《程序员的自我修养》第三章——目标文件里有什么

示例代码(main.c)

int printf(const char *format, ...);

int global_init_var = 84;
int global_uninit_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;
}

查看目标文件

gcc -c main.c
objdump  -s -d  main.o

目标文件内容

代码与目标文件对应关系

代码段

存放func1函数和main函数的代码

Contents of section .text:
 0000 f30f1efa 554889e5 4883ec10 897dfc8b  ....UH..H....}..
 0010 45fc89c6 488d3d00 000000b8 00000000  E...H.=.........
 0020 e8000000 0090c9c3 f30f1efa 554889e5  ............UH..
 0030 4883ec10 c745f801 0000008b 15000000  H....E..........
 0040 008b0500 00000001 c28b45f8 01c28b45  ..........E....E
 0050 fc01d089 c7e80000 00008b45 f8c9c3    ...........E...
 
 ......

Disassembly of section .text:

0000000000000000 <func1>:
   0:   f3 0f 1e fa             endbr64
   4:   55                      push   %rbp
   5:   48 89 e5                mov    %rsp,%rbp
   8:   48 83 ec 10             sub    $0x10,%rsp
   c:   89 7d fc                mov    %edi,-0x4(%rbp)
   f:   8b 45 fc                mov    -0x4(%rbp),%eax
  12:   89 c6                   mov    %eax,%esi
  14:   48 8d 3d 00 00 00 00    lea    0x0(%rip),%rdi        # 1b <func1+0x1b>
  1b:   b8 00 00 00 00          mov    $0x0,%eax
  20:   e8 00 00 00 00          callq  25 <func1+0x25>
  25:   90                      nop
  26:   c9                      leaveq
  27:   c3                      retq

0000000000000028 <main>:
  28:   f3 0f 1e fa             endbr64
  2c:   55                      push   %rbp
  2d:   48 89 e5                mov    %rsp,%rbp
  30:   48 83 ec 10             sub    $0x10,%rsp
  34:   c7 45 f8 01 00 00 00    movl   $0x1,-0x8(%rbp)
  3b:   8b 15 00 00 00 00       mov    0x0(%rip),%edx        # 41 <main+0x19>
  41:   8b 05 00 00 00 00       mov    0x0(%rip),%eax        # 47 <main+0x1f>
  47:   01 c2                   add    %eax,%edx
  49:   8b 45 f8                mov    -0x8(%rbp),%eax
  4c:   01 c2                   add    %eax,%edx
  4e:   8b 45 fc                mov    -0x4(%rbp),%eax
  51:   01 d0                   add    %edx,%eax
  53:   89 c7                   mov    %eax,%edi
  55:   e8 00 00 00 00          callq  5a <main+0x32>
  5a:   8b 45 f8                mov    -0x8(%rbp),%eax
  5d:   c9                      leaveq
  5e:   c3                      retq

数据段

存放初始化了的全局静态变量和局部静态变量，比如这里0x54000000和0x55000000和就分别是代码中的int global_init_var = 84;和static int static_var = 85;，注意用的是大端序。

Contents of section .data:
 0000 54000000 55000000                    T...U...

只读数据段

存放只读数据，比如const变量和字符串常量，比如这里0x25640a00就是printf函数用到的字符串常量"%d\n"

Contents of section .rodata:
 0000 25640a00                             %d..

BSS段

存放未初始化的全局静态变量和局部静态变量，比如代码中的global_uninit_var和static_var2

总结