In my program,I want to write sth in system dirs,but i donot have enough privilege.
I think obtainning root privilege maybe the only way.But how to do?
Thanks.

for the program to have root privelages it must either be executed as root or be setuid root. being setuid root means that the program binary is owned by root, and executes as root even when a normal user runs it. to make and executable setuid root do this:

chown root executable
chmod a+xs executable

obviously you have to do those commands as root. i'm not sure if that's the only way of getting root permissions for your program, but it's pretty standard for programs that need root privelages but want to be run by ordinary users.