How to write memory of a dll? - winapi

Question

I'm trying to replace a specific string in memory belongs to a dll. Here's the code.
I can read it, and it gives me correct result, but when writing VC++ shows 'Access violation writing location'.
HMODULE HMODULE1 = LoadLibrary(L"my.dll");
std::string x1(8, '\0');
std::string x2 = "CIFCDMEY";
auto startPos = (void*)((char*)(HMODULE1)+0x1158A0 + 9);
// Correct, I can read the memory
memcpy_s((void*)x1.data(), x1.size(), startPos, x1.size());
// Access violation writing location
memcpy_s(startPos, x2.size(), x2.data(), x2.size());
auto handle = GetCurrentProcess();
SIZE_T num;
auto ret = WriteProcessMemory(handle, startPos, x2.data(), x2.size(), &num);
auto lastError1 = GetLastError();
LPVOID lpMessageBuffer1 = NULL;
size_t size1 = ::FormatMessage(
FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
NULL,
lastError1,
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
(LPTSTR)&lpMessageBuffer1,
0,
NULL);
std::wstring errorMessage1;
if (size1 > 0) {
// errorMessage1: Invalid access to memory location.
errorMessage1 = std::wstring((LPCTSTR)lpMessageBuffer1, size1);
}
In 'Watch' window, the value of variable startPos is my.dll!0x0f2a58a9 (load symbols for additional information).
I know people use 'WriteProcessMemory' to write memory of a process, how about a dll?

Answer 1

If the target memory page does not have write permissions you need to take them using VirtualProtect()
I use a simple wrapper function for all my patching, it uses the PAGE_EXECUTE_READWRITE memory protection constant because if you are modifying a code page this will avoid crashes when the instruction pointer lands in the same memory page and you only used PAGE_READWRITE
void Patch(char* dst, char* src, const intptr_t size)
{
DWORD oldprotect;
VirtualProtect(dst, size, PAGE_EXECUTE_READWRITE, &oldprotect);
memcpy(dst, src, size);
VirtualProtect(dst, size, oldprotect, &oldprotect);
}