C必须包含main函数吗？

herberteuler · 发表于 2004-10-8 17:19:26

看来 C 的编译器对于 main 函数在开始前和结束后都做了一些工作，要想只用 C 来指定特定的函数作为入口点是比较困难的。

kj501 · 发表于 2004-10-8 17:22:52

第二个程序的段错误我觉得和进程加载时的初始化有关。
以一个最简单的程序为例：

int main()
{
exit(0);
}

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然后再用objdump反汇编：

[kj501@s2023 c]$ objdump -d a.out
a.out: file format elf32-i386
Disassembly of section .init:
0804824c <_init>:
804824c: 55 push %ebp
804824d: 89 e5 mov %esp,%ebp
804824f: 83 ec 08 sub $0x8,%esp
8048252: e8 6d 00 00 00 call 80482c4 <call_gmon_start>
8048257: e8 d4 00 00 00 call 8048330 <frame_dummy>
804825c: e8 cf 01 00 00 call 8048430 <__do_global_ctors_aux>
8048261: c9 leave
8048262: c3 ret
Disassembly of section .plt:
08048264 <.plt>:
8048264: ff 35 68 95 04 08 pushl 0x8049568
804826a: ff 25 6c 95 04 08 jmp *0x804956c
8048270: 00 00 add %al,(%eax)
8048272: 00 00 add %al,(%eax)
8048274: ff 25 70 95 04 08 jmp *0x8049570
804827a: 68 00 00 00 00 push $0x0
804827f: e9 e0 ff ff ff jmp 8048264 <_init+0x18>
8048284: ff 25 74 95 04 08 jmp *0x8049574
804828a: 68 08 00 00 00 push $0x8
804828f: e9 d0 ff ff ff jmp 8048264 <_init+0x18>
Disassembly of section .text:
080482a0 <_start>:
80482a0: 31 ed xor %ebp,%ebp
80482a2: 5e pop %esi
80482a3: 89 e1 mov %esp,%ecx
80482a5: 83 e4 f0 and $0xfffffff0,%esp
80482a8: 50 push %eax
80482a9: 54 push %esp
80482aa: 52 push %edx
80482ab: 68 d0 83 04 08 push $0x80483d0
80482b0: 68 80 83 04 08 push $0x8048380
80482b5: 51 push %ecx
80482b6: 56 push %esi
80482b7: 68 5c 83 04 08 push $0x804835c
80482bc: e8 b3 ff ff ff call 8048274 <_init+0x28>
80482c1: f4 hlt
80482c2: 90 nop
80482c3: 90 nop
080482c4 <call_gmon_start>:
80482c4: 55 push %ebp
80482c5: 89 e5 mov %esp,%ebp
80482c7: 53 push %ebx
80482c8: e8 00 00 00 00 call 80482cd <call_gmon_start+0x9>
80482cd: 5b pop %ebx
80482ce: 81 c3 97 12 00 00 add $0x1297,%ebx
80482d4: 52 push %edx
80482d5: 8b 83 14 00 00 00 mov 0x14(%ebx),%eax
80482db: 85 c0 test %eax,%eax
80482dd: 74 02 je 80482e1 <call_gmon_start+0x1d>
80482df: ff d0 call *%eax
80482e1: 58 pop %eax
80482e2: 5b pop %ebx
80482e3: c9 leave
80482e4: c3 ret
80482e5: 90 nop
80482e6: 90 nop
80482e7: 90 nop
80482e8: 90 nop
80482e9: 90 nop
80482ea: 90 nop
80482eb: 90 nop
80482ec: 90 nop
80482ed: 90 nop
80482ee: 90 nop
80482ef: 90 nop
080482f0 <__do_global_dtors_aux>:
80482f0: 55 push %ebp
80482f1: 89 e5 mov %esp,%ebp
80482f3: 50 push %eax
80482f4: 50 push %eax
80482f5: 80 3d 7c 95 04 08 00 cmpb $0x0,0x804957c
80482fc: 75 2e jne 804832c <__do_global_dtors_aux+0x3c>
80482fe: a1 84 94 04 08 mov 0x8049484,%eax
8048303: 8b 10 mov (%eax),%edx
8048305: 85 d2 test %edx,%edx
8048307: 74 1c je 8048325 <__do_global_dtors_aux+0x35>
8048309: 8d b4 26 00 00 00 00 lea 0x0(%esi,1),%esi
8048310: 83 c0 04 add $0x4,%eax
8048313: a3 84 94 04 08 mov %eax,0x8049484
8048318: ff d2 call *%edx
804831a: a1 84 94 04 08 mov 0x8049484,%eax
804831f: 8b 10 mov (%eax),%edx
8048321: 85 d2 test %edx,%edx
8048323: 75 eb jne 8048310 <__do_global_dtors_aux+0x20>
8048325: c6 05 7c 95 04 08 01 movb $0x1,0x804957c
804832c: c9 leave
804832d: c3 ret
804832e: 89 f6 mov %esi,%esi
08048330 <frame_dummy>:
8048330: 55 push %ebp
8048331: 89 e5 mov %esp,%ebp
8048333: 51 push %ecx
8048334: 51 push %ecx
8048335: 8b 15 60 95 04 08 mov 0x8049560,%edx
804833b: 85 d2 test %edx,%edx
804833d: 74 19 je 8048358 <frame_dummy+0x28>
804833f: b8 00 00 00 00 mov $0x0,%eax
8048344: 85 c0 test %eax,%eax
8048346: 74 10 je 8048358 <frame_dummy+0x28>
8048348: 83 ec 0c sub $0xc,%esp
804834b: 68 60 95 04 08 push $0x8049560
8048350: e8 ab 7c fb f7 call 0 <_init-0x804824c>
8048355: 83 c4 10 add $0x10,%esp
8048358: c9 leave
8048359: c3 ret
804835a: 90 nop
804835b: 90 nop
0804835c <main>:
804835c: 55 push %ebp
804835d: 89 e5 mov %esp,%ebp
804835f: 83 ec 08 sub $0x8,%esp
