这个工具和objdump命令提供的功能类似，但是它显示的信息更为具体，并且它不依赖BFD库(BFD库是一个GNU项目，它的目标就是希望通过一种统一的接口来处理不同的目标文件）；
ELF文件类型
ELF(Executable and Linking Format)是一种对象文件的格式，用于定义不同类型的对象文件(Object files)中都放了什么东西、以及都以什么样的格式去放这些东西。它自最早在 System V 系统上出现后，被 xNIX 世界所广泛接受，作为缺省的二进制文件格式来使用。可以说，ELF是构成众多xNIX系统的基础之一。ELF文件有三种类型：
- 可重定位的对象文件(Relocatable file) 由汇编器汇编生成的 .o 文件
- 可执行的对象文件(Executable file) 可执行应用程序
- 可被共享的对象文件(Shared object file) 动态库文件，也即 .so 文件
- .text section 里装载了可执行代码；
- .data section 里面装载了被初始化的数据；
- .bss section 里面装载了未被初始化的数据；
- 以 .rec 打头的 sections 里面装载了重定位条目；
- .symtab 或者 .dynsym section 里面装载了符号信息；
- .strtab 或者 .dynstr section 里面装载了字符串信息；
13.1. 参数说明
- -a –all 全部 Equivalent to: -h -l -S -s -r -d -V -A -I
- -h –file-header 文件头 Display the ELF file header
- -l –program-headers 程序 Display the program headers
- –segments An alias for –program-headers
- -S –section-headers 段头 Display the sections’ header
-
--sections An alias for –section-headers - -e –headers 全部头 Equivalent to: -h -l -S
- -s –syms 符号表 Display the symbol table
-
--symbols An alias for –syms - -n –notes 内核注释 Display the core notes (if present)
- -r –relocs 重定位 Display the relocations (if present)
- -u –unwind Display the unwind info (if present)
- -d –dynamic 动态段 Display the dynamic segment (if present)
- -V –version-info 版本 Display the version sections (if present)
- -A –arch-specific CPU构架 Display architecture specific information (if any).
- -D –use-dynamic 动态段 Use the dynamic section info when displaying symbols
- -x –hex-dump=
显示 段内内容Dump the contents of section - -w[liaprmfFso] or
- -I –histogram Display histogram of bucket list lengths
- -W –wide 宽行输出 Allow output width to exceed 80 characters
- -H –help Display this information
- -v –version Display the version number of readelf
13.2. 示例
想知道一个应用程序的可运行的架构平台:
$readelf -h main| grep Machine
-h选项将显示文件头的概要信息，从里面可以看到，有很多有用的信息：
$readelf -h main
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2 s complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x400790
Start of program headers: 64 (bytes into file)
Start of section headers: 5224 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 8
Size of section headers: 64 (bytes)
Number of section headers: 29
Section header string table index: 26
一个编译好的应用程序，想知道其编译时是否使用了-g选项（加入调试信息）:
$readelf -S main| grep debug
用-S选项是显示所有段信息；如果编译时使用了-g选项，则会有debug段;
查看.o文件是否编入了调试信息（编译的时候是否加了-g):
$readelf -S Shpos.o | grep debug
13.3. 完整输出
readelf输出的完整内容:
$readelf -all a.out
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x8048330
Start of program headers: 52 (bytes into file)
Start of section headers: 4412 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 9
Size of section headers: 40 (bytes)
Number of section headers: 30
Section header string table index: 27
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .interp PROGBITS 08048154 000154 000013 00 A 0 0 1
[ 2] .note.ABI-tag NOTE 08048168 000168 000020 00 A 0 0 4
[ 3] .note.gnu.build-i NOTE 08048188 000188 000024 00 A 0 0 4
[ 4] .gnu.hash GNU_HASH 080481ac 0001ac 000020 04 A 5 0 4
[ 5] .dynsym DYNSYM 080481cc 0001cc 000050 10 A 6 1 4
[ 6] .dynstr STRTAB 0804821c 00021c 00004c 00 A 0 0 1
[ 7] .gnu.version VERSYM 08048268 000268 00000a 02 A 5 0 2
[ 8] .gnu.version_r VERNEED 08048274 000274 000020 00 A 6 1 4
