为什么要动态链接

• 浪费内存和和磁盘空间
  • 每个程序都得拥有相同的拷贝
• 程序的发布、更新和部署困难
  • 更新任何一个小的模块，整个程序都需要重新链接

动态链接：将链接的过程推迟到运行时

• 节省内存
• 减少物理页面的换入换出、增加CPU的缓存命中率
• 更新升级方便，各个模块开发相互独立，耦合度更小
• 可扩展性和兼容性
  • 兼容性体现在能够在操作系统和应用层增加中间层，消除平台的依赖性

动态链接的例子

• lib.so 保留了完整的符号信息，
• 链接器在链接时将符号标记为动态符号，在装载时在进行地址重定位

// lib.h
#ifndef __LIB_H__
#define __LIB_H__
void foobar(int i);
#endif
// lib.c
#include <stdio.h>
void foobar(int i) {printf("Printing from lib.so %d\n", i);
}
// program1.c
#include "lib.h"
int main() {foobar(1);return 0;
}
// program2.c
#include "lib.h"
int main() {foobar(2);return 0;
}$ gcc -fPIC -shared -o lib.so lib.c
$ gcc -c program1 program1.c ./lib.so
$ gcc -o program2 program2.c ./lib.so

动态链接程序运行时地址空间分布

$ ./program1 &
[1] 3966895
Printing from lib.so 1
$ cat /proc/3966895/maps
563546b06000-563546b07000 r--p 00000000 fc:02 2601194                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/program1
563546b07000-563546b08000 r-xp 00001000 fc:02 2601194                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/program1
563546b08000-563546b09000 r--p 00002000 fc:02 2601194                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/program1
563546b09000-563546b0a000 r--p 00002000 fc:02 2601194                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/program1
563546b0a000-563546b0b000 rw-p 00003000 fc:02 2601194                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/program1
563547e2e000-563547e4f000 rw-p 00000000 00:00 0                          [heap]
7f4d5c20a000-7f4d5c20d000 rw-p 00000000 00:00 0 
7f4d5c20d000-7f4d5c22f000 r--p 00000000 fc:02 139834                     /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f4d5c22f000-7f4d5c3a7000 r-xp 00022000 fc:02 139834                     /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f4d5c3a7000-7f4d5c3f5000 r--p 0019a000 fc:02 139834                     /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f4d5c3f5000-7f4d5c3f9000 r--p 001e7000 fc:02 139834                     /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f4d5c3f9000-7f4d5c3fb000 rw-p 001eb000 fc:02 139834                     /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f4d5c3fb000-7f4d5c3ff000 rw-p 00000000 00:00 0 
7f4d5c40a000-7f4d5c40b000 r--p 00000000 fc:02 2601193                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/lib.so
7f4d5c40b000-7f4d5c40c000 r-xp 00001000 fc:02 2601193                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/lib.so
7f4d5c40c000-7f4d5c40d000 r--p 00002000 fc:02 2601193                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/lib.so
7f4d5c40d000-7f4d5c40e000 r--p 00002000 fc:02 2601193                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/lib.so
7f4d5c40e000-7f4d5c40f000 rw-p 00003000 fc:02 2601193                    /home/ubuntu/cpp_learn/effective_cpp/xiuyang07/lib.so
7f4d5c40f000-7f4d5c411000 rw-p 00000000 00:00 0 
7f4d5c411000-7f4d5c412000 r--p 00000000 fc:02 130951                     /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f4d5c412000-7f4d5c435000 r-xp 00001000 fc:02 130951                     /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f4d5c435000-7f4d5c43d000 r--p 00024000 fc:02 130951                     /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f4d5c43e000-7f4d5c43f000 r--p 0002c000 fc:02 130951                     /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f4d5c43f000-7f4d5c440000 rw-p 0002d000 fc:02 130951                     /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f4d5c440000-7f4d5c441000 rw-p 00000000 00:00 0 
7fff3218c000-7fff321ad000 rw-p 00000000 00:00 0                          [stack]
7fff321dd000-7fff321e0000 r--p 00000000 00:00 0                          [vvar]
7fff321e0000-7fff321e1000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 --xp 00000000 00:00 0                  [vsyscall]

lib.so 的装载属性

$ readelf -l lib.so Elf file type is DYN (Shared object file)
Entry point 0x1080
There are 11 program headers, starting at offset 64Program Headers:Type           Offset             VirtAddr           PhysAddrFileSiz            MemSiz              Flags  AlignLOAD           0x0000000000000000 0x0000000000000000 0x00000000000000000x0000000000000560 0x0000000000000560  R      0x1000LOAD           0x0000000000001000 0x0000000000001000 0x00000000000010000x000000000000017d 0x000000000000017d  R E    0x1000LOAD           0x0000000000002000 0x0000000000002000 0x00000000000020000x00000000000000dc 0x00000000000000dc  R      0x1000LOAD           0x0000000000002e10 0x0000000000003e10 0x0000000000003e100x0000000000000220 0x0000000000000228  RW     0x1000DYNAMIC        0x0000000000002e20 0x0000000000003e20 0x0000000000003e200x00000000000001c0 0x00000000000001c0  RW     0x8NOTE           0x00000000000002a8 0x00000000000002a8 0x00000000000002a80x0000000000000020 0x0000000000000020  R      0x8NOTE           0x00000000000002c8 0x00000000000002c8 0x00000000000002c80x0000000000000024 0x0000000000000024  R      0x4GNU_PROPERTY   0x00000000000002a8 0x00000000000002a8 0x00000000000002a80x0000000000000020 0x0000000000000020  R      0x8GNU_EH_FRAME   0x000000000000201c 0x000000000000201c 0x000000000000201c0x000000000000002c 0x000000000000002c  R      0x4GNU_STACK      0x0000000000000000 0x0000000000000000 0x00000000000000000x0000000000000000 0x0000000000000000  RW     0x10GNU_RELRO      0x0000000000002e10 0x0000000000003e10 0x0000000000003e100x00000000000001f0 0x00000000000001f0  R      0x1Section to Segment mapping:Segment Sections...00     .note.gnu.property .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt 01     .init .plt .plt.got .plt.sec .text .fini 02     .rodata .eh_frame_hdr .eh_frame 03     .init_array .fini_array .dynamic .got .got.plt .data .bss 04     .dynamic 05     .note.gnu.property 06     .note.gnu.build-id 07     .note.gnu.property 08     .eh_frame_hdr 09     10     .init_array .fini_array .dynamic .got