linux下是这样定义的：

在linux的实现中pthread_t被定义为 "unsigned long int",參考这里

Windows下这样定义：

    /* 
     * Generic handle type - intended to extend uniqueness beyond 
     * that available with a simple pointer. It should scale for either 
     * IA-32 or IA-64. 
     */  
    typedef struct {  
        void * p;                   /* Pointer to actual object */  
        unsigned int x;             /* Extra information - reuse count etc */  
    } ptw32_handle_t;  
    typedef ptw32_handle_t pthread_t;