Date: 2018.7.18
1、参考：
https://www.jianshu.com/p/863b279c941e
https://sourceware.org/binutils/docs/binutils/nm.html
2、Linux中目标文件解析的三个命令
所谓的目标文件，就是对C文件或者C++文件或者其他高级语言文件，采用编译器进行编译之后生成的中间文件，对这些目标文件采用链接器链接起来，就形成了可执行文件。
对于windows系统下，目标文件为.obj（cl.exe test.c test.obj）；对于Linux系统下，目标文件为.o（gcc -c test.c ）。
实质上，目标文件的类型为ELF文件。目标文件只是ELF文件的可重定位文件(Relocatable file)，ELF文件一共有4种类型：Relocatable file、Executable file、Shared object file和Core Dump file。
在Linux下，可以通过file命令查看文件类型。
（1）readelf命令：
man readelf： Displays information about ELF files，e.g header info.
常用命令：
-h -header-files: display info about elf format obj files
-S: Display source code intermixed with dissembly ,Implies -d.
-d: Display the assember mnemonics for the machine instrunctions from obj files.
（2）objdump命令：
man objdump: Display information about object files.
基本功能与readelf差不多。
objdump -S objfiles :查看汇编源代码信息
（3）nm命令：
man nm: List the symbols from object files.
nm [-A|-o|–print-file-name] [-a|–debug-syms]
[-B|–format=bsd] [-C|–demangle[=style]]
[-D|–dynamic] [-fformat|–format=format]
[-g|–extern-only] [-h|–help]
[-l|–line-numbers] [–inlines]
[-n|-v|–numeric-sort]
[-P|–portability] [-p|–no-sort]
[-r|–reverse-sort] [-S|–print-size]
[-s|–print-armap] [-t radix|–radix=radix]
[-u|–undefined-only] [-V|–version]
[-X 32_64] [–defined-only] [–no-demangle]
[–plugin name] [–size-sort] [–special-syms]
[–synthetic] [–with-symbol-versions] [–target=bfdname]
[objfile…]
GNU nm lists the symbols from object files objfile…. If no object files are listed as arguments, nm assumes the file a.out.
For each symbol, nm shows:
The symbol value, in the radix selected by options (see below), or hexadecimal by default.
The symbol type. At least the following types are used; others are, as well, depending on the object file format. If lowercase, the symbol is usually local; if uppercase, the symbol is global (external). There are however a few lowercase symbols that are shown for special global symbols (u, v and w).
A
The symbol’s value is absolute, and will not be changed by further linking.
B
b
The symbol is in the uninitialized data section (known as BSS).
C
The symbol is common. Common symbols are uninitialized data. When linking, multiple common symbols may appear with the same name. If the symbol is defined anywhere, the common symbols are treated as undefined references. For more details on common symbols, see the discussion of –warn-common in Linker options in The GNU linker.
D
d
The symbol is in the initialized data section.
G
g
The symbol is in an initialized data section for small objects. Some object file formats permit more efficient access to small data objects, such as a global int variable as opposed to a large global array.
i
For PE format files this indicates that the symbol is in a section specific to the implementation of DLLs. For ELF format files this indicates that the symbol is an indirect function. This is a GNU extension to the standard set of ELF symbol types. It indicates a symbol which if referenced by a relocation does not evaluate to its address, but instead must be invoked at runtime. The runtime execution will then return the value to be used in the relocation.
I
The symbol is an indirect reference to another symbol.
N
The symbol is a debugging symbol.
p
The symbols is in a stack unwind section.
R
r
The symbol is in a read only data section.
S
s
The symbol is in an uninitialized data section for small objects.
T
t
The symbol is in the text (code) section.
U
The symbol is undefined.
u
The symbol is a unique global symbol. This is a GNU extension to the standard set of ELF symbol bindings. For such a symbol the dynamic linker will make sure that in the entire process there is just one symbol with this name and type in use.
V
v
The symbol is a weak object. When a weak defined symbol is linked with a normal defined symbol, the normal defined symbol is used with no error. When a weak undefined symbol is linked and the symbol is not defined, the value of the weak symbol becomes zero with no error. On some systems, uppercase indicates that a default value has been specified.
W
w
The symbol is a weak symbol that has not been specifically tagged as a weak object symbol. When a weak defined symbol is linked with a normal defined symbol, the normal defined symbol is used with no error. When a weak undefined symbol is linked and the symbol is not defined, the value of the symbol is determined in a system-specific manner without error. On some systems, uppercase indicates that a default value has been specified.
