Why does the RegQueryValueEx() function return ERROR_FILE_NOT_FOUND while trying to read from a registry key?

Question

System: Windows 7 32bit
Language: C++

I have tried to access register HKEY_LOCAL_MACHINE\\HARDWARE\\DEVICEMAP\\Scsi\\Scsi Port 0, key Driver (type REG_SZ) -- no problem.

The same for reading from HKEY_LOCAL_MACHINE\\HARDWARE\\DEVICEMAP\\SERIALCOMM, all keys (types REG_SZ) got slashes, for example \Device\Serial0.

While reading such keys it always returns 2 (no such file) with following example code:

HKEY hKey = 0;
DWORD dwType = REG_SZ;
char buf[255] = {0};
DWORD dwBufSize = sizeof(buf);

if( RegOpenKeyEx( HKEY_LOCAL_MACHINE, TEXT("HARDWARE\\DEVICEMAP\\SERIALCOMM"), 0, KEY_QUERY_VALUE, &hKey ) == ERROR_SUCCESS ) 
{
  auto ret = RegQueryValueEx( hKey, TEXT("\Device\Serial0"), 0, &dwType, (LPBYTE)buf, &dwBufSize );
  // ret always == 2 for key with slashes
--- CUT ---

What is the proper way to read key values with slashes in name?

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Above has been properly answered by Cody Gray.

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Below another issue.

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Im getting the same problem when Im using variable instead of a text string.
Iv considered both approaches with single and double slashes:

HKEY hKey = 0;
DWORD keyType = REG_SZ;
TCHAR buf[255] = {0};
DWORD bufSize = sizeof(buf);

QSettings winReg("HKEY_LOCAL_MACHINE\\HARDWARE\\DEVICEMAP\\SERIALCOMM", QSettings::NativeFormat);
auto comsKey = winReg.allKeys();

FOREACH( auto com, comsKey )
{
  // FOREACH - boost macro
  // comsKey = QList<QString> (list of key names) [from Qt framework]
  // com = QString (single key name) [from Qt framework]
  if( RegOpenKeyEx(HKEY_LOCAL_MACHINE, TEXT("HARDWARE\\DEVICEMAP\\SERIALCOMM"), 0, KEY_QUERY_VALUE, &hKey ) == ERROR_SUCCESS )
  {
    wchar_t* keyw = new wchar_t();
    //com.replace("/", "\\\\"); <- checked both variants commented and not commented; com == /Device/Serial0 so im converting to \\Device\\Serial0
    int size = com.size();
    mbstowcs( keyw, com.toStdString().data(), size );
    //auto ret = RegQueryValueEx( hKey, TEXT("\\Device\\Serial0"), 0, &keyType, (LPBYTE)buf, &bufSize ); // <- this works!
    auto ret = RegQueryValueExW( hKey, (LPCWSTR)&keyw, 0, &keyType, (LPBYTE)buf, &bufSize ); // <- this one not works!

I have tried all variants with "\Device..", "/Device", "\Device", etc.

Answer 1

You have to escape the slashes, just like you did in the first line...

if( RegOpenKeyEx( HKEY_LOCAL_MACHINE, TEXT("HARDWARE\\DEVICEMAP\\SERIALCOMM"), 0, KEY_QUERY_VALUE, &hKey ) == ERROR_SUCCESS ) 
{
  auto ret = RegQueryValueEx( hKey, TEXT("\\Device\\Serial0"), 0, &dwType, (LPBYTE)buf, &dwBufSize );
  // ret always == 2 for key with slashes

If you don't, the RegQueryValueEx function can't find the specified key, and it returns ERROR_FILE_NOT_FOUND (== 2).

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But there's another problem. You should be declaring the buffer array as type wchar_t (or TCHAR), rather than char:

TCHAR buf[255] = {0};

Otherwise, the RegQueryValueEx function is going to attempt to fill the array with a Unicode string read from the specified registry key, and you're going to get something unreadable.