正如从该输出中看到的，简单的 a.out Hello World 文件中包含了大量有价值的细节信息，包括版本信息、柱状图、各种符号类型的表格，等等。通过使用本文中介绍的这几种工具分析目标文件，您可以慢慢地对可执行程序进行研究。
除了所有这些段之外，编译器可以将调试信息放入到目标文件中，并且还可以显示这些信息。输入下面的命令，仔细分析编译器的输出（假设您扮演了调试程序的角色）：
readelf --debug-dump a.out | less这个命令生成的输出如清单 5 所示。调试工具，如 GDB，可以读取这些调试信息，并且当程序在调试器中运行的同时，您可以使用该工具显示更具描述性的标记，而不是对代码进行反汇编时的原始地址值。
清单 5. 该程序中的调试信息The section .debug_aranges contains:
Length: 28
Version: 2
Offset into .debug_info: 0
Pointer Size: 4
Segment Size: 0
Address Length
080482c0 34
Length: 52
Version: 2
Offset into .debug_info: 10b
Pointer Size: 4
Segment Size: 0
Address Length
08048308 4
08048458 18
08048278 11
080482e4 36
Length: 44
Version: 2
Offset into .debug_info: 19b
Pointer Size: 4
Segment Size: 0
Address Length
08048308 4
0804846f 6
0804828d 2
Contents of the .debug_pubnames section:
Length: 33
Version: 2
Offset into .debug_info section: 122
Size of area in .debug_info section: 145
Offset Name
121 _IO_stdin_used
The section .debug_info contains:
Compilation Unit @ offset 0x0:
Length: 118
Version: 2
Abbrev Offset: 0
Pointer Size: 4
<0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
DW_AT_stmt_list : 0
DW_AT_low_pc : 0x80482c0
DW_AT_high_pc : 0x80482e2
DW_AT_name : ../sysdeps/i386/elf/start.S
DW_AT_comp_dir : /build/buildd/glibc-2.3.6/build-tree/glibc-2.3.6/csu
DW_AT_producer : GNU AS 2.16.91
DW_AT_language : 32769 (MIPS assembler)
Compilation Unit @ offset 0x7a:
Length: 141
Version: 2
Abbrev Offset: 20
Pointer Size: 4
<0><85>: Abbrev Number: 1 (DW_TAG_compile_unit)
DW_AT_stmt_list : 0x5b
DW_AT_high_pc : 0x80482e4
DW_AT_low_pc : 0x80482e4
DW_AT_producer : (indirect string, offset: 0x62): GNU C 3.4.6
DW_AT_language : 1 (ANSI C)
DW_AT_name : (indirect string, offset: 0x0): init.c
DW_AT_comp_dir : (indirect string, offset: 0x11): /build/buildd/...
<1><9f>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x90): unsigned int
DW_AT_byte_size : 4
DW_AT_encoding : 7 (unsigned)
<1><a6>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x54): unsigned char
DW_AT_byte_size : 1
DW_AT_encoding : 8 (unsigned char)
<1><ad>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x9d): short unsigned int
DW_AT_byte_size : 2
DW_AT_encoding : 7 (unsigned)
<1><b4>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x8b): long unsigned int
DW_AT_byte_size : 4
DW_AT_encoding : 7 (unsigned)
<1><bb>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x56): signed char
DW_AT_byte_size : 1
DW_AT_encoding : 6 (signed char)
<1><c2>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x7): short int
DW_AT_byte_size : 2
DW_AT_encoding : 5 (signed)
<1><c9>: Abbrev Number: 3 (DW_TAG_base_type)
DW_AT_name : int
DW_AT_byte_size : 4
DW_AT_encoding : 5 (signed)
<1><d0>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x46): long long int
DW_AT_byte_size : 8
DW_AT_encoding : 5 (signed)
<1><d7>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x86): long long unsigned int
DW_AT_byte_size : 8
DW_AT_encoding : 7 (unsigned)
<1><de>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x4b): long int
DW_AT_byte_size : 4
DW_AT_encoding : 5 (signed)
<1><e5>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x90): unsigned int
DW_AT_byte_size : 4
DW_AT_encoding : 7 (unsigned)
<1><ec>: Abbrev Number: 2 (DW_TAG_base_type)
DW_AT_name : (indirect string, offset: 0x5d): char
DW_AT_byte_size : 1
DW_AT_encoding : 6 (signed char)
<1><f3>: Abbrev Number: 4 (DW_TAG_variable)
