OpenClipboard crashes with CLIPBRD_E_CANT_OPEN - c++

Question

I have a problem with opening clipboard multiple times. It opens up fine on the first call, however, my program needs to access the clipboard several times, but shuts down on the second call of OpenClipboard(NULL) with the CLIPBRD_E_CANT_OPEN error).
This is a function that opens the clipboard
void Translator::process_copied()
{
//cout << "processing" << endl;
if (OpenClipboard(NULL))
{
HANDLE clip;
clip = GetClipboardData(CF_UNICODETEXT);
Relayout((WCHAR*)clip);
HANDLE res = GlobalAlloc(GMEM_FIXED, sizeof(WCHAR)*wcslen((WCHAR*)clip)+1);
res = clip;
EmptyClipboard();
SetClipboardData(CF_UNICODETEXT, res);
CloseClipboard();
}
else
{
std::cout << GetLastError();
//throw clipboard_did_not_open;
//cout << "error in opening clipboard" << endl;
}
}
I suspect that the problem appears in the SetClipboardData() function, because if I remove it, clipboard doesn't have any problem opening multiple times. The same goes to res = clip; line.
I am new to WinAPI and C++ in general, so I apologize if my question is silly.

Answer 1

When storing a memory block on the clipboard, the HANDLE given to SetClipboardData() must be allocated with the GMEM_MOVEABLE flag, per the SetClipboardData() documentation:
If the hMem parameter identifies a memory object, the object must have been allocated using the [GlobalAlloc] function with the GMEM_MOVEABLE flag.
As such, you must call GlobalLock() on the HANDLE returned by GetClipboardData() in order to access the data, per the GlobalAlloc() documentation:
GMEM_MOVEABLE
0x0002
Allocates movable memory. Memory blocks are never moved in physical memory, but they can be moved within the default heap.
The return value is a handle to the memory object. To translate the handle into a pointer, use the GlobalLock function.
Don't just type-cast the HANDLE as-is, like you can for a memory object allocated with the GMEM_FIXED flag.
You are also not putting any character data in the res memory that you allocate with GlobalAlloc(). Which is moot, since you are not putting your allocated memory on the clipboard at all, you are actually putting the original HANDLE from GetClipboardData() back onto the Clipboard, and then leaking your res memory.
Try something more like this instead:
void Translator::process_copied()
{
//cout << "processing" << endl;
HWND hwnd = ...; // a window that YOU own
if (OpenClipboard(hwnd))
{
HANDLE clip = GetClipboardData(CF_UNICODETEXT);
if (clip)
{
WCHAR *text = (WCHAR*) GlobalLock(clip);
Relayout(text);
HANDLE res = GlobalAlloc(GMEM_MOVABLE, (wcslen(text)+1) * sizeof(WCHAR));
if (res)
{
WCHAR *newText = (WCHAR*) GlobalLock(res);
// put characters in newText as needed...
wcscpy(newText, text);
GlobalUnlock(res);
GlobalUnlock(clip);
EmptyClipboard();
if (!SetClipboardData(CF_UNICODETEXT, res))
{
std::cout << GetLastError();
GlobalFree(res);
//throw clipboard_did_not_set;
//cout << "error in setting clipboard data" << endl;
}
}
else
{
std::cout << GetLastError();
//throw global_memory_not_allocated;
//cout << "error in allocating memory" << endl;
GlobalUnlock(clip);
}
}
else
{
std::cout << GetLastError();
//throw clipboard_data_not_get;
//cout << "error in getting clipboard data" << endl;
}
CloseClipboard();
}
else
{
std::cout << GetLastError();
//throw clipboard_did_not_open;
//cout << "error in opening clipboard" << endl;
}
}