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linux的thread有两种属性，分别是

joinable和detached。

我们看下man对于pthread_create说明里的notes部分：

A thread may either be joinable or detached.  If a thread is joinable, then another thread can call pthread_join(3) to wait  for  the
       thread  to terminate and fetch its exit status.  Only when a terminated joinable thread has been joined are the last of its resources
       released back to the system.  When a detached thread terminates, its resources are automatically released back to the system:

这两者的区别在于，joinable的thread只有被别的线程执行pthread_join，资源才能被系统回收，另外可以获取该线程的退出码。

detached的thread在退出时，如thread_exit，资源会自动被系统回收。

默认创建的thread都是joinable的，除非在thread_create里特殊指定，或者在thread运行后执行thread_detach，就可以变为detached线程。