DW_AT_name : (indirect string, offset: 0xb0): _IO_stdin_used
DW_AT_decl_file : 1
DW_AT_decl_line : 25
DW_AT_type : <105>
DW_AT_external : 1
DW_AT_location : 5 byte block: 3 7c 84 4 8 (DW_OP_addr: 804847c)
<1><105>: Abbrev Number: 5 (DW_TAG_const_type)
DW_AT_type : <c9>
Compilation Unit @ offset 0x10b:
Length: 140
Version: 2
Abbrev Offset: 86
Pointer Size: 4
<0><116>: Abbrev Number: 1 (DW_TAG_compile_unit)
DW_AT_stmt_list : 0x82
DW_AT_name : /build/buildd/glibc-2.3.6/build-tree/i386-libc/csu/crti.S
DW_AT_comp_dir : /build/buildd/glibc-2.3.6/build-tree/glibc-2.3.6/csu
DW_AT_producer : GNU AS 2.16.91
DW_AT_language : 32769 (MIPS assembler)
Compilation Unit @ offset 0x19b:
Length: 140
Version: 2
Abbrev Offset: 102
Pointer Size: 4
<0><1a6>: Abbrev Number: 1 (DW_TAG_compile_unit)
DW_AT_stmt_list : 0x12f
DW_AT_name : /build/buildd/glibc-2.3.6/build-tree/i386-libc/csu/crtn.S
DW_AT_comp_dir : /build/buildd/glibc-2.3.6/build-tree/glibc-2.3.6/csu
DW_AT_producer : GNU AS 2.16.91
DW_AT_language : 32769 (MIPS assembler)
Contents of the .debug_abbrev section:
Number TAG
1 DW_TAG_compile_unit [no children]
DW_AT_stmt_list DW_FORM_data4
DW_AT_low_pc DW_FORM_addr
DW_AT_high_pc DW_FORM_addr
DW_AT_name DW_FORM_string
DW_AT_comp_dir DW_FORM_string
DW_AT_producer DW_FORM_string
DW_AT_language DW_FORM_data2
Number TAG
1 DW_TAG_compile_unit [has children]
DW_AT_stmt_list DW_FORM_data4
DW_AT_high_pc DW_FORM_addr
DW_AT_low_pc DW_FORM_addr
DW_AT_producer DW_FORM_strp
DW_AT_language DW_FORM_data1
DW_AT_name DW_FORM_strp
DW_AT_comp_dir DW_FORM_strp
2 DW_TAG_base_type [no children]
DW_AT_name DW_FORM_strp
DW_AT_byte_size DW_FORM_data1
DW_AT_encoding DW_FORM_data1
3 DW_TAG_base_type [no children]
DW_AT_name DW_FORM_string
DW_AT_byte_size DW_FORM_data1
DW_AT_encoding DW_FORM_data1
4 DW_TAG_variable [no children]
DW_AT_name DW_FORM_strp
DW_AT_decl_file DW_FORM_data1
DW_AT_decl_line DW_FORM_data1
DW_AT_type DW_FORM_ref4
DW_AT_external DW_FORM_flag
DW_AT_location DW_FORM_block1
5 DW_TAG_const_type [no children]
DW_AT_type DW_FORM_ref4
Number TAG
1 DW_TAG_compile_unit [no children]
DW_AT_stmt_list DW_FORM_data4
DW_AT_name DW_FORM_string
DW_AT_comp_dir DW_FORM_string
DW_AT_producer DW_FORM_string
DW_AT_language DW_FORM_data2
Number TAG
1 DW_TAG_compile_unit [no children]
DW_AT_stmt_list DW_FORM_data4
DW_AT_name DW_FORM_string
DW_AT_comp_dir DW_FORM_string
DW_AT_producer DW_FORM_string
DW_AT_language DW_FORM_data2
Dump of debug contents of section .debug_line:
Length: 87
DWARF Version: 2
Prologue Length: 50
Minimum Instruction Length: 1
Initial value of 'is_stmt': 1
Line Base: -5
Line Range: 14
Opcode Base: 13
(Pointer size: 4)
Opcodes:
Opcode 1 has 0 args
Opcode 2 has 1 args
Opcode 3 has 1 args
Opcode 4 has 1 args
Opcode 5 has 1 args
Opcode 6 has 0 args
Opcode 7 has 0 args
Opcode 8 has 0 args
Opcode 9 has 1 args
Opcode 10 has 0 args
Opcode 11 has 0 args
Opcode 12 has 1 args
The Directory Table:
../sysdeps/i386/elf
The File Name Table:
Entry Dir Time Size Name
1 1 0 0 start.S
Line Number Statements:
Extended opcode 2: set Address to 0x80482c0
Advance Line by 64 to 65
Copy
Special opcode 38: advance Address by 2 to 0x80482c2 and Line by 5 to 70
Special opcode 20: advance Address by 1 to 0x80482c3 and Line by 1 to 71
Special opcode 39: advance Address by 2 to 0x80482c5 and Line by 6 to 77
Special opcode 48: advance Address by 3 to 0x80482c8 and Line by 1 to 78
