如果返回的是一个struct对象,return 语句会如何做呢?下面是测试代码
#include
using namespace std;
struct Big
{
char buf[100];
int i;
long d;
}B,B2;
Big bigfun(Big b)
{
b.i=100;
return b;
}
int main()
{
B2=bigfun(B);
return 0;
}
在main开头和结尾设断点
8: int main()
19: {
004012A0 push ebp
004012A1 mov ebp,esp
004012A3 sub esp,118h
//一开始对这迷惑不解,分析了好久
//原来(118h-40h)剩下的内存块存放了两个Big的变量
//低地址放Bigfun()函数返回值的临时变量
//高地址放B2.operator=(Big &) 的参数
004012A9 push ebx
004012AA push esi
004012AB push edi
004012AC lea edi,[ebp-118h]
004012B2 mov ecx,46h
004012B7 mov eax,0CCCCCCCCh
004012BC rep stos dword ptr [edi]
20: B2=bigfun(B);
004012BE sub esp,6Ch//b参数入栈
004012C1 mov ecx,1Bh
004012C6 mov esi,offset B (00438490)//B的首地址
004012CB mov edi,esp
004012CD rep movs dword ptr [edi],dword ptr [esi]//上面这些指令完成对b的初始化工作
004012CF lea eax,[ebp-0D8h]//eax存放Big类型一个返回值临时变量的首地址
004012D5 push eax//注意一般栈调用没有这条指令
004012D6 call @ILT+0(bigfun) (00401005)
/*
004012DB add esp,70h
004012DE mov esi,eax
004012E0 mov ecx,1Bh
004012E5 lea edi,[ebp-6Ch]
004012E8 rep movs dword ptr [edi],dword ptr [esi]
004012EA mov ecx,1Bh
004012EF lea esi,[ebp-6Ch]
004012F2 mov edi,offset B2 (00438500)
004012F7 rep movs dword ptr [edi],dword ptr [esi]
21:
22: return 0;
004012F9 xor eax,eax
23:
24: }
004012FB pop edi
004012FC pop esi
004012FD pop ebx
004012FE add esp,118h
00401304 cmp ebp,esp
00401306 call __chkesp (004081e0)
0040130B mov esp,ebp
0040130D pop ebp
0040130E ret
*/
@ILT+0(?bigfun@@YA?AUBig@@U1@@Z):
00401005 jmp bigfun (00401250)
11: Big bigfun(Big b)
12: {
00401250 push ebp
00401251 mov ebp,esp
00401253 sub esp,40h
00401256 push ebx
00401257 push esi
00401258 push edi
00401259 lea edi,[ebp-40h]
0040125C mov ecx,10h
00401261 mov eax,0CCCCCCCCh
00401266 rep stos dword ptr [edi]
13: b.i=100;
00401268 mov dword ptr [ebp+70h],64h
/*
0012FE50 00 00 00 00 64 00 00 00 00 00 00 00
*/
14: return b;
0040126F mov ecx,1Bh
00401274 lea esi,[ebp+0Ch]//esi=b的首地址
00401277 mov edi,dword ptr [ebp+8]//edi=返回值临时变量的首地址
0040127A rep movs dword ptr [edi],dword ptr [esi]
0040127C mov eax,dword ptr [ebp+8]
15: }
0040127F pop edi
00401280 pop esi
00401281 pop ebx
00401282 mov esp,ebp
00401284 pop ebp
00401285 ret
返回到main函数
004012DB add esp,70h//销毁局部参数
004012DE mov esi,eax//bigfunction()返会值临时变量的首地址
004012E0 mov ecx,1Bh
004012E5 lea edi,[ebp-6Ch]//B2.operator=(Big &)参数的首地址
004012E8 rep movs dword ptr [edi],dword ptr [esi]
004012EA mov ecx,1Bh
004012EF lea esi,[ebp-6Ch]
004012F2 mov edi,offset B2 (00438500)//B2的首地址
004012F7 rep movs dword ptr [edi],dword ptr [esi]
21:
22: return 0;
004012F9 xor eax,eax
23:
24: }
004012FB pop edi
004012FC pop esi
004012FD pop ebx
004012FE add esp,118h
00401304 cmp ebp,esp
00401306 call __chkesp (004081e0)
0040130B mov esp,ebp
0040130D pop ebp
0040130E ret
总结:
1.一般function frame的结构是
函数内局部变量
ebp
eip
函数参数
如果返回值是一个struct对象function frame
函数内局部变量
ebp
eip
返回值临时变量的首地址//特别注意这个
函数参数
(由于寄存器太小,不可能放一块struct内存,所以保存了临时变量的首地址).
2.struct对象的临时变量是在栈上的,而不是在堆上.
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