反省：pthread_barrier_init(),pthread_barrier_destroy()和pthread_barrier_wait()

     #include <pthread.h>

     int pthread_barrier_destroy(pthread_barrier_t *barrier);

     int pthread_barrier_init(pthread_barrier_t *barrier, const pthread_barrierattr_t *attr, int count);

     int pthread_barrier_wait(pthread_barrier_t *barrier);

     The pthread_barrier_init() function will initialize barrier with

     attributes specified in attr, or if it is NULL, with default attributes.

     The number of threads that must call pthread_barrier_wait() before any of

     the waiting threads can be released is specified by count.  The

     pthread_barrier_destroy() function will destroy barrier and release any

     resources that may have been allocated on its behalf.

     The pthread_barrier_wait() function will synchronize calling threads at

     barrier.  The threads will be blocked from making further progress until

     a sufficient number of threads calls this function.  The number of

     threads that must call it before any of them will be released is deter-

     mined by the count argument to pthread_barrier_init().  Once the threads

     have been released the barrier will be reset

确实这几个函数平常都没有用到，一般的作线程间的同步也就用到了mutex和condition_variable，还真的没有注意到这些个函数。虽然现在知道了这几个函数的功能，但是还没有能和实际情况联系起来，不知道在什么样的环境中需要用到。 按照描述，这个barrier的作用就像集体接力赛一样，一群人各自按照各自的路线像同一地点进发(pthread_barrier_wait())，当每个人到达该地点时就停下来等待其他的人到达，当所有的人(count个人)都到达后就再次出发干各自的事去了(可能是另一个目的地，也可能比赛就结束了，也可能……，总之是全新的一个开始)。

 

另外几个关于barrier的属性的几个函数：

      #include <pthread.h>

     int pthread_barrierattr_destroy(pthread_barrierattr_t *attr);

     int pthread_barrierattr_getpshared(const pthread_barrierattr_t *attr, int *pshared);

     int pthread_barrierattr_init(pthread_barrierattr_t *attr);

     int pthread_barrierattr_setpshared(pthread_barrierattr_t *attr, int pshared);

     The pthread_barrierattr_init() function will initialize attr with default

     attributes.  The pthread_barrierattr_destroy() function will destroy attr

     and release any resources that may have been allocated on its behalf.

     The pthread_barrierattr_getpshared() function will put the value of the

     process-shared attribute from attr into the memory area pointed to by

     pshared.  The pthread_barrierattr_setpshared() function will set the

     process-shared attribute of attr to the value specified in pshared.  The

     argument pshared may have one of the following values:

     PTHREAD_PROCESS_PRIVATE  The barrier object it is attached to may only be

            accessed by threads in the same process as the 

   one that created the object.

     PTHREAD_PROCESS_SHARED   The barrier object it is attached to may be 

   accessed by threads in processes other than the 

   one that created the object.

问题：如何在其他的进程的线程中来访问某个属性值为PTHREAD_PROCESS_SHARED   的 barrier呢？？？