Why is CreateProcess API call causing memory access violations? - c++

Question

I have a function that is supposed to launch another process:
DWORD WINAPI StartCalc(LPVOID lpParam) {
STARTUPINFOW info;
PROCESS_INFORMATION processInfo;
std::wstring cmd = L"C:\\Windows\\System32\\calc.exe";
BOOL hR = CreateProcessW(NULL, (LPWSTR)cmd.c_str(), NULL, NULL, TRUE, 0, NULL, NULL,
&info, &processInfo);
if (hR == 0) {
DWORD errorMessageID = ::GetLastError();
printf("Error creating process\n");
return 1;
}
return 0;
}
I get an exception in ntdll.dll "Access violation reading location 0xFFFFFFFFFFFFFFFF". I know there are a few common mistakes that might cause this:
Inconsistent calling conventions. I am using __stdcall
Encoding issues. I storing strings as wide chars
The problem happens in both x64 and x86 builds
The problems happens when I try to create other Windows processes
What am i doing wrong?
EDIT: This actually isn't a problem with casting cmd.c_str() as a (LPWSTR), that part appears to be fine. I needed to initialize the STARTUPINFO struct: STARTUPINFO info = { 0 };

Answer 1

BOOL hR = CreateProcessW(NULL, (LPWSTR)cmd.c_str(), NULL, NULL, TRUE, 0, NULL, NULL, &info, &processInfo);
^^^^^
That's a cast. A great rule of thumb is "spot the cast, you spot the error".
It's true here. CreateProcessW must be passed a writable string. That means, no literal, and also no c_str() result.
From the documentation:
The Unicode version of this function, CreateProcessW, can modify the contents of this string. Therefore, this parameter cannot be a pointer to read-only memory (such as a const variable or a literal string). If this parameter is a constant string, the function may cause an access violation.
Pass a real non-const pointer, don't play hide-the-const. &cmd[0] should work, that's guaranteed to be a writable string. To be super-safe, increase your wstring capacity beyond just what you needed, because CreateProcessW is going to use it as a working buffer.