因为elf格式中不同的section，不同的segment有不同的作用。这里只是注重整体ELF的作用，具体详细的用到了再看。

1  Elf files have a dual nature:

• Compilers, assemblers, and linkers treat the file as a set of logical sections described by a section header table.
• The system loader treats the file as a set of segments described by a program header table.

2 Elf defines the format of executable binary files. There are four different types:

• Relocatable ---- Created by compilers or assemblers. Need to be processed by the linker before running.
• Executable ---- Have all relocation done and all symbol resolved except perhaps shared library symbols that must be resolved at run time.
• Shared object ---- Shared library containing both symbol information for the linker and directly runnable code for run time.
• Core file ---- A core dump file.

格式为：

elf header------

segment table---------

sections/segments--------(一般一个segment可以包括n个section)

section table

具体细节可以使用objdump 或者 readelf命令查看。

更加细节的内容可以查看unit elf PPT.