linux online source code:

http://elixir.free-electrons.com/linux/latest/source

virtual judge:

https://vjudge.net/

cppreference:

http://en.cppreference.com/w/

cplusplus:

http://www.cplusplus.com/reference/

gnu mirrors:

http://mirrors.ustc.edu.cn/gnu/

ibiblio public‘s library and digital archive:

https://www.ibiblio.org/

http://www.ibiblio.org/pub/

http://www.ibiblio.org/pub/Linux/system/daemons/watchdog/

工具类url汇总