整体功能：如果调用的进程不是一个组的组长时，setsid创建一个新会话。调用进程将成为该新会话的组长，新进程组的组长，并且没有控制终端。-The overall function: if the call process is not a group leader, setsid create a new session. Call process will become the new session of the group leader, the new process group leader, and there is no control terminal.