥锁的功能，所以就是 0 | 1 , 二值信号量

    /* Initialize all members (could be done with SETALL) */
    for(cntr=0; cntr<members; cntr++)
        semctl(semid, cntr, SETVAL, semopts); //cntr 这里是信号量集里面得信号灯的索引！

    
    return semid;
    
}

int open_sem(key_t key)
{
    int semid;
/* Open the semaphore set - do not create! */
    if((semid = semget(key, 0, 0666)) == -1)
    {
        jprintf("Semaphore set does not exist!\n");
    }
    return semid;
}



int lock_sem(int semid, int member)
{
    struct sembuf sem_lock={ member, -1, SEM_UNDO};//-1 :decrease, 1:increase

    int semval;
    jprintf(" ----------------------------------in begin to get semaphore ---------------------------------\n");
    semval = semctl(semid, member, GETVAL, 0);
    jprintf("current value = %d\n",semval);
    if( semop(semid, &sem_lock, 1) == -1) //it will be blocked until get the access

    {
        fprintf(stderr, "Lock failed\n");
        return -1;
    }
    else
        jprintf("Semaphore resources decremented by one (locked)\n");
    semval = semctl(semid, member, GETVAL, 0);
    jprintf("semval for member %d is %d\n", member, semval);
    jprintf("-------------------------------------Has get semaphore -------------------------------------\n");
    return 0;
}

int unlock_sem(int semid, int member)
{
    struct sembuf sem_unlock={ member, 1, SEM_UNDO };
    int semval;
    
    /* Is the semaphore set locked? */
    semval = semctl(semid, member, GETVAL, 0);
    jprintf("\n-----------------------in unlock_sem ----------------------------\n");
    jprintf("current semaphore value = %d\n",semval);
        
    if(semval > 0) {
        jprintf("it has been unlocked , I did nothing \n");
        return 0;
    }
/* Attempt to lock the semaphore set */
    if((semop(semid, &sem_unlock, 1)) == -1)
    {
        fprintf(stderr, "Unlock failed\n");
        return -1;
    }
    else
        printf("Semaphore resources incremented by one (unlocked)\n");
    semval = semctl(semid, member, GETVAL, 0);
    jprintf("semval for member %d is %d\n", member, semval);
    jprintf("----------------------------unlock sem --------------------------------\n");
    return 0;
}


例子：

 

 

 

 

/**
 * query those files which are downloading from share memory to get status info
 * generally , conf_name is /etc/shm.conf
    for CGI , should have a version for Socket query
    int QueryShmForClient(DownloadFileInfoList *DList ,const char *conf_name , const char *client_ip);
    
 **/
int QueryShm(DownloadFileInfoList *DList, const char *conf_name) //etc/shm.conf

{
    char ptr[128];
    FILE *fp = NULL;
    
    int fd = 0;
    char *seg = NULL;
    key_t key;
    int shmid;    //share memory ID

    int semid;    //semaphore ID

    void *segptr = NULL;
    int downloading_num = 0;
    int rc = 0;
    
    assert(DList);
    fp = fopen(conf_name,"rt");
    if(!fp) {
        fatal_err("can't open /etc/shm.conf\n");
        LogMesg("can't open /etc/shm.conf , maybe /etc/shm.conf doesn't exist any longer \n");
        
        return SERVER_CANNOT_OPEN_SHM_CONF;
    }
    assert(fp);
    fd = fileno(fp);
    //lockf(fd,F_LOCK,0);

    readw_lock(fd,0,SEEK_SET,0);
    while(fgets(ptr,sizeof(ptr),fp)) {    //get a line

        DownloadFileInfo *doingJob_entry = (DownloadFileInfo *)malloc(sizeof(DownloadFileInfo));
        
        //assert(doingJob_entry);

        if(!doingJob_entry) {
            fatal_err("can't malloc DownloadFileInfo struct entry \n");
            continue;
        }
                
        jprintf("ptr = %s\n",ptr);
        seg = strrchr(ptr,':');
        if(seg == NULL) {
            jprintf("can't find \':\' in %s , maybe this line in shm.conf has problem ,please check %s\n",ptr,SHM_CONF);
            continue;
        }
        *seg = '\0';
        //now ptr should be fullpathname ,ie, /harddisk/hd~2/data/public/bob/linux-2.4.21.tar.gz

        
        //---------------------------Start reading share memory----------------------------

        key = ftok(ptr,0);
        
    // ----------------------------------semaphore mutex lock -------------------------------------    

    #if 0
        semid = open_sem(key);    //get a ID of a existing semaphore set

        if(semid < 0) {
            fatal_err("get semaphore failure , job name (%s) \n",ptr);
            continue;
        }
        rc = lock_sem(semid ,0);
        if(rc < 0) {
            fatal_err("Sorry ,lock failed for(%s) \n",ptr);
            continue;
        }
        jprintf("haha , you have get the mutex lock , you can read the share memory segment !\n");
    #endif    
    // ----------------------------------------------------------------------------------------------

        
             if((shmid = shmget(key, SEGSIZE, 0444)) == -1) {
                    fatal_err("can't get share memory ,key(%s) \n",key);
                    LogMesg("the key (%s) of file (%s) 's share memory looks that has problem(shmget error ) ,please check \n",key,SHM_CONF);
                      //return SERVER_SHMGET_FAIL;

                      continue;
                }
          else {
            jprintf("shmid = %d\n",shmid);
                    jprintf("has attached memory segment\n");
          }
        /* Attach (map) the shared memory segment into the current process */
        if((segptr = shmat(shmid, 0, 0)) == NULL) {
            fatal_err("can't shmat share memory ,key (%s) \n",key);
            LogMesg("the key (%s) of file (%s) 's share memory looks that has problem(shmat error ) ,please check \n",key,SHM_CONF);
            //return SERVER_SHMAT_ERROR;

            continue;
        }
        //memcpy(doingJob_entry,(DownloadFileInfo *)segptr,sizeof(DownloadFileInfo));    ////fill the doingJob_entry , if problem ,single assign ,bob 2005-12-29 10:51

        *doingJob_entry = *((DownloadFileInfo *)segptr);
        shmdt(segptr);
        
    //unlock the mutex lock

    #if 0
        unlock_sem(semid,0);
    #endif
        AddDownloadFileInfoList(DList,doingJob_entry);    //add the node into the List

        downloading_num++;
    } //endof while(fgets(ptr,sizeof(ptr),fp))

    
    //lockf(fd,F_ULOCK,0);

    un_lock(fd,0,SEEK_SET,0);
    fclose(fp);
    if(downloading_num==0) {
        jprintf("there is no job downloading in share memory \n");
    }
    else {
        jprintf("there is %d files downloading \n",downloading_num);
    }
    
    return 0;
}