c++ - Purpose of rep stos assembly command in this code from Visual Studio -

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this question has answer here:

take @ following code:

void f() { }

i compiled in visual studio 2013, debug, 32-bit mode , looked @ dissassembly.

void f() { 00304cb0  push        ebp   00304cb1  mov         ebp,esp   00304cb3  sub         esp,0c0h   00304cb9  push        ebx   00304cba  push        esi   00304cbb  push        edi   00304cbc  lea         edi,[ebp-0c0h]   00304cc2  mov         ecx,30h   00304cc7  mov         eax,0cccccccch   00304ccc  rep stos    dword ptr es:[edi]   } 00304cce  pop         edi   00304ccf  pop         esi   00304cd0  pop         ebx   00304cd1  mov         esp,ebp   00304cd3  pop         ebp   00304cd4  ret

what purpose of rep stos instruction?

i'm curious.

the rep stos instruction writes value in eax starting @ address pointed edi (your local stack in case), ecx (0x30) times. value in eax 0xcccccccc magic number chosen microsoft indicate uninitialized memory. debugger catch if try , dereference pointer memory. diagnostic checking enabled /rtcu option.

now might ask why, empty function body, memory reserved on local stack. because have edit , continue turned on /zi option. compiler setting aside space in case decide use in debug session.


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