linux下信号量的使用方法:
http://www.ibm.com/developerworks/cn/linux/l-ipc/part4/
http://blog.youkuaiyun.com/wwwsq/archive/2006/09/04/1175215.aspx
http://www.ibm.com/developerworks/cn/linux/l-ipc/part4/
http://blog.youkuaiyun.com/wwwsq/archive/2006/09/04/1175215.aspx