The modern art of computer programming combines a special kind of human personality with a special set of tools to produce a rather ghostly product -- software -- that other human beings find useful. Computer programmers are detail-oriented folks who are able to deal with the difficulties of computers. Computers are exacting in their demands and don't tolerate deviation from these demands at all. No doubt about it, computers are difficult to program no matter what your personality, and many tools have been created to assist you in making the task easier.
In UNIX® and Linux®, everything is a file. You could say that the very sine qua non of UNIX and Linux programming is writing code to deal with files. Many types of files make up the system, but object files have a special design that provides for flexible, multipurpose use.
Object files are roadmaps that contain mnemonic symbols with attached addresses and values. The symbols are used for naming various sections of code and data, both initialized and uninitialized. They are also used for locating embedded debugging information and, just like the semantic Web, are fully readable by programs.
Tools of the trade
The tools of the computer programming trade begin with a code editor, such as vi or Emacs, with which you can type and edit the instructions you want the computer to follow to carry out the required tasks, and end with the compilers and linkers that produce the machine code that actually accomplishes these goals.
High-level tools, known as Integrated Debugging Environments (IDEs), integrate the functionality of individual tools with a common look and feel. An IDE can vastly blur the lines between editor, compiler, linker, and debugger. So for the purpose of studying and learning the system with greater depth, it's often advisable to work with the tools separately before working with the integrated suite. (Note: IDEs are often called Integrated Development Environments, too.)
The compiler transforms the text that you create in the code editor into an object file. The object file was originally known as an intermediate representation of code, because it served as the input to link editors (in other words, linkers) that finish the task and produce an executable program as output.
The transformation process that proceeds from code to executable is well-defined and automated, and object files are an integral link in the chain. During the transformation process, the object files serve as a map to the link editors, enabling them to resolve the symbols and stitch together the various code and data sections into a unified whole.
History
Many notable object file formats exist in the world of computer programming. The DOS family includes the COM, OBJ, and EXE formats. UNIX and Linux use a.out, COFF, and ELF. Microsoft® Windows® uses the portable executable (PE) format and Macintosh uses PEF, Mach-O, and others.
Originally, each type of computer had its own unique object file format but, with the advent of UNIX and other operating systems designed to be portable among different hardware platforms, some common file formats ascended to the level of a common standard. Among these are the a.out, COFF, and ELF formats.
Understanding object files requires a set of tools that can read the various portions of the object file and display them in a more readable format. This article discusses some of the more important aspects of those tools. But first, you must create a workbench and put a victim -- er, a patient -- on it.
The workbench
Fire up an xterm session, and let's begin to explore object files by creating aclean workbench. The following commands create a useful place to play withobject files:
cd mkdir src cd src mkdir hw cd hw
Then, by using your favorite code editor, type the program shown in Listing 1in the $HOME/src/hw directory, and call it hw.c.
Listing 1. The hw.c program
#include <stdio.h>
int main(void)
{
printf("Hello World!\n");
return 0;
}
This simple "Hello World" program serves as a patient to study with the varioustools available in the UNIX arsenal. Instead of taking any shortcuts to creatingthe executable (and there are many shortcuts), you'll take your time to build andexamine just the object file output.
File formats
The normal output of a C compiler is assembler code forwhatever processor you specify as the target. The assembler code is input to theassembler, which by default produces the grandfather of all object files, thea.out file. The name itself stands for Assembler Output. To create thea.out file, type the following command in the xterm window:
cc hw.c
Note: If you experience any errors or the a.out file wasn't created, you mightneed to examine your system or source file (hw.c) for errors. Check also to seewhether cc is defined to run your C/C++ compiler.
Modern C compilers combine the compile and assemble stepsinto one step. You can invoke switches to see just the assembler output of theC compiler. By typing the following command, you can seewhat the assembler output from the C compiler looks like:
cc -S hw.c
This command has generated a new file -- hw.s -- that contains the assembler input text that you typically would not have seen, because the compiler defaults to producing the a.out file. As expected, the UNIX assembler program can assemble this type of input file to produce the a.out file.
UNIX-specific tools
Assuming that all went well with the compile and you have an a.out file in the directory, let's examine it. Among the list of useful tools for examining object files, the following set exists:
- nm: Lists symbols from object files.
