Why is my LogError function resetting GetLastError? - c++

Question

I created a library that gets the last error as a string and then outputs that error to a file but for some reason my SetLogLocation and LogError function makes GetLastError() return 0. It should not return 0 though because in the test below I am forcing GetModuleHandle to fail. Why is GetLastError() returning 0? Also, is handling errors such as fopen or GetModuleHandle failing in a library not recommended?
// test.exe
int main()
{
if (!GetModuleHandleA("xxx"))
{
SetLogLocation("c:\\users\\admin\\desktop\\error.log");
LogError("test");
}
// outputs 0
DWORD dwError = GetLastError();
std::cout << dwError;
std::cin.get();
return 0;
}
// test.dll
EXPORT void SetLogLocation(std::string path)
{
g_LogFile = std::ofstream(path, std::ios::app);
}
EXPORT void LogError(std::string errorText)
{
if (!g_LogFile.good())
return;
DWORD dwErrorCode = GetLastError();
if (dwErrorCode == 0)
{
Log(errorText);
}
else
{
std::stringstream ss;
ss << errorText << std::endl
<< "Last error: " << GetErrorAsString(dwErrorCode) << std::endl
<< "Error code: " << dwErrorCode;
Log(ss.str());
}
}
EXPORT std::string GetErrorAsString(DWORD dwErrorCode)
{
LPSTR lpBuffer = NULL;
DWORD dwSize = FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
NULL, dwErrorCode, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&lpBuffer, 0, NULL);
std::string errorMessage(lpBuffer, dwSize);
if (errorMessage[errorMessage.length() - 1] == '\n')
errorMessage.erase(errorMessage.length() - 1, 1);
LocalFree(lpBuffer);
return errorMessage;
}

Answer 1

A call to most windows functions will reset GetLastError, you need to store the value of GetLastError as soon as possible after your failing function then use that value rather than calling GetLastError again.
SetLogLocation will clear GetLastError as it is opening a file. FormatMessageA inside LogError will also clear GetLastError.
If you want your logging functions to not alter GetLastError you can always save and restore the value at the end of your functions:
struct LastError
{
LastError()
: value(getLastError())
{
}
~LastError()
{
SetLastError(value);
}
DWORD value;
};
EXPORT void SetLogLocation(std::string path)
{
LastError lastError;
g_LogFile = std::ofstream(path, std::ios::app);
}
EXPORT void LogError(std::string errorText)
{
LastError lastError;
if (!g_LogFile.good())
return;
if (lastError.value == 0)
{
Log(errorText);
}
else
{
std::stringstream ss;
ss << errorText << std::endl
<< "Last error: " << GetErrorAsString(lastError.value) << std::endl
<< "Error code: " << lastError.value;
Log(ss.str());
}
}