線程鎖并不是想象的那樣可靠

今天測試時發現程式經常崩潰，代碼大意如下：

static  void  fext_queue(ImageBuffer* pSrc)
{
    //>=0表示初始化完成。
    if (g_FextThreadQueue == NULL
        && g_FextThreadQueue->queue_length < GH_FEXT_QUEUE_LENGTH)
    {
        pthread_mutex_lock(&(g_FextThreadQueue->queue_mutex));
 
　　　　　//------------------
 
        g_FextThreadQueue->queue_length ++;
 
        pthread_mutex_unlock(&(g_FextThreadQueue->queue_mutex));
    }
}      

這是怎麼回事？後來把queue_length輸出，發現已經越界了．也就是在加鎖期間，值又變化了．是以正确寫法是：

static  void  fext_queue(ImageBuffer* pSrc)
{
    //>=0表示初始化完成。
    if (g_FextThreadQueue == NULL
        && g_FextThreadQueue->queue_length < GH_FEXT_QUEUE_LENGTH)
    {
        pthread_mutex_lock(&(g_FextThreadQueue->queue_mutex));
 
        //從外界進來的時候，有可能增加了．
        if (g_FextThreadQueue->queue_length >= GH_FEXT_QUEUE_LENGTH)
        {
            g_FextThreadQueue->queue_length = 0;
        }
 
        //-------------------------
 
        g_FextThreadQueue->queue_length ++;
 
        pthread_mutex_unlock(&(g_FextThreadQueue->queue_mutex));
    }
}      

或者這樣寫：

static  void  fext_queue(ImageBuffer* pSrc)
{
    //>=0表示初始化完成。
    if (g_FextThreadQueue == NULL)
    {
        return;
    }
 
    pthread_mutex_lock(&(g_FextThreadQueue->queue_mutex));
 
    if (g_FextThreadQueue->queue_length >= GH_FEXT_QUEUE_LENGTH)
    {
        g_FextThreadQueue->queue_length = 0;
    }
 
    //-------------------------
 
    g_FextThreadQueue->queue_length ++;
 
    pthread_mutex_unlock(&(g_FextThreadQueue->queue_mutex));
}