- objdump: Displays detailed information from object files.
- readelf: Displays information about ELF object files.
The first tool on the list is nm, which lists the symbols in an object file. If you type the nm command, you'll notice that it defaults to looking for a file named a.out If the file isn't found, the tool complains. If, however, the tool did find the a.out file that your compiler created, it presents a listing similar to Listing 2.
Listing 2. Output of the nm command
08049594 A __bss_start
080482e4 t call_gmon_start
08049594 b completed.4463
08049498 d __CTOR_END__
08049494 d __CTOR_LIST__
08049588 D __data_start
08049588 W data_start
0804842c t __do_global_ctors_aux
0804830c t __do_global_dtors_aux
0804958c D __dso_handle
080494a0 d __DTOR_END__
0804949c d __DTOR_LIST__
080494a8 d _DYNAMIC
08049594 A _edata
08049598 A _end
08048458 T _fini
08049494 a __fini_array_end
08049494 a __fini_array_start
08048478 R _fp_hw
0804833b t frame_dummy
08048490 r __FRAME_END__
08049574 d _GLOBAL_OFFSET_TABLE_
w __gmon_start__
08048308 T __i686.get_pc_thunk.bx
08048278 T _init
08049494 a __init_array_end
08049494 a __init_array_start
0804847c R _IO_stdin_used
080494a4 d __JCR_END__
080494a4 d __JCR_LIST__
w _Jv_RegisterClasses
080483e1 T __libc_csu_fini
08048390 T __libc_csu_init
U __libc_start_main@@GLIBC_2.0
08048360 T main
08049590 d p.4462
U puts@@GLIBC_2.0
080482c0 T _start
The sections that contain executable code are known as text sections or segments. Likewise, there are data sections or segments for containing non-executable information or data. Another type of section, known by the BSS designation, contains blocks started by symbol data.
For each symbol that the nm command lists, the symbol's value in hexadecimal (by default) and the symbol type with a coded character precede the symbol. Various codes that you commonly see include A for absolute, which means that the value will not change by further linking; B for a symbol found in the BSS section; or C for common symbols that reference uninitialized data.
Object files contain many different parts that are divided into sections. Sections can contain executable code, symbol names, initialized data values, and many other types of data. For detailed information on all of these types of data, consider reading the UNIX man page on nm, where each type is described by the character codes shown in the output of the command.
Details, details . . .
Even a simple Hello World program contains a vast array of details when it reaches the object file stage. The nm program is good for listing symbols and their types and values but, for examining in greater detail the contents of those named sections of the object file, more powerful tools are necessary.
Two of these more powerful tools are the objdump and readelf programs. By typing the following command, you can see an assembly listing of every section in the object file that contains executable code. Isn't it amazing how much code the compiler actually generates for such a tiny program?
objdump -d a.out
This command produces the output you see in Listing 3. Each section of executable code is run when a particular event becomes necessary, including events like the initialization of a library and the main starting entry point of the program itself.
Listing 3: Output of the objdump command
a.out: file format elf32-i386
Disassembly of section .init:
08048278 <_init>:
8048278: 55 push %ebp
8048279: 89 e5 mov %esp,%ebp
804827b: 83 ec 08 sub $0x8,%esp
804827e: e8 61 00 00 00 call 80482e4 <call_gmon_start>
8048283: e8 b3 00 00 00 call 804833b <frame_dummy>
8048288: e8 9f 01 00 00 call 804842c <__do_global_ctors_aux>
804828d: c9 leave
804828e: c3 ret
Disassembly of section .plt:
08048290 <puts@plt-0x10>:
8048290: ff 35 78 95 04 08 pushl 0x8049578
8048296: ff 25 7c 95 04 08 jmp *0x804957c
804829c: 00 00 add %al,(%eax)
...
