实验检测编译过程中的链接作用

00000000004004ed : //（test_a:在最终可执行文件中开始的地址） 00000000004004fb : //（test_b:在最终可执行文件中开始的地址） 0000000000400516

: //（main:在最终可执行文件中开始的地址）同时通过反汇编程序，也能了解到每一种可执行文件确实有差别（window下的和linux等不同操作系统下的）。每个可执行文件的开头还有一些初始化部分等。

//文件test_a中的代码： void test_a() { int a = 10; return ; } //文件test_a.o的反汇编 test_a.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 : 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: c7 45 fc 0a 00 00 00 movl $0xa,-0x4(%rbp) b: 90 nop c: 5d pop %rbp d: c3 retq 代码说明:虽然具体的汇编指令看不懂什么意思，但第一行是第一条指令的逻辑地址。各列的意思大概是，（地址，指令所对应的机器代码，最后一列是汇编代码）

//文件test_a中的代码： void test_b() { int b = 5; return ; } //文件test_b.o的反汇编 test_b.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 : 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: c7 45 fc 05 00 00 00 movl $0x5,-0x4(%rbp) b: 90 nop c: 5d pop %rbp d: c3 retq 代码说明:可以看到第一条指令也是从地址0x00开始的。

//文件main中的代码： #include "test.h" void test_c() { int c = 30; } int main() { int m = 20; test_a(); test_b(); return 0; } //文件main.o的反汇编 main.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 : 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: c7 45 fc 1e 00 00 00 movl $0x1e,-0x4(%rbp) b: 5d pop %rbp c: c3 retq 000000000000000d

: d: 55 push %rbp e: 48 89 e5 mov %rsp,%rbp 11: 48 83 ec 10 sub $0x10,%rsp 15: c7 45 fc 14 00 00 00 movl $0x14,-0x4(%rbp) 1c: b8 00 00 00 00 mov $0x0,%eax 21: e8 00 00 00 00 callq 26 0x19> 26: b8 00 00 00 00 mov $0x0,%eax 2b: e8 00 00 00 00 callq 30 0x23> 30: b8 00 00 00 00 mov $0x0,%eax 35: c9 leaveq 36: c3 retq 代码说明：main函数第一条指令也是从0x00开始的（这里的第一条指令不是main函数而是test_c的相关内容）

