例如：想要查看 c 中 sleep 这个函数的文档
$ man 3 sleep
Section 列表
Section Description
1 General commands
2 System calls
3 Library functions, covering in particular the C standard library
4 Special files (usually devices, those found in /dev) and drivers
5 File formats and conventions
6 Games and screensavers
7 Miscellanea
8 System administration commands and daemons