C++ RegEnumValue throws ERROR_MORE_DATA

The buffer is likely big enough, you're just lying to the function about its actual size.

When you use the sizeof operator on a pointer, it returns the size of the pointer, not the size of the array to which it points. And because key is passed to the function as a char*, it is merely a pointer to the array, not the array itself.

You have to pass the actual size of the buffer to your function as a parameter along with the pointer:

DWORD regGetValue(char *key, size_t length, int index)
{
   HKEY hKey; 
   RegOpenKeyEx(HKEY_LOCAL_MACHINE,"HARDWARE\\DEVICEMAP\\SERIALCOMM",0,KEY_ALL_ACCESS,&hKey);
   DWORD chars = length;
   LONG result = RegEnumValue(hKey, index, key, &chars, 0,0,0,0 );
   RegCloseKey(hKey);
   return result;
}

char key[512];
int index = 0;
regGetValue(key, sizeof(key), index);

The sizeof operator works as expected here because it's operating on the array itself, not merely a pointer to it. This approach, however common in C APIs, is cumbersome.

You can simplify things at the call site a bit by using a function template that automatically deduces the size of your buffer and passes it to an internal helper function that actually does the work:

// Helper function, implemented as a private member of your class
DWORD regGetValue(char *key, size_t length int index)
{
   HKEY hKey; 
   RegOpenKeyEx(HKEY_LOCAL_MACHINE,"HARDWARE\\DEVICEMAP\\SERIALCOMM",0,KEY_ALL_ACCESS,&hKey);
   DWORD chars = length;
   LONG result = RegEnumValue(hKey, index, key, &chars, 0,0,0,0 );
   RegCloseKey(hKey);
   return result;
}

// The public function that you will actually call
template <size_t N>
DWORD regGetValue(char (&key)[N], int index)
{
   return regGetValue(key, N, index);
}

char key[512];
int index = 0;
regGetValue(key, index);  // length determined and passed automatically

This works because you pass a reference to the array, rather than a pointer.

Aside from all of that, I have to seriously question why you're calling ANSI functions and using char* for your strings. All modern Windows applications should be Unicode. That means calling the W-suffixed versions of the Windows API functions and using wchar_t* as your string type. Make sure that both the UNICODE and _UNICODE symbols are defined for your project, and you'll get a compile-time error if you use the wrong type.

MSDN says: If the lpName buffer is too small to receive the name of the key, the function returns ERROR_MORE_DATA. Before you call RegEnumKeyEx, you can call function RegQueryInfoKey to determine the size of the largest subkey for the key identified by hKey. The usage of RegQueryInfoKey can find here: "http://msdn.microsoft.com/en-us/library/windows/desktop/ms724902(v=vs.85).aspx"