Special opcode 24: advance Address by 1 to 0x80482c9 and Line by 5 to 83
Special opcode 21: advance Address by 1 to 0x80482ca and Line by 2 to 85
Advance Line by 24 to 109
Special opcode 19: advance Address by 1 to 0x80482cb and Line by 0 to 109
Special opcode 76: advance Address by 5 to 0x80482d0 and Line by 1 to 110
Special opcode 77: advance Address by 5 to 0x80482d5 and Line by 2 to 112
Special opcode 20: advance Address by 1 to 0x80482d6 and Line by 1 to 113
Special opcode 21: advance Address by 1 to 0x80482d7 and Line by 2 to 115
Special opcode 79: advance Address by 5 to 0x80482dc and Line by 4 to 119
Special opcode 78: advance Address by 5 to 0x80482e1 and Line by 3 to 122
Advance PC by 1 to 0x80482e2
Extended opcode 1: End of Sequence
Length: 35
DWARF Version: 2
Prologue Length: 29
Minimum Instruction Length: 1
Initial value of 'is_stmt': 1
Line Base: -5
Line Range: 14
Opcode Base: 13
(Pointer size: 4)
Opcodes:
Opcode 1 has 0 args
Opcode 2 has 1 args
Opcode 3 has 1 args
Opcode 4 has 1 args
Opcode 5 has 1 args
Opcode 6 has 0 args
Opcode 7 has 0 args
Opcode 8 has 0 args
Opcode 9 has 1 args
Opcode 10 has 0 args
Opcode 11 has 0 args
Opcode 12 has 1 args
The Directory Table is empty.
The File Name Table:
Entry Dir Time Size Name
1 0 0 0 init.c
Line Number Statements:
Length: 169
DWARF Version: 2
Prologue Length: 80
Minimum Instruction Length: 1
Initial value of 'is_stmt': 1
Line Base: -5
Line Range: 14
Opcode Base: 13
(Pointer size: 4)
Opcodes:
Opcode 1 has 0 args
Opcode 2 has 1 args
Opcode 3 has 1 args
Opcode 4 has 1 args
Opcode 5 has 1 args
Opcode 6 has 0 args
Opcode 7 has 0 args
Opcode 8 has 0 args
Opcode 9 has 1 args
Opcode 10 has 0 args
Opcode 11 has 0 args
Opcode 12 has 1 args
The Directory Table:
/build/buildd/glibc-2.3.6/build-tree/i386-libc/csu
The File Name Table:
Entry Dir Time Size Name
1 1 0 0 crti.S
Line Number Statements:
Extended opcode 2: set Address to 0x8048308
Advance Line by 64 to 65
Copy
Special opcode 48: advance Address by 3 to 0x804830b and Line by 1 to 66
Advance PC by 1 to 0x804830c
Extended opcode 1: End of Sequence
Extended opcode 2: set Address to 0x8048458
Advance Line by 46 to 47
Copy
Special opcode 20: advance Address by 1 to 0x8048459 and Line by 1 to 48
Special opcode 34: advance Address by 2 to 0x804845b and Line by 1 to 49
Special opcode 20: advance Address by 1 to 0x804845c and Line by 1 to 50
Special opcode 76: advance Address by 5 to 0x8048461 and Line by 1 to 51
Special opcode 90: advance Address by 6 to 0x8048467 and Line by 1 to 52
Advance PC by 3 to 0x804846a
Extended opcode 1: End of Sequence
Extended opcode 2: set Address to 0x8048278
Advance Line by 31 to 32
Copy
Special opcode 20: advance Address by 1 to 0x8048279 and Line by 1 to 33
Special opcode 34: advance Address by 2 to 0x804827b and Line by 1 to 34
Special opcode 48: advance Address by 3 to 0x804827e and Line by 1 to 35
Advance PC by 5 to 0x8048283
Extended opcode 1: End of Sequence
Extended opcode 2: set Address to 0x80482e4
Advance Line by 10 to 11
Copy
Special opcode 20: advance Address by 1 to 0x80482e5 and Line by 1 to 12
Special opcode 34: advance Address by 2 to 0x80482e7 and Line by 1 to 13
Special opcode 20: advance Address by 1 to 0x80482e8 and Line by 1 to 14
Special opcode 76: advance Address by 5 to 0x80482ed and Line by 1 to 15
Special opcode 90: advance Address by 6 to 0x80482f3 and Line by 1 to 16