-
The symbol is a stabs symbol in an a.out object file. In this case, the next values printed are the stabs other field, the stabs desc field, and the stab type. Stabs symbols are used to hold debugging information.
?
The symbol type is unknown, or object file format specific.
The symbol name.
The long and short forms of options, shown here as alternatives, are equivalent.
-A
-o
–print-file-name
Precede each symbol by the name of the input file (or archive member) in which it was found, rather than identifying the input file once only, before all of its symbols.
-a
–debug-syms
Display all symbols, even debugger-only symbols; normally these are not listed.
-B
The same as –format=bsd (for compatibility with the MIPS nm).
-C
–demangle[=style]
Decode (demangle) low-level symbol names into user-level names. Besides removing any initial underscore prepended by the system, this makes C++ function names readable. Different compilers have different mangling styles. The optional demangling style argument can be used to choose an appropriate demangling style for your compiler. See c++filt, for more information on demangling.
–no-demangle
Do not demangle low-level symbol names. This is the default.
-D
–dynamic
Display the dynamic symbols rather than the normal symbols. This is only meaningful for dynamic objects, such as certain types of shared libraries.
-f format
–format=format
Use the output format format, which can be bsd, sysv, or posix. The default is bsd. Only the first character of format is significant; it can be either upper or lower case.
-g
–extern-only
Display only external symbols.
-h
–help
Show a summary of the options to nm and exit.
-l
–line-numbers
For each symbol, use debugging information to try to find a filename and line number. For a defined symbol, look for the line number of the address of the symbol. For an undefined symbol, look for the line number of a relocation entry which refers to the symbol. If line number information can be found, print it after the other symbol information.
–inlines
When option -l is active, if the address belongs to a function that was inlined, then this option causes the source information for all enclosing scopes back to the first non-inlined function to be printed as well. For example, if main inlines callee1 which inlines callee2, and address is from callee2, the source information for callee1 and main will also be printed.
-n
-v
–numeric-sort
Sort symbols numerically by their addresses, rather than alphabetically by their names.
-p
–no-sort
Do not bother to sort the symbols in any order; print them in the order encountered.
-P
–portability
Use the POSIX.2 standard output format instead of the default format. Equivalent to ‘-f posix’.
-r
–reverse-sort
Reverse the order of the sort (whether numeric or alphabetic); let the last come first.
-S
–print-size
Print both value and size of defined symbols for the bsd output style. This option has no effect for object formats that do not record symbol sizes, unless ‘–size-sort’ is also used in which case a calculated size is displayed.
-s
–print-armap
When listing symbols from archive members, include the index: a mapping (stored in the archive by ar or ranlib) of which modules contain definitions for which names.
-t radix
–radix=radix
Use radix as the radix for printing the symbol values. It must be ‘d’ for decimal, ‘o’ for octal, or ‘x’ for hexadecimal.
-u
–undefined-only
Display only undefined symbols (those external to each object file).
-V
–version
Show the version number of nm and exit.
-X
This option is ignored for compatibility with the AIX version of nm. It takes one parameter which must be the string 32_64. The default mode of AIX nm corresponds to -X 32, which is not supported by GNU nm.
–defined-only
Display only defined symbols for each object file.
–plugin name
Load the plugin called name to add support for extra target types. This option is only available if the toolchain has been built with plugin support enabled.
If –plugin is not provided, but plugin support has been enabled then nm iterates over the files in ${libdir}/bfd-plugins in alphabetic order and the first plugin that claims the object in question is used.
Please note that this plugin search directory is not the one used by ld’s -plugin option. In order to make nm use the linker plugin it must be copied into the ${libdir}/bfd-plugins directory. For GCC based compilations the linker plugin is called liblto_plugin.so.0.0.0. For Clang based compilations it is called LLVMgold.so. The GCC plugin is always backwards compatible with earlier versions, so it is sufficient to just copy the newest one.
–size-sort
Sort symbols by size. For ELF objects symbol sizes are read from the ELF, for other object types the symbol sizes are computed as the difference between the value of the symbol and the value of the symbol with the next higher value. If the bsd output format is used the size of the symbol is printed, rather than the value, and ‘-S’ must be used in order both size and value to be printed.
–special-syms
Display symbols which have a target-specific special meaning. These symbols are usually used by the target for some special processing and are not normally helpful when included in the normal symbol lists. For example for ARM targets this option would skip the mapping symbols used to mark transitions between ARM code, THUMB code and data.
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