080482a0 <puts@plt>:
80482a0: ff 25 80 95 04 08 jmp *0x8049580
80482a6: 68 00 00 00 00 push $0x0
80482ab: e9 e0 ff ff ff jmp 8048290 <_init+0x18>
080482b0 <__libc_start_main@plt>:
80482b0: ff 25 84 95 04 08 jmp *0x8049584
80482b6: 68 08 00 00 00 push $0x8
80482bb: e9 d0 ff ff ff jmp 8048290 <_init+0x18>
Disassembly of section .text:
080482c0 <_start>:
80482c0: 31 ed xor %ebp,%ebp
80482c2: 5e pop %esi
80482c3: 89 e1 mov %esp,%ecx
80482c5: 83 e4 f0 and $0xfffffff0,%esp
80482c8: 50 push %eax
80482c9: 54 push %esp
80482ca: 52 push %edx
80482cb: 68 e1 83 04 08 push $0x80483e1
80482d0: 68 90 83 04 08 push $0x8048390
80482d5: 51 push %ecx
80482d6: 56 push %esi
80482d7: 68 60 83 04 08 push $0x8048360
80482dc: e8 cf ff ff ff call 80482b0 <__libc_start_main@plt>
80482e1: f4 hlt
80482e2: 90 nop
80482e3: 90 nop
080482e4 <call_gmon_start>:
80482e4: 55 push %ebp
80482e5: 89 e5 mov %esp,%ebp
80482e7: 53 push %ebx
80482e8: e8 1b 00 00 00 call 8048308 <__i686.get_pc_thunk.bx>
80482ed: 81 c3 87 12 00 00 add $0x1287,%ebx
80482f3: 83 ec 04 sub $0x4,%esp
80482f6: 8b 83 fc ff ff ff mov 0xfffffffc(%ebx),%eax
80482fc: 85 c0 test %eax,%eax
80482fe: 74 02 je 8048302 <call_gmon_start+0x1e>
8048300: ff d0 call *%eax
8048302: 83 c4 04 add $0x4,%esp
8048305: 5b pop %ebx
8048306: 5d pop %ebp
8048307: c3 ret
08048308 <__i686.get_pc_thunk.bx>:
8048308: 8b 1c 24 mov (%esp),%ebx
804830b: c3 ret
0804830c <__do_global_dtors_aux>:
804830c: 55 push %ebp
804830d: 89 e5 mov %esp,%ebp
804830f: 83 ec 08 sub $0x8,%esp
8048312: 80 3d 94 95 04 08 00 cmpb $0x0,0x8049594
8048319: 74 0c je 8048327 <__do_global_dtors_aux+0x1b>
804831b: eb 1c jmp 8048339 <__do_global_dtors_aux+0x2d>
804831d: 83 c0 04 add $0x4,%eax
8048320: a3 90 95 04 08 mov %eax,0x8049590
8048325: ff d2 call *%edx
8048327: a1 90 95 04 08 mov 0x8049590,%eax
804832c: 8b 10 mov (%eax),%edx
804832e: 85 d2 test %edx,%edx
8048330: 75 eb jne 804831d <__do_global_dtors_aux+0x11>
8048332: c6 05 94 95 04 08 01 movb $0x1,0x8049594
8048339: c9 leave
804833a: c3 ret
0804833b <frame_dummy>:
804833b: 55 push %ebp
804833c: 89 e5 mov %esp,%ebp
804833e: 83 ec 08 sub $0x8,%esp
8048341: a1 a4 94 04 08 mov 0x80494a4,%eax
8048346: 85 c0 test %eax,%eax
8048348: 74 12 je 804835c <frame_dummy+0x21>
804834a: b8 00 00 00 00 mov $0x0,%eax
804834f: 85 c0 test %eax,%eax
8048351: 74 09 je 804835c <frame_dummy+0x21>
8048353: c7 04 24 a4 94 04 08 movl $0x80494a4,(%esp)
804835a: ff d0 call *%eax
804835c: c9 leave
804835d: c3 ret
804835e: 90 nop
804835f: 90 nop
08048360 <main>:
8048360: 55 push %ebp
8048361: 89 e5 mov %esp,%ebp
8048363: 83 ec 08 sub $0x8,%esp
8048366: 83 e4 f0 and $0xfffffff0,%esp
8048369: b8 00 00 00 00 mov $0x0,%eax
804836e: 83 c0 0f add $0xf,%eax
8048371: 83 c0 0f add $0xf,%eax
8048374: c1 e8 04 shr $0x4,%eax
8048377: c1 e0 04 shl $0x4,%eax
