请问 为啥不能运行 python多线程 multiprocessing

我做了一个简单的多线程测试，代码如下：

import multiprocessing

def start():
    print('------这是多线程函数-----')

p = multiprocessing.Process(target=start)
p.start()

结果却报错如下：

RuntimeError: 
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

python的多线程使用部分要用main包裹,不然会出错
(而且这个包是多进程)