多线程的start_routine指明了CPU的一个可执行流；
多线程的上线文说明了执行流现在的运行情况；
多线程的属性说明了上线文的数据结构。
https://computing.llnl.gov/tutorials/pthreads/
- pthread_create arguments:
- thread: An opaque, unique identifier for the new thread returned by the subroutine.
- attr: An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values.
- start_routine: the C routine that the thread will execute once it is created.
- arg: A single argument that may be passed to start_routine. It must be passed by reference as a pointer cast of type void. NULL may be used if no argument is to be passed.