804837a: 29 c4 sub %eax,%esp
804837c: c7 04 24 80 84 04 08 movl $0x8048480,(%esp)
8048383: e8 18 ff ff ff call 80482a0 <puts@plt>
8048388: b8 00 00 00 00 mov $0x0,%eax
804838d: c9 leave
804838e: c3 ret
804838f: 90 nop
08048390 <__libc_csu_init>:
8048390: 55 push %ebp
8048391: 89 e5 mov %esp,%ebp
8048393: 57 push %edi
8048394: 56 push %esi
8048395: 31 f6 xor %esi,%esi
8048397: 53 push %ebx
8048398: e8 6b ff ff ff call 8048308 <__i686.get_pc_thunk.bx>
804839d: 81 c3 d7 11 00 00 add $0x11d7,%ebx
80483a3: 83 ec 0c sub $0xc,%esp
80483a6: e8 cd fe ff ff call 8048278 <_init>
80483ab: 8d 83 20 ff ff ff lea 0xffffff20(%ebx),%eax
80483b1: 8d 93 20 ff ff ff lea 0xffffff20(%ebx),%edx
80483b7: 89 45 f0 mov %eax,0xfffffff0(%ebp)
80483ba: 29 d0 sub %edx,%eax
80483bc: c1 f8 02 sar $0x2,%eax
80483bf: 39 c6 cmp %eax,%esi
80483c1: 73 16 jae 80483d9 <__libc_csu_init+0x49>
80483c3: 89 d7 mov %edx,%edi
80483c5: ff 14 b2 call *(%edx,%esi,4)
80483c8: 8b 45 f0 mov 0xfffffff0(%ebp),%eax
80483cb: 83 c6 01 add $0x1,%esi
80483ce: 29 f8 sub %edi,%eax
80483d0: 89 fa mov %edi,%edx
80483d2: c1 f8 02 sar $0x2,%eax
80483d5: 39 c6 cmp %eax,%esi
80483d7: 72 ec jb 80483c5 <__libc_csu_init+0x35>
80483d9: 83 c4 0c add $0xc,%esp
80483dc: 5b pop %ebx
80483dd: 5e pop %esi
80483de: 5f pop %edi
80483df: 5d pop %ebp
80483e0: c3 ret
080483e1 <__libc_csu_fini>:
80483e1: 55 push %ebp
80483e2: 89 e5 mov %esp,%ebp
80483e4: 83 ec 18 sub $0x18,%esp
80483e7: 89 5d f4 mov %ebx,0xfffffff4(%ebp)
80483ea: e8 19 ff ff ff call 8048308 <__i686.get_pc_thunk.bx>
80483ef: 81 c3 85 11 00 00 add $0x1185,%ebx
80483f5: 89 75 f8 mov %esi,0xfffffff8(%ebp)
80483f8: 89 7d fc mov %edi,0xfffffffc(%ebp)
80483fb: 8d b3 20 ff ff ff lea 0xffffff20(%ebx),%esi
8048401: 8d bb 20 ff ff ff lea 0xffffff20(%ebx),%edi
8048407: 29 fe sub %edi,%esi
8048409: c1 fe 02 sar $0x2,%esi
804840c: eb 03 jmp 8048411 <__libc_csu_fini+0x30>
804840e: ff 14 b7 call *(%edi,%esi,4)
8048411: 83 ee 01 sub $0x1,%esi
8048414: 83 fe ff cmp $0xffffffff,%esi
8048417: 75 f5 jne 804840e <__libc_csu_fini+0x2d>
8048419: e8 3a 00 00 00 call 8048458 <_fini>
804841e: 8b 5d f4 mov 0xfffffff4(%ebp),%ebx
8048421: 8b 75 f8 mov 0xfffffff8(%ebp),%esi
8048424: 8b 7d fc mov 0xfffffffc(%ebp),%edi
8048427: 89 ec mov %ebp,%esp
8048429: 5d pop %ebp
804842a: c3 ret
804842b: 90 nop
0804842c <__do_global_ctors_aux>:
804842c: 55 push %ebp
804842d: 89 e5 mov %esp,%ebp
804842f: 53 push %ebx
8048430: 83 ec 04 sub $0x4,%esp
8048433: a1 94 94 04 08 mov 0x8049494,%eax
8048438: 83 f8 ff cmp $0xffffffff,%eax
804843b: 74 12 je 804844f <__do_global_ctors_aux+0x23>
804843d: bb 94 94 04 08 mov $0x8049494,%ebx
8048442: ff d0 call *%eax
8048444: 8b 43 fc mov 0xfffffffc(%ebx),%eax
8048447: 83 eb 04 sub $0x4,%ebx
804844a: 83 f8 ff cmp $0xffffffff,%eax