[ 9] .rel.dyn REL 08048294 000294 000008 08 A 5 0 4
[10] .rel.plt REL 0804829c 00029c 000018 08 A 5 12 4
[11] .init PROGBITS 080482b4 0002b4 00002e 00 AX 0 0 4
[12] .plt PROGBITS 080482f0 0002f0 000040 04 AX 0 0 16
[13] .text PROGBITS 08048330 000330 00018c 00 AX 0 0 16
[14] .fini PROGBITS 080484bc 0004bc 00001a 00 AX 0 0 4
[15] .rodata PROGBITS 080484d8 0004d8 000011 00 A 0 0 4
[16] .eh_frame_hdr PROGBITS 080484ec 0004ec 000034 00 A 0 0 4
[17] .eh_frame PROGBITS 08048520 000520 0000c4 00 A 0 0 4
[18] .ctors PROGBITS 08049f14 000f14 000008 00 WA 0 0 4
[19] .dtors PROGBITS 08049f1c 000f1c 000008 00 WA 0 0 4
[20] .jcr PROGBITS 08049f24 000f24 000004 00 WA 0 0 4
[21] .dynamic DYNAMIC 08049f28 000f28 0000c8 08 WA 6 0 4
[22] .got PROGBITS 08049ff0 000ff0 000004 04 WA 0 0 4
[23] .got.plt PROGBITS 08049ff4 000ff4 000018 04 WA 0 0 4
[24] .data PROGBITS 0804a00c 00100c 000008 00 WA 0 0 4
[25] .bss NOBITS 0804a014 001014 000008 00 WA 0 0 4
[26] .comment PROGBITS 00000000 001014 00002a 01 MS 0 0 1
[27] .shstrtab STRTAB 00000000 00103e 0000fc 00 0 0 1
[28] .symtab SYMTAB 00000000 0015ec 000410 10 29 45 4
[29] .strtab STRTAB 00000000 0019fc 0001f9 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x08048034 0x08048034 0x00120 0x00120 R E 0x4
INTERP 0x000154 0x08048154 0x08048154 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
LOAD 0x000000 0x08048000 0x08048000 0x005e4 0x005e4 R E 0x1000
LOAD 0x000f14 0x08049f14 0x08049f14 0x00100 0x00108 RW 0x1000
DYNAMIC 0x000f28 0x08049f28 0x08049f28 0x000c8 0x000c8 RW 0x4
NOTE 0x000168 0x08048168 0x08048168 0x00044 0x00044 R 0x4
GNU_EH_FRAME 0x0004ec 0x080484ec 0x080484ec 0x00034 0x00034 R 0x4
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x4
GNU_RELRO 0x000f14 0x08049f14 0x08049f14 0x000ec 0x000ec R 0x1
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.dyn .rel.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
03 .ctors .dtors .jcr .dynamic .got .got.plt .data .bss
04 .dynamic
05 .note.ABI-tag .note.gnu.build-id
06 .eh_frame_hdr
07
08 .ctors .dtors .jcr .dynamic .got
Dynamic section at offset 0xf28 contains 20 entries:
Tag Type Name/Value
0x00000001 (NEEDED) Shared library: [libc.so.6]
0x0000000c (INIT) 0x80482b4
0x0000000d (FINI) 0x80484bc
0x6ffffef5 (GNU_HASH) 0x80481ac
0x00000005 (STRTAB) 0x804821c
0x00000006 (SYMTAB) 0x80481cc
0x0000000a (STRSZ) 76 (bytes)
0x0000000b (SYMENT) 16 (bytes)
0x00000015 (DEBUG) 0x0
0x00000003 (PLTGOT) 0x8049ff4
0x00000002 (PLTRELSZ) 24 (bytes)
0x00000014 (PLTREL) REL
0x00000017 (JMPREL) 0x804829c
0x00000011 (REL) 0x8048294
0x00000012 (RELSZ) 8 (bytes)
0x00000013 (RELENT) 8 (bytes)
0x6ffffffe (VERNEED) 0x8048274
0x6fffffff (VERNEEDNUM) 1
0x6ffffff0 (VERSYM) 0x8048268
0x00000000 (NULL) 0x0
Relocation section '.rel.dyn' at offset 0x294 contains 1 entries:
Offset Info Type Sym.Value Sym. Name
08049ff0 00000206 R_386_GLOB_DAT 00000000 __gmon_start__
Relocation section '.rel.plt' at offset 0x29c contains 3 entries:
Offset Info Type Sym.Value Sym. Name
0804a000 00000107 R_386_JUMP_SLOT 00000000 printf
0804a004 00000207 R_386_JUMP_SLOT 00000000 __gmon_start__
0804a008 00000307 R_386_JUMP_SLOT 00000000 __libc_start_main
There are no unwind sections in this file.