//最后链接完成的可执行文件的反汇编 main: file format elf64-x86-64 Disassembly of section .init: 00000000004003a8 <_init>: 4003a8: 48 83 ec 08 sub $0x8,%rsp 4003ac: 48 8b 05 45 0c 20 00 mov 0x200c45(%rip),%rax # 600ff8 <_DYNAMIC+0x1d0> 4003b3: 48 85 c0 test %rax,%rax 4003b6: 74 05 je 4003bd <_init+0x15> 4003b8: e8 33 00 00 00 callq 4003f0 <__gmon_start__@plt> 4003bd: 48 83 c4 08 add $0x8,%rsp 4003c1: c3 retq Disassembly of section .plt: 00000000004003d0 <__libc_start_main@plt-0x10>: 4003d0: ff 35 32 0c 20 00 pushq 0x200c32(%rip) # 601008 <_GLOBAL_OFFSET_TABLE_+0x8> 4003d6: ff 25 34 0c 20 00 jmpq *0x200c34(%rip) # 601010 <_GLOBAL_OFFSET_TABLE_+0x10> 4003dc: 0f 1f 40 00 nopl 0x0(%rax) 00000000004003e0 <__libc_start_main@plt>: 4003e0: ff 25 32 0c 20 00 jmpq *0x200c32(%rip) # 601018 <_GLOBAL_OFFSET_TABLE_+0x18> 4003e6: 68 00 00 00 00 pushq $0x0 4003eb: e9 e0 ff ff ff jmpq 4003d0 <_init+0x28> 00000000004003f0 <__gmon_start__@plt>: 4003f0: ff 25 2a 0c 20 00 jmpq *0x200c2a(%rip) # 601020 <_GLOBAL_OFFSET_TABLE_+0x20> 4003f6: 68 01 00 00 00 pushq $0x1 4003fb: e9 d0 ff ff ff jmpq 4003d0 <_init+0x28> Disassembly of section .text: 0000000000400400 <_start>: 400400: 31 ed xor %ebp,%ebp 400402: 49 89 d1 mov %rdx,%r9 400405: 5e pop %rsi 400406: 48 89 e2 mov %rsp,%rdx 400409: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp 40040d: 50 push %rax 40040e: 54 push %rsp 40040f: 49 c7 c0 b0 05 40 00 mov $0x4005b0,%r8 400416: 48 c7 c1 40 05 40 00 mov $0x400540,%rcx 40041d: 48 c7 c7 16 05 40 00 mov $0x400516,%rdi 400424: e8 b7 ff ff ff callq 4003e0 <__libc_start_main@plt> 400429: f4 hlt 40042a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) 0000000000400430 : 400430: b8 3f 10 60 00 mov $0x60103f,%eax 400435: 55 push %rbp 400436: 48 2d 38 10 60 00 sub $0x601038,%rax 40043c: 48 83 f8 0e cmp $0xe,%rax 400440: 48 89 e5 mov %rsp,%rbp 400443: 77 02 ja 400447 0x17> 400445: 5d pop %rbp 400446: c3 retq 400447: b8 00 00 00 00 mov $0x0,%eax 40044c: 48 85 c0 test %rax,%rax 40044f: 74 f4 je 400445 0x15> 400451: 5d pop %rbp 400452: bf 38 10 60 00 mov $0x601038,%edi 400457: ff e0 jmpq *%rax 400459: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 0000000000400460 : 400460: b8 38 10 60 00 mov $0x601038,%eax 400465: 55 push %rbp 400466: 48 2d 38 10 60 00 sub $0x601038,%rax 40046c: 48 c1 f8 03 sar $0x3,%rax 400470: 48 89 e5 mov %rsp,%rbp 400473: 48 89 c2 mov %rax,%rdx 400476: 48 c1 ea 3f shr $0x3f,%rdx 40047a: 48 01 d0 add %rdx,%rax 40047d: 48 d1 f8 sar %rax 400480: 75 02 jne 400484 0x24> 400482: 5d pop %rbp 400483: c3 retq 400484: ba 00 00 00 00 mov $0x0,%edx 400489: 48 85 d2 test %rdx,%rdx 40048c: 74 f4 je 400482 0x22> 40048e: 5d pop %rbp 40048f: 48 89 c6 mov %rax,%rsi 400492: bf 38 10 60 00 mov $0x601038,%edi 400497: ff e2 jmpq *%rdx 400499: 0f 1f 80 00 00 00 00 nopl 0x0(%rax) 00000000004004a0 <__do_global_dtors_aux>: 4004a0: 80 3d 91 0b 20 00 00 cmpb $0x0,0x200b91(%rip) # 601038 <__TMC_END__> 4004a7: 75 11 jne 4004ba <__do_global_dtors_aux+0x1a> 4004a9: 55 push %rbp 4004aa: 48 89 e5 mov %rsp,%rbp 4004ad: e8 7e ff ff ff callq 400430 4004b2: 5d pop %rbp 4004b3: c6 05 7e 0b 20 00 01 movb $0x1,0x200b7e(%rip) # 601038 <__TMC_END__> 4004ba: f3 c3 repz retq 4004bc: 0f 1f 40 00 nopl 0x0(%rax) 00000000004004c0 : 4004c0: 48 83 3d 58 09 20 00 cmpq $0x0,0x200958(%rip) # 600e20 <__JCR_END__> 4004c7: 00 4004c8: 74 1e je 4004e8 0x28> 4004ca: b8 00 00 00 00 mov $0x0,%eax 4004cf: 48 85 c0 test %rax,%rax 4004d2: 74 14 je 4004e8 0x28> 4004d4: 55 push %rbp 4004d5: bf 20 0e 60 00 mov $0x600e20,%edi 4004da: 48 89 e5 mov %rsp,%rbp 4004dd: ff d0 callq *%rax 4004df: 5d pop %rbp 4004e0: e9 7b ff ff ff jmpq 400460 4004e5: 0f 1f 00 nopl (%rax) 4004e8: e9 73 ff ff ff jmpq 400460 00000000004004ed : 4004ed: 55 push %rbp 4004ee: 48 89 e5 mov %rsp,%rbp 4004f1: c7 45 fc 0a 00 00 00 movl $0xa,-0x4(%rbp) 4004f8: 90 nop 4004f9: 5d pop %rbp 4004fa: c3 retq 00000000004004fb : 4004fb: 55 push %rbp 4004fc: 48 89 e5 mov %rsp,%rbp 4004ff: c7 45 fc 05 00 00 00 movl $0x5,-0x4(%rbp) 400506: 90 nop 400507: 5d pop %rbp 400508: c3 retq 0000000000400509 : 400509: 55 push %rbp 40050a: 48 89 e5 mov %rsp,%rbp 40050d: c7 45 fc 1e 00 00 00 movl $0x1e,-0x4(%rbp) 400514: 5d pop %rbp 400515: c3 retq 0000000000400516

