How can I convert a PWCHAR into a FILETIME structure? - winapi

Question

The idea I have is to use the STL and convert a PWCHAR timestamp into a wstring, load it into a stream and use the >> operator to assign a given split into the appropriate member of the SYSTEMTIME structure and convert it with a Win32 function to a FILETIME structure.
VOID StringToFileTime(const PWCHAR timeString, FILETIME &ft) {
if(NULL == timeString)
return;
wstring time(L"2013-May-14 20:33:13.132814");
std::wistringstream stream(time);
SYSTEMTIME st = {0};
stream >> st.wYear;
stream.ignore(1, '-');
stream >> st.wMonth;
stream.ignore(1, '-');
stream >> st.wDay;
stream.ignore(1, ' ');
stream >> st.wHour;
stream.ignore(1, ':');
stream >> st.wMinute;
stream.ignore(1, ':');
stream >> st.wSecond;
stream.ignore(1, '.');
stream >> st.wMilliseconds;
SystemTimeToFileTime(&st, &ft);
}
The problem I'm running into is the SYSTEMTIME structure does not get filled out after st.wYear. So the year gets set with 2013, but members after that are 0.
Looking at the variable contents I get more confusion. The hardcoded timestamp does not appear in the wstring time variable in the debuggers variable view. Instead there is an nPOS number and errors listed for the time variable.
How can I convert the string to a FILETIME structure?
I'm using Visual Studio 2013 Update 3 on Windows 10.

Answer 1

The problem is that stream >> st.wMonth fails, because it is trying to read the string value May into a numeric WORD, which is not a supported conversion. You are not resetting the stream error state, so subsequent uses of >> and ignore() also fail, thus the SYSTEMTIME does not get filled in after wYear.
If you have a C++11 compiler, you can use std::get_time() to parse timeString:
#include <sstream>
#include <iomanip>
bool StringToFileTime(LPCWSTR timeString, FILETIME &ft)
{
SYSTEMTIME st = {0};
std::wistringstream stream(timeString);
std::tm tmb;
// std::get_time() does not support reading milliseconds...
if (!(stream >> std::get_time(&tmb, L"%Y-%b-%d %H:%M:%S")))
return false;
st.wYear = 1900 + tmb.tm_year;
st.wMonth = tmb.tm_mon + 1;
st.wDayOfWeek = tmb.tm_wday;
st.wDay = tmb.tm_mday;
st.wHour = tmb.tm_hour;
st.wMinute = tmb.tm_min;
st.wSecond = tmb.tm_sec;
if (!stream.eof())
{
stream.ignore(1, L'.');
// st.wMilliseconds is a 16bit WORD, so it can only go up to 65535.
// There are only 1000 ms in a second, so 132814 is clearly not
// expressed in milliseconds. Is it nanoseconds? 100-nanoseconds?
// Whatever it is, convert it to st.wMilliseconds as needed...
int iValue;
if (!(stream >> iValue))
return false;
st.wMilliseconds = ...;
}
return SystemTimeToFileTime(&st, &ft);
}
If you don't have a C++11 compiler, you can still use std::wistringstream, you just have to extract the values manually:
#include <sstream>
bool StringToFileTime(LPCWSTR timeString, FILETIME &ft)
{
SYSTEMTIME st = {0};
std::wistringstream stream(timeString);
std::string sMonth;
if (!(stream >> st.wYear)) return false;
if (!stream.ignore(1, L'-')) return false;
// st.wMonth is a 16bit WORD, but a month name is given instead.
// parse it into st.wMonth as needed...
if (!std::getline(stream, sMonth, L'-')) return false;
st.wMonth = ...;
if (!stream.ignore(1, L'-')) return false;
if (!(stream >> st.wDay)) return false;
if (!stream.ignore(1, L' ')) return false;
if (!(stream >> st.wHour)) return false;
if (!stream.ignore(1, L':')) return false;
if (!(stream >> st.wMinute)) return false;
if (!stream.ignore(1, L':')) return false;
if (!(stream >> st.wSecond)) return false;
if (!stream.eof())
{
stream.ignore(1, L'.');
// convert to st.wMilliseconds as needed...
int iValue;
if (!(stream >> iValue))
return false;
st.wMilliseconds = ...;
}
return SystemTimeToFileTime(&st, &ft);
}
Alternatively, you can use swscanf() instead of std::wistringstream:
bool StringToFileTime(LPCWSTR timeString, FILETIME &ft)
{
SYSTEMTIME st = {0};
WCHAR szMonth[12];
int iValue = 0;
int numRead = swscanf(timeString, L"%hu-%[^-]-%hu &hu:&hu:&hu.%d", &st.wYear, szMonth, &st.wDay, &st.wHour, &st.wMinute, st.wSecond, &iValue);
if (numRead < 6) return false;
// parse szMonth into st.wMonth as needed...
if (numRead == 7)
{
// convert iValue into st.wMilliseconds as needed...
}
return SystemTimeToFileTime(&st, &ft);
}