804844d: 75 f3 jne 8048442 <__do_global_ctors_aux+0x16>
804844f: 83 c4 04 add $0x4,%esp
8048452: 5b pop %ebx
8048453: 5d pop %ebp
8048454: c3 ret
8048455: 90 nop
8048456: 90 nop
8048457: 90 nop
Disassembly of section .fini:
08048458 <_fini>:
8048458: 55 push %ebp
8048459: 89 e5 mov %esp,%ebp
804845b: 53 push %ebx
804845c: e8 a7 fe ff ff call 8048308 <__i686.get_pc_thunk.bx>
8048461: 81 c3 13 11 00 00 add $0x1113,%ebx
8048467: 83 ec 04 sub $0x4,%esp
804846a: e8 9d fe ff ff call 804830c <__do_global_dtors_aux>
804846f: 83 c4 04 add $0x4,%esp
8048472: 5b pop %ebx
8048473: 5d pop %ebp
8048474: c3 ret
For a programmer who is fascinated by the low-level details of programming, this is a powerful tool for studying the output of compilers and assemblers. Details, such as those shown in this code, reveal a lot about how the native processor itself operates. When studied hand-in-hand with the processor manufacturer's technical documentation, you can glean valuable insights into how such things work to a greater degree because of the clarity of output from a functioning program.
Likewise, the readelf program can list the contents of the object file with similar lucidity. You can see this by typing the following command:
readelf -all a.out
This command produces the output shown in Listing 4. The ELF header shows a nice summary of all the section entries in the file. Before enumerating the contents of those headers, you can see how many there are. This information can be useful when exploring a rather large object file.
Listing 4. Output of the readelf command
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: 0x80482c0
Start of program headers: 52 (bytes into file)
Start of section headers: 3504 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 7
Size of section headers: 40 (bytes)
Number of section headers: 34
Section header string table index: 31
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 08048114 000114 000013 00 A 0 0 1
[ 2] .note.ABI-tag NOTE 08048128 000128 000020 00 A 0 0 4
[ 3] .hash HASH 08048148 000148 00002c 04 A 4 0 4
[ 4] .dynsym DYNSYM 08048174 000174 000060 10 A 5 1 4
[ 5] .dynstr STRTAB 080481d4 0001d4 00005e 00 A 0 0 1
[ 6] .gnu.version VERSYM 08048232 000232 00000c 02 A 4 0 2
[ 7] .gnu.version_r VERNEED 08048240 000240 000020 00 A 5 1 4
[ 8] .rel.dyn REL 08048260 000260 000008 08 A 4 0 4
[ 9] .rel.plt REL 08048268 000268 000010 08 A 4 11 4
[10] .init PROGBITS 08048278 000278 000017 00 AX 0 0 1
[11] .plt PROGBITS 08048290 000290 000030 04 AX 0 0 4
[12] .text PROGBITS 080482c0 0002c0 000198 00 AX 0 0 4
[13] .fini PROGBITS 08048458 000458 00001d 00 AX 0 0 1
[14] .rodata PROGBITS 08048478 000478 000015 00 A 0 0 4
[15] .eh_frame PROGBITS 08048490 000490 000004 00 A 0 0 4
[16] .ctors PROGBITS 08049494 000494 000008 00 WA 0 0 4
