头文件中结构体互相引用的问题

先上代码看下错误的例子：

typedef struct _thread{
    int       id;                        /* friendly id               */
    pthread_t pthread;                   /* pointer to actual thread  */
    thpool_handle_t thpool_p;            /* access to thpool          */
} thread_t;

/* Threadpool
 * threadpool has many threads, and he should to access each threads, threads pointer array to save all threads pointer */
typedef struct _thpool{
    thread_t**   threads;                  /* pointer to threads        */
    volatile int num_threads_alive;      /* threads currently alive   */
    volatile int num_threads_working;    /* threads currently working */
    pthread_mutex_t  thcount_lock;       /* used for thread count etc */
    jobqueue*  jobqueue_p;               /* pointer to the job queue  */
} thpool_t, *thpool_handle_t;

编译提示：
./include/thread_pool.h:31:5: error: unknown type name ‘thpool_handle_t’

修改如下解决：

struct _thread;
struct _thpool;
typedef struct _thread thread_t;
typedef struct _thpool thpool_t, *thpool_handle_t;

typedef struct _thread{
    int       id;                        /* friendly id               */
    pthread_t pthread;                   /* pointer to actual thread  */
    thpool_handle_t thpool_p;            /* access to thpool          */
} thread_t;

/* Threadpool
 * threadpool has many threads, and he should to access each threads, threads pointer array to save all threads pointer */
typedef struct _thpool{
    thread_t**   threads;                  /* pointer to threads        */
    volatile int num_threads_alive;      /* threads currently alive   */
    volatile int num_threads_working;    /* threads currently working */
    pthread_mutex_t  thcount_lock;       /* used for thread count etc */
    jobqueue*  jobqueue_p;               /* pointer to the job queue  */
} thpool_t, *thpool_handle_t;