Symbol table '.dynsym' contains 5 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 FUNC GLOBAL DEFAULT UND printf@GLIBC_2.0 (2)
2: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
3: 00000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.0 (2)
4: 080484dc 4 OBJECT GLOBAL DEFAULT 15 _IO_stdin_used
Symbol table '.symtab' contains 65 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 08048154 0 SECTION LOCAL DEFAULT 1
2: 08048168 0 SECTION LOCAL DEFAULT 2
3: 08048188 0 SECTION LOCAL DEFAULT 3
4: 080481ac 0 SECTION LOCAL DEFAULT 4
5: 080481cc 0 SECTION LOCAL DEFAULT 5
6: 0804821c 0 SECTION LOCAL DEFAULT 6
7: 08048268 0 SECTION LOCAL DEFAULT 7
8: 08048274 0 SECTION LOCAL DEFAULT 8
9: 08048294 0 SECTION LOCAL DEFAULT 9
10: 0804829c 0 SECTION LOCAL DEFAULT 10
11: 080482b4 0 SECTION LOCAL DEFAULT 11
12: 080482f0 0 SECTION LOCAL DEFAULT 12
13: 08048330 0 SECTION LOCAL DEFAULT 13
14: 080484bc 0 SECTION LOCAL DEFAULT 14
15: 080484d8 0 SECTION LOCAL DEFAULT 15
16: 080484ec 0 SECTION LOCAL DEFAULT 16
17: 08048520 0 SECTION LOCAL DEFAULT 17
18: 08049f14 0 SECTION LOCAL DEFAULT 18
19: 08049f1c 0 SECTION LOCAL DEFAULT 19
20: 08049f24 0 SECTION LOCAL DEFAULT 20
21: 08049f28 0 SECTION LOCAL DEFAULT 21
22: 08049ff0 0 SECTION LOCAL DEFAULT 22
23: 08049ff4 0 SECTION LOCAL DEFAULT 23
24: 0804a00c 0 SECTION LOCAL DEFAULT 24
25: 0804a014 0 SECTION LOCAL DEFAULT 25
26: 00000000 0 SECTION LOCAL DEFAULT 26
27: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
28: 08049f14 0 OBJECT LOCAL DEFAULT 18 __CTOR_LIST__
29: 08049f1c 0 OBJECT LOCAL DEFAULT 19 __DTOR_LIST__
30: 08049f24 0 OBJECT LOCAL DEFAULT 20 __JCR_LIST__
31: 08048360 0 FUNC LOCAL DEFAULT 13 __do_global_dtors_aux
32: 0804a014 1 OBJECT LOCAL DEFAULT 25 completed.6086
33: 0804a018 4 OBJECT LOCAL DEFAULT 25 dtor_idx.6088
34: 080483c0 0 FUNC LOCAL DEFAULT 13 frame_dummy
35: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
36: 08049f18 0 OBJECT LOCAL DEFAULT 18 __CTOR_END__
37: 080485e0 0 OBJECT LOCAL DEFAULT 17 __FRAME_END__
38: 08049f24 0 OBJECT LOCAL DEFAULT 20 __JCR_END__
39: 08048490 0 FUNC LOCAL DEFAULT 13 __do_global_ctors_aux
40: 00000000 0 FILE LOCAL DEFAULT ABS a.c
41: 08049f14 0 NOTYPE LOCAL DEFAULT 18 __init_array_end
42: 08049f28 0 OBJECT LOCAL DEFAULT 21 _DYNAMIC
43: 08049f14 0 NOTYPE LOCAL DEFAULT 18 __init_array_start