[17] .dtors PROGBITS 0804949c 00049c 000008 00 WA 0 0 4
[18] .jcr PROGBITS 080494a4 0004a4 000004 00 WA 0 0 4
[19] .dynamic DYNAMIC 080494a8 0004a8 0000c8 08 WA 5 0 4
[20] .got PROGBITS 08049570 000570 000004 04 WA 0 0 4
[21] .got.plt PROGBITS 08049574 000574 000014 04 WA 0 0 4
[22] .data PROGBITS 08049588 000588 00000c 00 WA 0 0 4
[23] .bss NOBITS 08049594 000594 000004 00 WA 0 0 4
[24] .comment PROGBITS 00000000 000594 000126 00 0 0 1
[25] .debug_aranges PROGBITS 00000000 0006c0 000088 00 0 0 8
[26] .debug_pubnames PROGBITS 00000000 000748 000025 00 0 0 1
[27] .debug_info PROGBITS 00000000 00076d 00022b 00 0 0 1
[28] .debug_abbrev PROGBITS 00000000 000998 000076 00 0 0 1
[29] .debug_line PROGBITS 00000000 000a0e 0001bb 00 0 0 1
[30] .debug_str PROGBITS 00000000 000bc9 0000bf 01 MS 0 0 1
[31] .shstrtab STRTAB 00000000 000c88 000127 00 0 0 1
[32] .symtab SYMTAB 00000000 001300 000520 10 33 63 4
[33] .strtab STRTAB 00000000 001820 0002d2 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), 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 0x000e0 0x000e0 R E 0x4
INTERP 0x000114 0x08048114 0x08048114 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
LOAD 0x000000 0x08048000 0x08048000 0x00494 0x00494 R E 0x1000
LOAD 0x000494 0x08049494 0x08049494 0x00100 0x00104 RW 0x1000
DYNAMIC 0x0004a8 0x080494a8 0x080494a8 0x000c8 0x000c8 RW 0x4
NOTE 0x000128 0x08048128 0x08048128 0x00020 0x00020 R 0x4
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x4
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version
.gnu.version_r .rel.dyn .rel.plt .init .plt .text .fini .rodata .eh_frame
03 .ctors .dtors .jcr .dynamic .got .got.plt .data .bss
04 .dynamic
05 .note.ABI-tag
06
Dynamic section at offset 0x4a8 contains 20 entries:
Tag Type Name/Value
0x00000001 (NEEDED) Shared library: [libc.so.6]
0x0000000c (INIT) 0x8048278
0x0000000d (FINI) 0x8048458
0x00000004 (HASH) 0x8048148
0x00000005 (STRTAB) 0x80481d4
0x00000006 (SYMTAB) 0x8048174
0x0000000a (STRSZ) 94 (bytes)
0x0000000b (SYMENT) 16 (bytes)
0x00000015 (DEBUG) 0x0
0x00000003 (PLTGOT) 0x8049574
0x00000002 (PLTRELSZ) 16 (bytes)
0x00000014 (PLTREL) REL
0x00000017 (JMPREL) 0x8048268
0x00000011 (REL) 0x8048260
0x00000012 (RELSZ) 8 (bytes)
0x00000013 (RELENT) 8 (bytes)
0x6ffffffe (VERNEED) 0x8048240
0x6fffffff (VERNEEDNUM) 1
0x6ffffff0 (VERSYM) 0x8048232
0x00000000 (NULL) 0x0
Relocation section '.rel.dyn' at offset 0x260 contains 1 entries:
Offset Info Type Sym.Value Sym. Name
08049570 00000506 R_386_GLOB_DAT 00000000 __gmon_start__
Relocation section '.rel.plt' at offset 0x268 contains 2 entries:
Offset Info Type Sym.Value Sym. Name
08049580 00000107 R_386_JUMP_SLOT 00000000 puts
08049584 00000207 R_386_JUMP_SLOT 00000000 __libc_start_main
There are no unwind sections in this file.
