工作了这么久，现在也终于有时间来写写这几年在程序世界中的感受。一时之间并不知道从哪里开始。想来想去，还是从大学入学开始吧。记得那是一个风和日丽的下午，一堆大学生抱着书跑进教室，那个时候并没有那么多逃课的，只知道相传C语言是一门学了就能找到工作的科目。从此我和我们内敛含蓄的hello world妹妹来了一次深入的体会。老师说main函数就是hello world的一切，我们的程序都是从main开始，虽然老师是好意，但是这确实导致未来很大一部分初级程序员都认为C语言的入口就是main函数。

下面这个Hello World 不知道坑害了多少善良无辜的程序员。现在我们就来解剖她，看她这么较小单纯，真舍不得让她一丝不挂的展现出来。

#include <stdio.h>

int main (int argc, char *argv[])
{
    printf ("Hello World\n");

    return 0;
}

保存为hello.c

我们可以通过gcc hello.c -o hello得到可执行程序hello.很多人会认为运行hello, CPU会首先跳转到main函数，执行printf. 至少绝大部分刚毕业的软件工程师是这样认为（因为老师就是这样说的）。下面我们就来详细讲讲CPU是怎么运行到main函数的。

先用objdump -axd hello > a.s来看看，实际使用的可执行程序hello到底包含了哪些：

hello: file format elf64-x86-64
hello
architecture: i386:x86-64, flags 0x00000112:
EXEC_P, HAS_SYMS, D_PAGED
start address 0x0000000000400430

省略其他段....

Disassembly of section .init:

00000000004003c8 <_init>:
  4003c8: 48 83 ec 08 sub $0x8,%rsp
  4003cc: 48 8b 05 25 0c 20 00 mov 0x200c25(%rip),%rax # 600ff8 <_DYNAMIC+0x1d0>
  4003d3: 48 85 c0 test %rax,%rax
  4003d6: 74 05 je 4003dd <_init+0x15>
  4003d8: e8 43 00 00 00 callq 400420 <__libc_start_main@plt+0x10>
  4003dd: 48 83 c4 08 add $0x8,%rsp
  4003e1: c3 retq

Disassembly of section .plt:

00000000004003f0 <puts@plt-0x10>:
  4003f0: ff 35 12 0c 20 00 pushq 0x200c12(%rip) # 601008 <_GLOBAL_OFFSET_TABLE_+0x8>
  4003f6: ff 25 14 0c 20 00 jmpq *0x200c14(%rip) # 601010 <_GLOBAL_OFFSET_TABLE_+0x10>
  4003fc: 0f 1f 40 00 nopl 0x0(%rax)

0000000000400400 <puts@plt>:
  400400: ff 25 12 0c 20 00 jmpq *0x200c12(%rip) # 601018 <_GLOBAL_OFFSET_TABLE_+0x18>
  400406: 68 00 00 00 00 pushq $0x0
  40040b: e9 e0 ff ff ff jmpq 4003f0 <_init+0x28>

0000000000400410 <__libc_start_main@plt>:
  400410: ff 25 0a 0c 20 00 jmpq *0x200c0a(%rip) # 601020 <_GLOBAL_OFFSET_TABLE_+0x20>
  400416: 68 01 00 00 00 pushq $0x1
  40041b: e9 d0 ff ff ff jmpq 4003f0 <_init+0x28>

Disassembly of section .plt.got:

0000000000400420 <.plt.got>:
  400420: ff 25 d2 0b 20 00 jmpq *0x200bd2(%rip) # 600ff8 <_DYNAMIC+0x1d0>
  400426: 66 90 xchg %ax,%ax

Disassembly of section .text:
0000000000400430 <_start>:
  400430: 31 ed xor %ebp,%ebp
  400432: 49 89 d1 mov %rdx,%r9
  400435: 5e pop %rsi
  400436: 48 89 e2 mov %rsp,%rdx
  400439: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
  40043d: 50 push %rax
  40043e: 54 push %rsp
  40043f: 49 c7 c0 c0 05 40 00 mov $0x4005c0,%r8
  400446: 48 c7 c1 50 05 40 00 mov $0x400550,%rcx
  40044d: 48 c7 c7 26 05 40 00 mov $0x400526,%rdi
  400454: e8 b7 ff ff ff callq 400410 <__libc_start_main@plt>
  400459: f4 hlt
  40045a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)

0000000000400460 <deregister_tm_clones>:
  400460: b8 3f 10 60 00 mov $0x60103f,%eax
  400465: 55 push %rbp
  400466: 48 2d 38 10 60 00 sub $0x601038,%rax
  40046c: 48 83 f8 0e cmp $0xe,%rax
  400470: 48 89 e5 mov %rsp,%rbp
  400473: 76 1b jbe 400490 <deregister_tm_clones+0x30>
  400475: b8 00 00 00 00 mov $0x0,%eax
  40047a: 48 85 c0 test %rax,%rax
  40047d: 74 11 je 400490 <deregister_tm_clones+0x30>
  40047f: 5d pop %rbp
  400480: bf 38 10 60 00 mov $0x601038,%edi
  400485: ff e0 jmpq *%rax
  400487: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
  40048e: 00 00
  400490: 5d pop %rbp
  400491: c3 retq
  400492: 0f 1f 40 00 nopl 0x0(%rax)
  400496: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
  40049d: 00 00 00