44: 08049ff4 0 OBJECT LOCAL DEFAULT 23 _GLOBAL_OFFSET_TABLE_
45: 08048480 2 FUNC GLOBAL DEFAULT 13 __libc_csu_fini
46: 08048482 0 FUNC GLOBAL HIDDEN 13 __i686.get_pc_thunk.bx
47: 0804a00c 0 NOTYPE WEAK DEFAULT 24 data_start
48: 00000000 0 FUNC GLOBAL DEFAULT UND printf@@GLIBC_2.0
49: 0804a014 0 NOTYPE GLOBAL DEFAULT ABS _edata
50: 080484bc 0 FUNC GLOBAL DEFAULT 14 _fini
51: 08049f20 0 OBJECT GLOBAL HIDDEN 19 __DTOR_END__
52: 0804a00c 0 NOTYPE GLOBAL DEFAULT 24 __data_start
53: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
54: 0804a010 0 OBJECT GLOBAL HIDDEN 24 __dso_handle
55: 080484dc 4 OBJECT GLOBAL DEFAULT 15 _IO_stdin_used
56: 00000000 0 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_
57: 08048410 97 FUNC GLOBAL DEFAULT 13 __libc_csu_init
58: 0804a01c 0 NOTYPE GLOBAL DEFAULT ABS _end
59: 08048330 0 FUNC GLOBAL DEFAULT 13 _start
60: 080484d8 4 OBJECT GLOBAL DEFAULT 15 _fp_hw
61: 0804a014 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
62: 080483e4 40 FUNC GLOBAL DEFAULT 13 main
63: 00000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
64: 080482b4 0 FUNC GLOBAL DEFAULT 11 _init
Histogram for `.gnu.hash' bucket list length (total of 2 buckets):
Length Number % of total Coverage
0 1 ( 50.0%)
1 1 ( 50.0%) 100.0%
Version symbols section '.gnu.version' contains 5 entries:
Addr: 0000000008048268 Offset: 0x000268 Link: 5 (.dynsym)
000: 0 (*local*) 2 (GLIBC_2.0) 0 (*local*) 2 (GLIBC_2.0)
004: 1 (*global*)
Version needs section '.gnu.version_r' contains 1 entries:
Addr: 0x0000000008048274 Offset: 0x000274 Link: 6 (.dynstr)
000000: Version: 1 File: libc.so.6 Cnt: 1
0x0010: Name: GLIBC_2.0 Flags: none Version: 2
Notes at offset 0x00000168 with length 0x00000020:
Owner Data size Description
GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag)
OS: Linux, ABI: 2.6.15
Notes at offset 0x00000188 with length 0x00000024:
Owner Data size Description
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: 17fb9651029b6a8543bfafec9eea23bd16454e65
关于ELF文件格式的参考：http://www.cnblogs.com/xmphoenix/archive/2011/10/23/2221879.html
下一节：在linux终端，面对命令不知道怎么用，或不记得命令的拼写及参数时，我们需要求助于系统的帮助文档； linux系统内置的帮助文档很详细，通常能解决我们的问题，我们需要掌握如何正确的去使用它们；
1. 在只记得部分命令关键字的场合，我们可通过man -k来搜索；
2. 需要知道某个命令的简要说明，可以使用whatis；而更详细的介绍，则可用info命令；
3. 查看命令在哪个位置，我们需要使用which；
4. 而对于命令的具体参数及使用方法，我们需要用到强大的man；
下面介绍这些命令；