CODEWITHSUNDEEP
c++visual-studiodisassembly
DinamicDigital
DinamicDigital

Reputation: 23

What is all of these weird assembler instructions on my main function disassembly?

So i have this main function, and it produces alot of weird instructions. I am using Visual Studio 2019, and i am in debug mode, so optimizations are disabled. What are these instructions doing?

int main()
{
00D340E0  push        ebp  
00D340E1  mov         ebp,esp  
00D340E3  sub         esp,104h  
00D340E9  push        ebx  
00D340EA  push        esi  
00D340EB  push        edi  
00D340EC  lea         edi,[ebp-104h]  
00D340F2  mov         ecx,41h  
00D340F7  mov         eax,0CCCCCCCCh  
00D340FC  rep stos    dword ptr es:[edi]  
00D340FE  mov         eax,dword ptr [__security_cookie (0D3A024h)]  
00D34103  xor         eax,ebp  
00D34105  mov         dword ptr [ebp-4],eax  
00D34108  mov         ecx,offset _842A6236_main@cpp (0D3C012h)  
00D3410D  call        @__CheckForDebuggerJustMyCode@4 (0D31208h) 

(the rest of the file...)

EDIT: By weird, i meant i dont understand what is happening here, not that its not standard.

Upvotes: 2

Views: 451

Answers (1)

catnip
catnip

Reputation: 25408

These are debugging helpers.

__security_cookie is used to check for writes past the end of the stack (if you overflow a stack-based buffer, say). The actual check is performed when exiting the function.

__CheckForDebuggerJustMyCode allows the debugger to step over "system, framework, library, and other non-user calls", see here.

And this code:

lea         edi,[ebp-104h]  
mov         ecx,41h  
mov         eax,0CCCCCCCCh  
rep stos    dword ptr es:[edi]

Fills the stack frame with garbage bytes that will (for example) cause an exception if you dereference an uninitialised pointer.

None of this stuff is present in Release builds, only in Debug builds.

Upvotes: 1

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