【what is stubs?】
A stub is a small program routine that substitutes for a longer program, possibly to be loaded later or that is located remotely. For example, a program that uses Remote Procedure Calls ( RPC ) is compiled with stubs that substitute for the program that provides a requested procedure. The stub accepts the request and then forwards it (through another program) to the remote procedure. When that procedure has completed its service, it returns the results or other status to the stub which passes it back to the program that made the request.
大意：stub是一个段小程序，用于代替更大的程序（譬如延时加载的动态库、RPC）。
参考：http://whatis.techtarget.com/definition/stub
In Mac OSX, A symbol_stubs section contains symbol stubs, which are sequences of machine instructions (all the same size) used for lazily binding undefined function calls at runtime.
参考：《OSX Assemler Reference》