source: Daodan/src/Daodan_Patch.c @ 272

#include "Daodan_Patch.h"
#include <windows.h>

bool DDrPatch_MakeJump(void* from, void* to)
{
        DWORD oldp;
        
        if (VirtualProtect(from, 5, PAGE_EXECUTE_READWRITE, &oldp))
        {
                *(char*)from++ = 0xe9; // jmp rel32
                *(int*)from = (unsigned int)to - (unsigned int)from - 4;
                VirtualProtect(from, 5, oldp, &oldp);
                return true;
        }
        else
                return false;
}

bool DDrPatch_MakeCall(void* from, void* to)
{
        DWORD oldp;
        
        if (VirtualProtect(from, 5, PAGE_EXECUTE_READWRITE, &oldp))
        {
                *(char*)from++ = 0xe8; // call rel32
                *(int*)from = (unsigned int)to - (unsigned int)from - 4;
                VirtualProtect(from, 5, oldp, &oldp);
                return true;
        }
        else
                return false;
}

bool DDrPatch_String(char* dest, const char* string, int length)
{
        DWORD oldp;
        
        if (VirtualProtect(dest, length, PAGE_EXECUTE_READWRITE, &oldp))
        {
                memcpy(dest, string, length);
                VirtualProtect(dest, length, oldp, &oldp);
                return true;
        }
        else
                return false;
}

bool DDrPatch_Byte(char* dest, char value)
{
        DWORD oldp;
        
        if (VirtualProtect(dest, 1, PAGE_EXECUTE_READWRITE, &oldp))
        {
                *dest = value;
                VirtualProtect(dest, 1, oldp, &oldp);
                return true;
        }
        else
                return false;
}

bool DDrPatch_Int32(int* dest, int value)
{
        DWORD oldp;
        
        if (VirtualProtect(dest, 4, PAGE_EXECUTE_READWRITE, &oldp))
        {
                *dest = value;
                VirtualProtect(dest, 4, oldp, &oldp);
                return true;
        }
        else
                return false;
}

bool DDrPatch_Int16(short* dest, short value)
{
        DWORD oldp;
        
        if (VirtualProtect(dest, 2, PAGE_EXECUTE_READWRITE, &oldp))
        {
                *dest = value;
                VirtualProtect(dest, 2, oldp, &oldp);
                return true;
        }
        else
                return false;
}