目标文件的格式是什么
流行的可执行文件格式主要是win下的PE(Portable Executable) 和 Linux下的ELF(Executable Linkable Format)，它们皆为COFF(Common file format)格式的变种
linux下的目标文件后缀为.o，终端中查看.o文件类型信息如下:[root@slave02 temp]# file hello.ohello.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
| 均按照可执行文件格式存储 | win下 | linux下 |
|---|---|---|
| 可执行文件 | .exe | ELF |
| 动态链接库 | .dll | .so |
| 静态链接库 | .lib | .a |
目标文件包含哪些内容
目标文件中的内容包括 编译后的机器指令代码、数据和供后续链接时所需要的一些信息，如符号表、调试信息、字符串等
目标文件内部分成不同的段, 简单例子如下:
指令和数据分开存放的好处
- 指令段设置为只读，数据段设置为可读写，提高安全性
- 有利于提高cache缓存命中率
- 当系统中运行多个该程序的副本时，只需保留一份指令段，当然，每个进程的数据段是私有的。有利于节省大量内存空间。
细探SimpleSection.o
在centos中编辑如下文件:
//SimpleSection.c文件内容
#include <stdio.h>
int global_init_var = 84;
int global_unint_var;
void func1(int i){
printf("%d\n", i);
}
int main(void){
static int static_var = 85;
static int static_var2;
int a=1;
int b;
func1(static_var+static_var2+a+b);
return a;
}
然后运行gcc -c SimpleSection.c得到目标文件SimpleSection.o
使用objdump命令或者readelf命令查看.o文件objdump -h SimpleSection.o
SimpleSection.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000054 0000000000000000 0000000000000000 00000040 2**0
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000008 0000000000000000 0000000000000000 00000094 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000004 0000000000000000 0000000000000000 0000009c 2**2
ALLOC
3 .rodata 00000004 0000000000000000 0000000000000000 0000009c 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .comment 0000002e 0000000000000000 0000000000000000 000000a0 2**0
CONTENTS, READONLY
5 .note.GNU-stack 00000000 0000000000000000 0000000000000000 000000ce 2**0
CONTENTS, READONLY
6 .eh_frame 00000058 0000000000000000 0000000000000000 000000d0 2**3
CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
代码段
Contents of section .text:
0000 554889e5 4883ec10 897dfc8b 45fc89c6 UH..H....}..E...
0010 bf000000 00b80000 0000e800 000000c9 ................
0020 c3554889 e54883ec 10c745fc 01000000 .UH..H....E.....
0030 8b150000 00008b05 00000000 01c28b45 ...............E
0040 fc01c28b 45f801d0 89c7e800 0000008b ....E...........
0050 45fcc9c3 E...
略去无关内容。。。
0000000000000000 :
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 10 sub $0x10,%rsp
8: 89 7d fc mov %edi,-0x4(%rbp)
b: 8b 45 fc mov -0x4(%rbp),%eax
e: 89 c6 mov %eax,%esi
10: bf 00 00 00 00 mov $0x0,%edi
15: b8 00 00 00 00 mov $0x0,%eax
1a: e8 00 00 00 00 callq 1f
1f: c9 leaveq
20: c3 retq
0000000000000021 :
21: 55 push %rbp
22: 48 89 e5 mov %rsp,%rbp
25: 48 83 ec 10 sub $0x10,%rsp
29: c7 45 fc 01 00 00 00 movl $0x1,-0x4(%rbp)
30: 8b 15 00 00 00 00 mov 0x0(%rip),%edx # 36
36: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 3c
3c: 01 c2 add %eax,%edx
3e: 8b 45 fc mov -0x4(%rbp),%eax
41: 01 c2 add %eax,%edx
43: 8b 45 f8 mov -0x8(%rbp),%eax
46: 01 d0 add %edx,%eax
48: 89 c7 mov %eax,%edi
4a: e8 00 00 00 00 callq 4f
4f: 8b 45 fc mov -0x4(%rbp),%eax
52: c9 leaveq
53: c3 retq
数据段和只读数据段
.bbs段
其他段
自定义段
ELF文件结构描述
ELF文件头
[cmd] readelf -h SimpleSection.o
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: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 1056 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 13
Section header string table index: 10
段表(Section Header Table)
它是除了文件头外最重要的结构，用于描述各个段的信息,比如:段名，段长,在文件中的位置偏移，读写权限和其它属性[cmd] readelf -S SimpleSection.o
There are 13 section headers, starting at offset 0x420:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
0000000000000054 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 00000378
0000000000000078 0000000000000018 I 11 1 8
[ 3] .data PROGBITS 0000000000000000 00000094
0000000000000008 0000000000000000 WA 0 0 4
[ 4] .bss NOBITS 0000000000000000 0000009c
0000000000000004 0000000000000000 WA 0 0 4
[ 5] .rodata PROGBITS 0000000000000000 0000009c
0000000000000004 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 000000a0
000000000000002e 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 000000ce
0000000000000000 0000000000000000 0 0 1
[ 8] .eh_frame PROGBITS 0000000000000000 000000d0
0000000000000058 0000000000000000 A 0 0 8
[ 9] .rela.eh_frame RELA 0000000000000000 000003f0
0000000000000030 0000000000000018 I 11 8 8
[10] .shstrtab STRTAB 0000000000000000 00000128
0000000000000061 0000000000000000 0 0 1
[11] .symtab SYMTAB 0000000000000000 00000190
0000000000000180 0000000000000018 12 11 8
[12] .strtab STRTAB 0000000000000000 00000310
0000000000000065 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
重定位表
字符串表
链接的Interface–符号
链接本质上就是将不同的目标文件”粘”到一起,
C++名称修饰机制实现函数重载
int func(int);
float func(float);
class C{
int func(int);
class C2{
int func(int);
};
};
namespace N{
int func(int);
class C{
int func(int);
};
}
值得注意的是，visual studio的名称修饰规则和上述规则不同，但目的都相同。
extern “C”
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