Editing assembly bytes with C++ code

Question

I have running process (*.exe file). At address 0x496A42 I have this asm code

496A42 cmp     al, 14h

I found it in IDA software. How can i change this value to

al, 18h

in C++, using VirtualProtect..

I unlocked memory with VirtualProtect, but i don't know how to change this value. Would you tell me how to edit this assembly code with C++ code?

I hope to see you answer. Thank you.

Answer 1

Assuming that ASLR is not enabled, and the .exe is loaded into the default imagebase of 0x400000 and your address is inside the .exe module

The "cmp al, 14" instruction has the corresponding bytes 0x3C 0x14

0x3C would be at address 0x496A42 and the 0x14 operand would be at address 0x496A43

So you overwrite the byte at address 0x496A43 with 0x18

void Patch(char* dst, char* src, int size)
{
    DWORD oldprotect;
    VirtualProtect(dst, size, PAGE_EXECUTE_READWRITE, &oldprotect);
    memcpy(dst, src, size);
    VirtualProtect(dst, size, oldprotect, &oldprotect);
}

Patch((char*)0x496A43, "\x18", 1);