Static Linking就是将多个relocatable object file 组合转换成一个executable object file。
这个过程中，linker主要完成两件事情：
1. Symbol resolution.
Object files define and reference symbols. The purpose of symbol resolution is to
associate each symbol reference with exactly one symbol definition.
2. Relocation
. Compilers and assemblers generate code and data sections that start at address zero. The
linker relocates these sections by associating a memory location with each symbol definition, and
then modifying all of the references to those symbols so that they point to this memory location.