I want to see the man page of some c standard library functions in Mac OS X. When I typed in man setsockopt, it returns: No manual entry for setsockopt. Does that mean I haven't installed man pages for c standard? How should I install it?
macOS is not feature complete out of the box when it comes to development. Installing the Xcode command line tools will give you what you need, as well as a compiler etc.
For macOS and newer versions of Mac OS X, installing the Command Line Tools package is done with the command xcode-select --install.
For older versions of Mac OS X you'll need to install Xcode (free in the Mac App Store) first. Then you can install the Command Line Tools module after you installed Xcode, by going into Xcode->Preferences->Downloads->Components