linux OSlab4 添加自定义系统调用

http://blog.csdn.net/ly01kongjian/article/details/8947285

http://www.cnblogs.com/hoys/archive/2011/04/07/2008854.html

http://blog.csdn.net/yangdelong/article/details/5479070

http://www.360doc.com/content/13/0308/14/11722082_270168965.shtml

getpid系统调用的实现

linux-2.6.31起，struct task_struct定义在include/linux/sched.h中，原来task_struct结构体定义有所改动，将uid和euid等挪到 cred中，见include/linux/cred.h 和include/linux/cred.h