8048362: 83 e4 f0 and $0xfffffff0,%esp
8048365: b8 00 00 00 00 mov $0x0,%eax
804836a: 29 c4 sub %eax,%esp
804836c: 83 ec 0c sub $0xc,%esp
804836f: 6a 00 push $0x0
8048371: e8 0e ff ff ff call 8048284 <_init+0x38>
8048376: 90 nop
8048377: 90 nop
8048378: 90 nop
8048379: 90 nop
804837a: 90 nop
804837b: 90 nop
804837c: 90 nop
804837d: 90 nop
804837e: 90 nop
804837f: 90 nop
08048380 <__libc_csu_init>:
8048380: 55 push %ebp
8048381: 89 e5 mov %esp,%ebp
8048383: 57 push %edi
8048384: 56 push %esi
8048385: 31 f6 xor %esi,%esi
8048387: 53 push %ebx
8048388: e8 00 00 00 00 call 804838d <__libc_csu_init+0xd>
804838d: 5b pop %ebx
804838e: 81 c3 d7 11 00 00 add $0x11d7,%ebx
8048394: 83 ec 0c sub $0xc,%esp
8048397: e8 b0 fe ff ff call 804824c <_init>
804839c: 8d 83 18 ff ff ff lea 0xffffff18(%ebx),%eax
80483a2: 8d 93 18 ff ff ff lea 0xffffff18(%ebx),%edx
80483a8: 29 c2 sub %eax,%edx
80483aa: c1 fa 02 sar $0x2,%edx
80483ad: 39 d6 cmp %edx,%esi
80483af: 73 10 jae 80483c1 <__libc_csu_init+0x41>
80483b1: 89 45 f0 mov %eax,0xfffffff0(%ebp)
80483b4: 89 d7 mov %edx,%edi
80483b6: ff 14 b0 call *(%eax,%esi,4)
80483b9: 8b 45 f0 mov 0xfffffff0(%ebp),%eax
80483bc: 46 inc %esi
80483bd: 39 fe cmp %edi,%esi
80483bf: 72 f5 jb 80483b6 <__libc_csu_init+0x36>
80483c1: 83 c4 0c add $0xc,%esp
80483c4: 5b pop %ebx
80483c5: 5e pop %esi
80483c6: 5f pop %edi
80483c7: 5d pop %ebp
80483c8: c3 ret
80483c9: 8d b4 26 00 00 00 00 lea 0x0(%esi,1),%esi
080483d0 <__libc_csu_fini>:
80483d0: 55 push %ebp
80483d1: 89 e5 mov %esp,%ebp
80483d3: 57 push %edi
80483d4: 56 push %esi
80483d5: 53 push %ebx
80483d6: e8 00 00 00 00 call 80483db <__libc_csu_fini+0xb>
80483db: 5b pop %ebx
80483dc: 81 c3 89 11 00 00 add $0x1189,%ebx
80483e2: 83 ec 0c sub $0xc,%esp
80483e5: 8d 83 18 ff ff ff lea 0xffffff18(%ebx),%eax
80483eb: 8d bb 18 ff ff ff lea 0xffffff18(%ebx),%edi
80483f1: 29 f8 sub %edi,%eax
80483f3: c1 f8 02 sar $0x2,%eax
80483f6: 85 c0 test %eax,%eax
80483f8: 8d 70 ff lea 0xffffffff(%eax),%esi
80483fb: 75 13 jne 8048410 <__libc_csu_fini+0x40>
80483fd: e8 52 00 00 00 call 8048454 <_fini>
8048402: 83 c4 0c add $0xc,%esp
8048405: 5b pop %ebx
8048406: 5e pop %esi
8048407: 5f pop %edi
8048408: 5d pop %ebp
8048409: c3 ret
804840a: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
8048410: ff 14 b7 call *(%edi,%esi,4)
8048413: 89 f0 mov %esi,%eax
8048415: 4e dec %esi
8048416: 85 c0 test %eax,%eax
8048418: 75 f6 jne 8048410 <__libc_csu_fini+0x40>
804841a: e8 35 00 00 00 call 8048454 <_fini>
804841f: 83 c4 0c add $0xc,%esp
8048422: 5b pop %ebx
8048423: 5e pop %esi
8048424: 5f pop %edi
8048425: 5d pop %ebp
8048426: c3 ret
8048427: 90 nop
8048428: 90 nop
8048429: 90 nop
804842a: 90 nop
804842b: 90 nop
804842c: 90 nop
804842d: 90 nop
804842e: 90 nop
804842f: 90 nop
08048430 <__do_global_ctors_aux>:
8048430: 55 push %ebp
8048431: 89 e5 mov %esp,%ebp
8048433: 53 push %ebx
8048434: 52 push %edx
8048435: bb 50 95 04 08 mov $0x8049550,%ebx
804843a: a1 50 95 04 08 mov 0x8049550,%eax
804843f: 83 f8 ff cmp $0xffffffff,%eax
8048442: 74 0c je 8048450 <__do_global_ctors_aux+0x20>
8048444: 83 eb 04 sub $0x4,%ebx
8048447: ff d0 call *%eax
8048449: 8b 03 mov (%ebx),%eax
804844b: 83 f8 ff cmp $0xffffffff,%eax
804844e: 75 f4 jne 8048444 <__do_global_ctors_aux+0x14>
8048450: 58 pop %eax
8048451: 5b pop %ebx
8048452: 5d pop %ebp
8048453: c3 ret
Disassembly of section .fini:
08048454 <_fini>:
8048454: 55 push %ebp
8048455: 89 e5 mov %esp,%ebp
8048457: 53 push %ebx
8048458: e8 00 00 00 00 call 804845d <_fini+0x9>
804845d: 5b pop %ebx
804845e: 81 c3 07 11 00 00 add $0x1107,%ebx
8048464: 50 push %eax
8048465: e8 86 fe ff ff call 80482f0 <__do_global_dtors_aux>
804846a: 59 pop %ecx
804846b: 5b pop %ebx
804846c: c9 leave
804846d: c3 ret
[kj501@s2023 c]$