00000000004004a0 <register_tm_clones>:
  4004a0: be 38 10 60 00 mov $0x601038,%esi
  4004a5: 55 push %rbp
  4004a6: 48 81 ee 38 10 60 00 sub $0x601038,%rsi
  4004ad: 48 c1 fe 03 sar $0x3,%rsi
  4004b1: 48 89 e5 mov %rsp,%rbp
  4004b4: 48 89 f0 mov %rsi,%rax
  4004b7: 48 c1 e8 3f shr $0x3f,%rax
  4004bb: 48 01 c6 add %rax,%rsi
  4004be: 48 d1 fe sar %rsi
  4004c1: 74 15 je 4004d8 <register_tm_clones+0x38>
  4004c3: b8 00 00 00 00 mov $0x0,%eax
  4004c8: 48 85 c0 test %rax,%rax
  4004cb: 74 0b je 4004d8 <register_tm_clones+0x38>
  4004cd: 5d pop %rbp
  4004ce: bf 38 10 60 00 mov $0x601038,%edi
  4004d3: ff e0 jmpq *%rax
  4004d5: 0f 1f 00 nopl (%rax)
  4004d8: 5d pop %rbp
  4004d9: c3 retq
  4004da: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)

00000000004004e0 <__do_global_dtors_aux>:
  4004e0: 80 3d 51 0b 20 00 00 cmpb $0x0,0x200b51(%rip) # 601038 <__TMC_END__>
  4004e7: 75 11 jne 4004fa <__do_global_dtors_aux+0x1a>
  4004e9: 55 push %rbp
  4004ea: 48 89 e5 mov %rsp,%rbp
  4004ed: e8 6e ff ff ff callq 400460 <deregister_tm_clones>
  4004f2: 5d pop %rbp
  4004f3: c6 05 3e 0b 20 00 01 movb $0x1,0x200b3e(%rip) # 601038 <__TMC_END__>
  4004fa: f3 c3 repz retq
  4004fc: 0f 1f 40 00 nopl 0x0(%rax)

0000000000400500 <frame_dummy>:
  400500: bf 20 0e 60 00 mov $0x600e20,%edi
  400505: 48 83 3f 00 cmpq $0x0,(%rdi)
  400509: 75 05 jne 400510 <frame_dummy+0x10>
  40050b: eb 93 jmp 4004a0 <register_tm_clones>
  40050d: 0f 1f 00 nopl (%rax)
  400510: b8 00 00 00 00 mov $0x0,%eax
  400515: 48 85 c0 test %rax,%rax
  400518: 74 f1 je 40050b <frame_dummy+0xb>
  40051a: 55 push %rbp
  40051b: 48 89 e5 mov %rsp,%rbp
  40051e: ff d0 callq *%rax
  400520: 5d pop %rbp
  400521: e9 7a ff ff ff jmpq 4004a0 <register_tm_clones>

0000000000400526 <main>:
  400526: 55 push %rbp
  400527: 48 89 e5 mov %rsp,%rbp
  40052a: 48 83 ec 10 sub $0x10,%rsp
  40052e: 89 7d fc mov %edi,-0x4(%rbp)
  400531: 48 89 75 f0 mov %rsi,-0x10(%rbp)
  400535: bf d4 05 40 00 mov $0x4005d4,%edi
  40053a: e8 c1 fe ff ff callq 400400 <puts@plt>
  40053f: b8 00 00 00 00 mov $0x0,%eax
  400544: c9 leaveq
  400545: c3 retq
  400546: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
  40054d: 00 00 00

0000000000400550 <__libc_csu_init>:
  400550: 41 57 push %r15
  400552: 41 56 push %r14
  400554: 41 89 ff mov %edi,%r15d
  400557: 41 55 push %r13
  400559: 41 54 push %r12
  40055b: 4c 8d 25 ae 08 20 00 lea 0x2008ae(%rip),%r12 # 600e10 <__frame_dummy_init_array_entry>
  400562: 55 push %rbp
  400563: 48 8d 2d ae 08 20 00 lea 0x2008ae(%rip),%rbp # 600e18 <__init_array_end>
  40056a: 53 push %rbx
  40056b: 49 89 f6 mov %rsi,%r14
  40056e: 49 89 d5 mov %rdx,%r13
  400571: 4c 29 e5 sub %r12,%rbp
  400574: 48 83 ec 08 sub $0x8,%rsp
  400578: 48 c1 fd 03 sar $0x3,%rbp
  40057c: e8 47 fe ff ff callq 4003c8 <_init>
  400581: 48 85 ed test %rbp,%rbp
  400584: 74 20 je 4005a6 <__libc_csu_init+0x56>
  400586: 31 db xor %ebx,%ebx
  400588: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
  40058f: 00
  400590: 4c 89 ea mov %r13,%rdx
  400593: 4c 89 f6 mov %r14,%rsi
  400596: 44 89 ff mov %r15d,%edi
  400599: 41 ff 14 dc callq *(%r12,%rbx,8)
  40059d: 48 83 c3 01 add $0x1,%rbx
  4005a1: 48 39 eb cmp %rbp,%rbx
  4005a4: 75 ea jne 400590 <__libc_csu_init+0x40>
  4005a6: 48 83 c4 08 add $0x8,%rsp
  4005aa: 5b pop %rbx
  4005ab: 5d pop %rbp
  4005ac: 41 5c pop %r12
  4005ae: 41 5d pop %r13
  4005b0: 41 5e pop %r14
  4005b2: 41 5f pop %r15
  4005b4: c3 retq
  4005b5: 90 nop
  4005b6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
  4005bd: 00 00 00

00000000004005c0 <__libc_csu_fini>:
  4005c0: f3 c3 repz retq

Disassembly of section .fini:

00000000004005c4 <_fini>:
  4005c4: 48 83 ec 08 sub $0x8,%rsp
  4005c8: 48 83 c4 08 add $0x8,%rsp
  4005cc: c3 retq

ITPub博客

万恶的Hello World 之应用程序

最新文章