使用man查看C函数文档

运行：

man man

摘取到下面内容：

The table below shows the section numbers of the manual followed by  the
       types of pages they contain.

       1   Executable programs or shell commands
       2   System calls (functions provided by the kernel)
       3   Library calls (functions within program libraries)
       4   Special files (usually found in /dev)
       5   File formats and conventions eg /etc/passwd
       6   Games
       7   Miscellaneous  (including  macro  packages  and  conventions),  e.g.
           man(7), groff(7)
       8   System administration commands (usually only for root)
       9   Kernel routines [Non standard]

测试一下：

man 3 fopen

成功！

如果文档不全的话，就安装把：

sudo apt-get install manpages-dev

有时候需要下面命令来更新man的索引：

mandb -c

再测试一下：

man fopen

成功。