Special opcode 48: advance Address by 3 to 0x80482f6 and Line by 1 to 17
Special opcode 90: advance Address by 6 to 0x80482fc and Line by 1 to 18
Special opcode 34: advance Address by 2 to 0x80482fe and Line by 1 to 19
Special opcode 34: advance Address by 2 to 0x8048300 and Line by 1 to 20
Special opcode 35: advance Address by 2 to 0x8048302 and Line by 2 to 22
Special opcode 48: advance Address by 3 to 0x8048305 and Line by 1 to 23
Special opcode 20: advance Address by 1 to 0x8048306 and Line by 1 to 24
Special opcode 20: advance Address by 1 to 0x8048307 and Line by 1 to 25
Advance PC by 1 to 0x8048308
Extended opcode 1: End of Sequence
Length: 136
DWARF Version: 2
Prologue Length: 80
Minimum Instruction Length: 1
Initial value of 'is_stmt': 1
Line Base: -5
Line Range: 14
Opcode Base: 13
(Pointer size: 4)
Opcodes:
Opcode 1 has 0 args
Opcode 2 has 1 args
Opcode 3 has 1 args
Opcode 4 has 1 args
Opcode 5 has 1 args
Opcode 6 has 0 args
Opcode 7 has 0 args
Opcode 8 has 0 args
Opcode 9 has 1 args
Opcode 10 has 0 args
Opcode 11 has 0 args
Opcode 12 has 1 args
The Directory Table:
/build/buildd/glibc-2.3.6/build-tree/i386-libc/csu
The File Name Table:
Entry Dir Time Size Name
1 1 0 0 crtn.S
Line Number Statements:
Extended opcode 2: set Address to 0x8048308
Advance Line by 33 to 34
Copy
Special opcode 48: advance Address by 3 to 0x804830b and Line by 1 to 35
Advance PC by 1 to 0x804830c
Extended opcode 1: End of Sequence
Extended opcode 2: set Address to 0x804846f
Advance Line by 18 to 19
Copy
Special opcode 48: advance Address by 3 to 0x8048472 and Line by 1 to 20
Special opcode 20: advance Address by 1 to 0x8048473 and Line by 1 to 21
Special opcode 20: advance Address by 1 to 0x8048474 and Line by 1 to 22
Advance PC by 1 to 0x8048475
Extended opcode 1: End of Sequence
Extended opcode 2: set Address to 0x804828d
Advance Line by 9 to 10
Copy
Special opcode 20: advance Address by 1 to 0x804828e and Line by 1 to 11
Advance PC by 1 to 0x804828f
Extended opcode 1: End of Sequence
Contents of the .debug_str section:
0x00000000 696e6974 2e630073 686f7274 20696e74 init.c.short int
0x00000010 002f6275 696c642f 6275696c 64642f67 ./build/buildd/g
0x00000020 6c696263 2d322e33 2e362f62 75696c64 libc-2.3.6/build
0x00000030 2d747265 652f676c 6962632d 322e332e -tree/glibc-2.3.
0x00000040 362f6373 75006c6f 6e67206c 6f6e6720 6/csu.long long
0x00000050 696e7400 756e7369 676e6564 20636861 int.unsigned cha
0x00000060 7200474e 55204320 332e342e 36202855 r.GNU C 3.4.6 (U
0x00000070 62756e74 7520332e 342e362d 31756275 buntu 3.4.6-1ubu
0x00000080 6e747532 29006c6f 6e67206c 6f6e6720 ntu2).long long
0x00000090 756e7369 676e6564 20696e74 0073686f unsigned int.sho
0x000000a0 72742075 6e736967 6e656420 696e7400 rt unsigned int.
0x000000b0 5f494f5f 73746469 6e5f7573 656400 _IO_stdin_used.
可执行文件是目标文件
在 Unix 中，可执行文件是 目标文件，并且您可以像对 a.out 文件那样对它们进行分析。可以进行一次有益的练习，更改到 /bin 或 /local/bin 目录，然后针对一些您最常用的命令，如 pwd、ps、cat 或 rm，运行 nm、objdump 和 readelf。通常，在您编写需要某种功能的程序时，如果标准的工具已经提供了这个功能，那么通过运行 objdump -d <command>，可以查看这些工具究竟如何完成这项任务。
如果您倾向于使用编译器和其他的语言工具，那么您可以对组成计算机系统的各种目标文件进行仔细研究，并且您将会发现这项工作是非常值得的。UNIX 操作系统具有许多层次，那些通过工具查看目标文件所公开的层次，非常接近底层硬件。通过这种方式，您可以真实地接触到系统。
结束语
研究目标文件可以极大地加深您对 UNIX 操作系统的认识，并且可以更深入地了解如何对软件的源代码进行汇编。我鼓励您使用本文中介绍的目标文件工具对系统中 /bin 或 /local/bin 目录中的程序进行分析，仔细研究其输出结果，并找出您的硬件制造商所提供的系统文档。
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