Symbol table '.dynsym' contains 6 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 378 FUNC GLOBAL DEFAULT UND puts@GLIBC_2.0 (2)
2: 00000000 230 FUNC GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.0 (2)
3: 0804847c 4 OBJECT GLOBAL DEFAULT 14 _IO_stdin_used
4: 00000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
5: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
Symbol table '.symtab' contains 82 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 08048114 0 SECTION LOCAL DEFAULT 1
2: 08048128 0 SECTION LOCAL DEFAULT 2
3: 08048148 0 SECTION LOCAL DEFAULT 3
4: 08048174 0 SECTION LOCAL DEFAULT 4
5: 080481d4 0 SECTION LOCAL DEFAULT 5
6: 08048232 0 SECTION LOCAL DEFAULT 6
7: 08048240 0 SECTION LOCAL DEFAULT 7
8: 08048260 0 SECTION LOCAL DEFAULT 8
9: 08048268 0 SECTION LOCAL DEFAULT 9
10: 08048278 0 SECTION LOCAL DEFAULT 10
11: 08048290 0 SECTION LOCAL DEFAULT 11
12: 080482c0 0 SECTION LOCAL DEFAULT 12
13: 08048458 0 SECTION LOCAL DEFAULT 13
14: 08048478 0 SECTION LOCAL DEFAULT 14
15: 08048490 0 SECTION LOCAL DEFAULT 15
16: 08049494 0 SECTION LOCAL DEFAULT 16
17: 0804949c 0 SECTION LOCAL DEFAULT 17
18: 080494a4 0 SECTION LOCAL DEFAULT 18
19: 080494a8 0 SECTION LOCAL DEFAULT 19
20: 08049570 0 SECTION LOCAL DEFAULT 20
21: 08049574 0 SECTION LOCAL DEFAULT 21
22: 08049588 0 SECTION LOCAL DEFAULT 22
23: 08049594 0 SECTION LOCAL DEFAULT 23
24: 00000000 0 SECTION LOCAL DEFAULT 24
25: 00000000 0 SECTION LOCAL DEFAULT 25
26: 00000000 0 SECTION LOCAL DEFAULT 26
27: 00000000 0 SECTION LOCAL DEFAULT 27
28: 00000000 0 SECTION LOCAL DEFAULT 28
29: 00000000 0 SECTION LOCAL DEFAULT 29
30: 00000000 0 SECTION LOCAL DEFAULT 30
31: 00000000 0 SECTION LOCAL DEFAULT 31
32: 00000000 0 SECTION LOCAL DEFAULT 32
33: 00000000 0 SECTION LOCAL DEFAULT 33
34: 00000000 0 FILE LOCAL DEFAULT ABS abi-note.S
35: 00000000 0 FILE LOCAL DEFAULT ABS ../sysdeps/i386/elf/start
36: 00000000 0 FILE LOCAL DEFAULT ABS init.c
37: 00000000 0 FILE LOCAL DEFAULT ABS initfini.c
38: 00000000 0 FILE LOCAL DEFAULT ABS /build/buildd/glibc-2.3.6
39: 080482e4 0 FUNC LOCAL DEFAULT 12 call_gmon_start
40: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
41: 08049494 0 OBJECT LOCAL DEFAULT 16 __CTOR_LIST__
42: 0804949c 0 OBJECT LOCAL DEFAULT 17 __DTOR_LIST__
43: 080494a4 0 OBJECT LOCAL DEFAULT 18 __JCR_LIST__
44: 08049594 1 OBJECT LOCAL DEFAULT 23 completed.4463
45: 08049590 0 OBJECT LOCAL DEFAULT 22 p.4462
46: 0804830c 0 FUNC LOCAL DEFAULT 12 __do_global_dtors_aux
47: 0804833b 0 FUNC LOCAL DEFAULT 12 frame_dummy
48: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
49: 08049498 0 OBJECT LOCAL DEFAULT 16 __CTOR_END__
50: 080494a0 0 OBJECT LOCAL DEFAULT 17 __DTOR_END__
51: 08048490 0 OBJECT LOCAL DEFAULT 15 __FRAME_END__
52: 080494a4 0 OBJECT LOCAL DEFAULT 18 __JCR_END__
53: 0804842c 0 FUNC LOCAL DEFAULT 12 __do_global_ctors_aux
54: 00000000 0 FILE LOCAL DEFAULT ABS initfini.c
55: 00000000 0 FILE LOCAL DEFAULT ABS /build/buildd/glibc-2.3.6
56: 00000000 0 FILE LOCAL DEFAULT ABS hw.c
57: 080494a8 0 OBJECT LOCAL HIDDEN 19 _DYNAMIC
58: 08049494 0 NOTYPE LOCAL HIDDEN ABS __fini_array_end
59: 08049494 0 NOTYPE LOCAL HIDDEN ABS __fini_array_start
60: 08049494 0 NOTYPE LOCAL HIDDEN ABS __init_array_end
61: 08049574 0 OBJECT LOCAL HIDDEN 21 _GLOBAL_OFFSET_TABLE_
62: 08049494 0 NOTYPE LOCAL HIDDEN ABS __init_array_start
63: 08048478 4 OBJECT GLOBAL DEFAULT 14 _fp_hw
64: 0804958c 0 OBJECT GLOBAL HIDDEN 22 __dso_handle
65: 080483e1 74 FUNC GLOBAL DEFAULT 12 __libc_csu_fini
66: 00000000 378 FUNC GLOBAL DEFAULT UND puts@@GLIBC_2.0
67: 08048278 0 FUNC GLOBAL DEFAULT 10 _init
68: 080482c0 0 FUNC GLOBAL DEFAULT 12 _start
69: 08048390 81 FUNC GLOBAL DEFAULT 12 __libc_csu_init
70: 08049594 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
