链接

典型的ELF可重定位文件格式

示例代码

/* main.c */
int sum(int *a, int n);
int array[2] = {1, 2};
int main()
{
    int value = sum(array, 2);
    return value;
}

/* sum.c */
int sum(int *a,int n){
    int i, sum = 0;
    for (i = 0; i < n; ++i)
    {
        sum += a[i];
    }
    return sum;
}

符号表

编译成elf可重定位文件后，使用 readelf -all main.o 打印出如下信息

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:          720 (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:         12
  Section header string table index: 11
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
       0000000000000021  0000000000000000  AX       0     0     1
  [ 2] .rela.text        RELA             0000000000000000  00000228
       0000000000000030  0000000000000018   I       9     1     8
  [ 3] .data             PROGBITS         0000000000000000  00000068
       0000000000000008  0000000000000000  WA       0     0     8
  [ 4] .bss              NOBITS           0000000000000000  00000070
       0000000000000000  0000000000000000  WA       0     0     1
  [ 5] .comment          PROGBITS         0000000000000000  00000070
       000000000000002a  0000000000000001  MS       0     0     1
  [ 6] .note.GNU-stack   PROGBITS         0000000000000000  0000009a
       0000000000000000  0000000000000000           0     0     1
  [ 7] .eh_frame         PROGBITS         0000000000000000  000000a0
       0000000000000038  0000000000000000   A       0     0     8
  [ 8] .rela.eh_frame    RELA             0000000000000000  00000258
       0000000000000018  0000000000000018   I       9     7     8
  [ 9] .symtab           SYMTAB           0000000000000000  000000d8
       0000000000000120  0000000000000018          10     8     8
  [10] .strtab           STRTAB           0000000000000000  000001f8
       000000000000002d  0000000000000000           0     0     1
  [11] .shstrtab         STRTAB           0000000000000000  00000270
       0000000000000059  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  l (large), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
There is no dynamic section in this file.
Relocation section '.rela.text' at offset 0x228 contains 2 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000010  000800000002 R_X86_64_PC32     0000000000000000 array - 4
000000000015  000b00000004 R_X86_64_PLT32    0000000000000000 sum - 4
Relocation section '.rela.eh_frame' at offset 0x258 contains 1 entry:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000020  000200000002 R_X86_64_PC32     0000000000000000 .text + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.symtab' contains 12 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS main.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3 
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    7 
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    5 
     8: 0000000000000000     8 OBJECT  GLOBAL DEFAULT    3 array
     9: 0000000000000000    33 FUNC    GLOBAL DEFAULT    1 main
    10: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_
    11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND sum
No version information found in this file.

重点关注符号表的最后4行，表示main是一个.text节（由Ndx=1代表text节）中偏移量为0的大小为33字节的函数，array为一个.data节（由Ndx=3代表data节）中偏移量为0的大小为8字节的对象，sum是一个对外部符号的引用。

符号解析

（c++中的符号重整）

强符号与弱符号

强符号： 函数与已初始化的全局变量
弱符号： 未初始化的全局变量
Linux处理规则

objdump -dx main.o 得到以下输出

0000000000000000 <main>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 83 ec 10             sub    $0x10,%rsp
   8:   be 02 00 00 00          mov    $0x2,%esi
   d:   48 8d 3d 00 00 00 00    lea    0x0(%rip),%rdi        # 14 <main+0x14>
                        10: R_X86_64_PC32       array-0x4
  14:   e8 00 00 00 00          callq  19 <main+0x19>
                        15: R_X86_64_PLT32      sum-0x4
  19:   89 45 fc                mov    %eax,-0x4(%rbp)
  1c:   8b 45 fc                mov    -0x4(%rbp),%eax
  1f:   c9                      leaveq 
  20:   c3                      retq

其中第6、7行是对array的重定位条目，8、9行是对sum的重定位条目。

创建静态库

代码部分

/* multvec.c */
/* $begin multvec */
int multcnt = 0;
void multvec(int *x, int *y,
             int *z, int n)
{
    int i;
    multcnt++;
    for (i = 0; i < n; i++)
        z[i] = x[i] * y[i];
}
/* $end multvec */

/* addvec.c */
/* $begin addvec */
int addcnt = 0;
void addvec(int *x, int *y,
            int *z, int n)
{
    int i;
    addcnt++;
    for (i = 0; i < n; i++)
        z[i] = x[i] + y[i];
}
/* $end addvec */

/* main.c */
/* $begin main */
#include <stdio.h>
#include "vector.h"
int x[2] = {1, 2};
int y[2] = {3, 4};
int z[2];
int main()
{
    addvec(x, y, z, 2);
    printf("z = [%d %d]\n", z[0], z[1]);
    return 0;
}
/* $end main2 */

利用AR工具创建静态库
gcc -c multvec.c addvec.c
ar rcs libvector.a addvec.o multvec.o
使用： gcc -o prog1 main.c -L. -lvector

创建动态库

命令 gcc -shared -fpic -o libvector.so addvec.c multvec.c
其中-fpic用于生成与位置无关的代码，-shared指示链接器创造一个共享的目标文件。
使用： gcc -o prog2 main.c -L. -lvector

可以看出动态链接比静态链接产生的代码体积小

Linux运行时加载动态库

主要包括函数 dlopen dlsym dlclose 与· dlerror
示例（来自CSAPP第七章7.11）：

/* $begin dll */
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
int x[2] = {1, 2};
int y[2] = {3, 4};
int z[2];
int main() 
{
    void *handle;
    void (*addvec)(int *, int *, int *, int);
    char *error; 
    /* Dynamically load the shared library that contains addvec() */
    handle = dlopen("./libvector.so", RTLD_LAZY);
    if (!handle) {
    fprintf(stderr, "%s\n", dlerror());
    exit(1);
    }
    /* Get a pointer to the addvec() function we just loaded */
    addvec = dlsym(handle, "addvec");
    if ((error = dlerror()) != NULL) {
    fprintf(stderr, "%s\n", error);
    exit(1);
    }
    /* Now we can call addvec() just like any other function */
    addvec(x, y, z, 2);
    printf("z = [%d %d]\n", z[0], z[1]);
    /* Unload the shared library */
    if (dlclose(handle) < 0) {
    fprintf(stderr, "%s\n", dlerror());
    exit(1);
    }
    return 0;
}
/* $end dll */