C_從匯編的角度理解被調函數返回不同類型的值給

被調函數返回不同類型得值給主調函數，根據返回值類型得大小，有不同得返回方式。

示例代碼：

int funcReturnInt(){ int a = 3; int b = 4; return a+b;}int main(){ int d = 0; d = funcReturnInt(); return 0;}

匯編：

13: int d = 0;004010A8 mov dword ptr [ebp-4],014: d = funcReturnInt();004010AF call 等ILT+20(funcReturnDouble) (00401019)004010B4 mov dword ptr [ebp-4],eax // int返回值保存在eax寄存器，mov到[ebp-4]

函數調用時：

9: return a+b;00401046 mov eax,dword ptr [ebp-4] // 這里得[ebp-4]是被調函數得局部變量a00401049 add eax,dword ptr [ebp-8]

如果是返回指針，也同樣是通過寄存器eax。

示例代碼：

double funcReturndouble(){ double a = 1.1; double b = 2.2; return a+b;}double main(){ double d = 0; d = funcReturndouble(); return 0;}

匯編：

13: double d = 0;00401088 mov dword ptr [ebp-8],00040108F mov dword ptr [ebp-4],014: d = funcReturndouble();00401096 call 等ILT+10(funcReturndouble) (0040100f)0040109B fstp qword ptr [ebp-8] // 浮點棧pop給[ebp-8]

調用函數時有一個壓浮點棧操作：

9: return a+b;00401054 fld qword ptr [ebp-8] // 浮點棧push被調函數得局部變量a00401057 fadd qword ptr [ebp-10h] // 先前push入得值再加入b

float也是同樣得fstp和fld指令。

示例代碼：

struct employee{ int id; char sex; double salary;};employee funcReturnCompoundValue(employee ee){ employee tmp(ee); return tmp;}int main(){ employee ee; ee.id=3; ee.sex = 'm'; ee.salary = 5555; employee ef; ef=funcReturnCompoundValue(ee); return 0;}

函數調用得匯編代碼：

19: employee ef;20: ef=funcReturnCompoundValue(ee);004010C4 sub esp,10h // 參數需要16個字節004010C7 mov eax,esp004010C9 mov ecx,dword ptr [ebp-10h]004010CC mov dword ptr [eax],ecx // 開始壓參004010CE mov edx,dword ptr [ebp-0Ch]004010D1 mov dword ptr [eax+4],edx004010D4 mov ecx,dword ptr [ebp-8]004010D7 mov dword ptr [eax+8],ecx004010DA mov edx,dword ptr [ebp-4]004010DD mov dword ptr [eax+0Ch],edx004010E0 lea eax,[ebp-40h]004010E3 push eax // 壓入存放返回值得起始地址（被調函數棧幀內）004010E4 call 等ILT+5(funcReturnCompoundValue) (0040100a) // 跳轉得同時會壓入返回地址004010E9

開始調用函數：

7: employee funcReturnCompoundValue(employee ee)8: {00401020 push ebp00401021 mov ebp,esp00401023 sub esp,50h00401026 push ebx00401027 push esi00401028 push edi00401029 lea edi,[ebp-50h]0040102C mov ecx,14h00401031 mov eax,0CCCCCCCCh00401036 rep stos dword ptr [edi] 9: employee tmp(ee);00401038 mov eax,dword ptr [ebp+0Ch]0040103B mov dword ptr [ebp-10h],eax0040103E mov ecx,dword ptr [ebp+10h]00401041 mov dword ptr [ebp-0Ch],ecx00401044 mov edx,dword ptr [ebp+14h]00401047 mov dword ptr [ebp-8],edx0040104A mov eax,dword ptr [ebp+18h]0040104D mov dword ptr [ebp-4],eax

此時得棧幀：

調用函數時返回得匯編

10: return tmp;00401050 mov ecx,dword ptr [ebp+8] // 將返回地值得起始地址讀入 ecx00401053 mov edx,dword ptr [ebp-10h] // 待返回值得起始地址得值讀入 edx00401056 mov dword ptr [ecx],edx // 300401058 mov eax,dword ptr [ebp-0Ch]0040105B mov dword ptr [ecx+4],eax // 'm'0040105E mov edx,dword ptr [ebp-8] // 5555得半段00401061 mov dword ptr [ecx+8],edx00401064 mov eax,dword ptr [ebp-4]00401067 mov dword ptr [ecx+0Ch],eax0040106A mov eax,dword ptr [ebp+8]11: }

回到主調函數：

004010E9 add esp,14h // 參數16個字節+4個字節得存入返回值得起始地址004010EC mov ecx,dword ptr [eax] // eax存儲得是存放返回值得起始地址004010EE mov dword ptr [ebp-30h],ecx // ebp-30 = 下圖得0012FF18004010F1 mov edx,dword ptr [eax+4]004010F4 mov dword ptr [ebp-2Ch],edx004010F7 mov ecx,dword ptr [eax+8]004010FA mov dword ptr [ebp-28h],ecx004010FD mov edx,dword ptr [eax+0Ch]00401100 mov dword ptr [ebp-24h],edx00401103 mov eax,dword ptr [ebp-30h]00401106 mov dword ptr [ebp-20h],eax // ebp-20 = 下圖得0012FF2800401109 mov ecx,dword ptr [ebp-2Ch]0040110C mov dword ptr [ebp-1Ch],ecx0040110F mov edx,dword ptr [ebp-28h]00401112 mov dword ptr [ebp-18h],edx00401115 mov eax,dword ptr [ebp-24h]00401118 mov dword ptr [ebp-14h],eax21: return 0;0040111B xor eax,eax

圖示：

當返回復合類型，需要在主調函數得棧幀內指定一塊內存空間。在被調函數得棧幀上，會保存指向主調函數存儲返回值得內存空間得起始地址。

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