71: 08048360 47 FUNC GLOBAL DEFAULT 12 main
72: 00000000 230 FUNC GLOBAL DEFAULT UND __libc_start_main@@GLIBC_
73: 08049588 0 NOTYPE WEAK DEFAULT 22 data_start
74: 08048458 0 FUNC GLOBAL DEFAULT 13 _fini
75: 08049594 0 NOTYPE GLOBAL DEFAULT ABS _edata
76: 08048308 0 FUNC GLOBAL HIDDEN 12 __i686.get_pc_thunk.bx
77: 08049598 0 NOTYPE GLOBAL DEFAULT ABS _end
78: 0804847c 4 OBJECT GLOBAL DEFAULT 14 _IO_stdin_used
79: 08049588 0 NOTYPE GLOBAL DEFAULT 22 __data_start
80: 00000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses
81: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
Histogram for bucket list length (total of 3 buckets):
Length Number % of total Coverage
0 0 ( 0.0%)
1 1 ( 33.3%) 20.0%
2 2 ( 66.7%) 100.0%
Version symbols section '.gnu.version' contains 6 entries:
Addr: 0000000008048232 Offset: 0x000232 Link: 4 (.dynsym)
000: 0 (*local*) 2 (GLIBC_2.0) 2 (GLIBC_2.0) 1 (*global*)
004: 0 (*local*) 0 (*local*)
Version needs section '.gnu.version_r' contains 1 entries:
Addr: 0x0000000008048240 Offset: 0x000240 Link to section: 5 (.dynstr)
000000: Version: 1 File: libc.so.6 Cnt: 1
0x0010: Name: GLIBC_2.0 Flags: none Version: 2
Notes at offset 0x00000128 with length 0x00000020:
Owner Data size Description
GNU 0x00000010 NT_VERSION (version)
As you can see from this output, a huge amount of useful detail resides in the simple a.out Hello World file -- version information, histograms, multiple tables of various symbol types, and so on. Yes, one can spend a great deal of time learning about executable programs by exploring object files with just the few tools presented here.
In addition to all these sections, the compiler can place debugging information in the object files, and such information can be displayed as well. Type the following command and take some time to see what the compiler is telling you (if you're a debugging program, that is):
readelf --debug-dump a.out | less
This command produces the output shown in Listing 5. Debugging tools, such as GDB, read in this debugging information, and you can get the tools to display more descriptive labels (for example) than raw address values when disassembling code while it's running under the debugger.
Listing 5. Debugging information in the program
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.
Executable files are object files
In the UNIX world, executable files are object files, and you can examine them as you did the a.out file. It is a useful exercise to change to the /bin or /local/bin directory and run nm, objdump, and readelf over some of your most commonly used commands, such as pwd, ps, cat, or rm. Often when you're writing a program that requires a certain functionality that one of the standard tools has, it's useful to see how those tools actually do their work by simply running objdump -d <command> over it.
If you're so inclined to work on compilers and other language tools, you'll find that time spent studying the various object files that make up your computer's system is time well spent. A UNIX operating system has many layers, and the layers that the tools examining its object files expose are close to the hardware. You can get a real feel for the system in this way.
Conclusion
Exploring object files can greatly deepen your knowledge of the UNIX operating system and provide greater insight into how the software is actually assembled from source code. I encourage you to study the output of the object file tools described in this article by running them over the programs found in the /bin or /local/bin directories on your system and seek out system documentation that your hardware manufacturer provides.
http://www.ibm.com/developerworks/aix/library/au-unixtools.html?S_TACT=105AGX52&S_CMP=cn-a-aix
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