: 400516: 55 push %rbp 400517: 48 89 e5 mov %rsp,%rbp 40051a: 48 83 ec 10 sub $0x10,%rsp 40051e: c7 45 fc 14 00 00 00 movl $0x14,-0x4(%rbp) 400525: b8 00 00 00 00 mov $0x0,%eax 40052a: e8 be ff ff ff callq 4004ed 40052f: b8 00 00 00 00 mov $0x0,%eax 400534: e8 c2 ff ff ff callq 4004fb 400539: b8 00 00 00 00 mov $0x0,%eax 40053e: c9 leaveq 40053f: c3 retq 0000000000400540 <__libc_csu_init>: 400540: 41 57 push %r15 400542: 41 89 ff mov %edi,%r15d 400545: 41 56 push %r14 400547: 49 89 f6 mov %rsi,%r14 40054a: 41 55 push %r13 40054c: 49 89 d5 mov %rdx,%r13 40054f: 41 54 push %r12 400551: 4c 8d 25 b8 08 20 00 lea 0x2008b8(%rip),%r12 # 600e10 <__frame_dummy_init_array_entry> 400558: 55 push %rbp 400559: 48 8d 2d b8 08 20 00 lea 0x2008b8(%rip),%rbp # 600e18 <__init_array_end> 400560: 53 push %rbx 400561: 4c 29 e5 sub %r12,%rbp 400564: 31 db xor %ebx,%ebx 400566: 48 c1 fd 03 sar $0x3,%rbp 40056a: 48 83 ec 08 sub $0x8,%rsp 40056e: e8 35 fe ff ff callq 4003a8 <_init> 400573: 48 85 ed test %rbp,%rbp 400576: 74 1e je 400596 <__libc_csu_init+0x56> 400578: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) 40057f: 00 400580: 4c 89 ea mov %r13,%rdx 400583: 4c 89 f6 mov %r14,%rsi 400586: 44 89 ff mov %r15d,%edi 400589: 41 ff 14 dc callq *(%r12,%rbx,8) 40058d: 48 83 c3 01 add $0x1,%rbx 400591: 48 39 eb cmp %rbp,%rbx 400594: 75 ea jne 400580 <__libc_csu_init+0x40> 400596: 48 83 c4 08 add $0x8,%rsp 40059a: 5b pop %rbx 40059b: 5d pop %rbp 40059c: 41 5c pop %r12 40059e: 41 5d pop %r13 4005a0: 41 5e pop %r14 4005a2: 41 5f pop %r15 4005a4: c3 retq 4005a5: 66 66 2e 0f 1f 84 00 data32 nopw %cs:0x0(%rax,%rax,1) 4005ac: 00 00 00 00 00000000004005b0 <__libc_csu_fini>: 4005b0: f3 c3 repz retq Disassembly of section .fini: 00000000004005b4 <_fini>: 4005b4: 48 83 ec 08 sub $0x8,%rsp 4005b8: 48 83 c4 08 add $0x8,%rsp 4005bc: c3 retq 代码说明：可以看到这里的test_a、test_b、main函数等内容已经不是从0x00开始了，而是按照指令的位置重新排序了。要说明的是，这个简短的程序的第一条指令并不是就运行main函数中的内容，而是要做一定的初始化工作。如切换上下文中的环境等。

实验检测编译过程中的链接作用

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