fork调用的内核实现:
http://www.cnblogs.com/huangwei/archive/2010/05/21/1740794.html
http://blog.csdn.net/hello_bravo_/article/details/52887184
http://www.cnblogs.com/huangwei/archive/2010/05/21/1740794.html
http://blog.csdn.net/hello_bravo_/article/details/52887184