复制代码

可以看到在main函数之前，一个进程是要作很多初始化工作的，进程结束时也一样要做清场工作。而对第二个程序x2的objdump反汇编结果，就能看出要少很多：

[kj501@s2023 c]$ objdump -d a.out
a.out: file format elf32-i386
Disassembly of section .plt:
080481b0 <.plt>:
80481b0: ff 35 b0 92 04 08 pushl 0x80492b0
80481b6: ff 25 b4 92 04 08 jmp *0x80492b4
80481bc: 00 00 add %al,(%eax)
80481be: 00 00 add %al,(%eax)
80481c0: ff 25 b8 92 04 08 jmp *0x80492b8
80481c6: 68 00 00 00 00 push $0x0
80481cb: e9 e0 ff ff ff jmp 80481b0 <x-0x30>
80481d0: ff 25 bc 92 04 08 jmp *0x80492bc
80481d6: 68 08 00 00 00 push $0x8
80481db: e9 d0 ff ff ff jmp 80481b0 <x-0x30>
Disassembly of section .text:
080481e0 <x>:
80481e0: 55 push %ebp
80481e1: 89 e5 mov %esp,%ebp
80481e3: 83 ec 08 sub $0x8,%esp
80481e6: 83 ec 08 sub $0x8,%esp
80481e9: 8b 45 0c mov 0xc(%ebp),%eax
80481ec: ff 30 pushl (%eax)
80481ee: 68 05 82 04 08 push $0x8048205
80481f3: e8 c8 ff ff ff call 80481c0 <x-0x20>
80481f8: 83 c4 10 add $0x10,%esp
80481fb: 83 ec 0c sub $0xc,%esp
80481fe: 6a 00 push $0x0
8048200: e8 cb ff ff ff call 80481d0 <x-0x10>
[kj501@s2023 c]$

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具体的原因还有等待于进一步研究。

herberteuler · 发表于 2004-10-8 17:27:43

比较了手工和自动的输入后，好像只要能在编译下面的程序时加入初始化和收尾的工作就可以手工指定入口点了：

int user_defined_function(int argc, char *argv[])
{
asm("andl $-16, %esp\n"
"movl $0, %eax\n"
"subl %eax, %esp");
/* regular codes here */
return 0;
}

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还有个问题，就是如果需要第 3 个参数 char *envp[] 的话 _init 是否能够做好。

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C必须包含main函数吗？

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