Is it safe to use LPCTSTR in a structure with the WM_COPYDATA message?

Question

I have this structure:

typedef struct tagCOPY_PACKET
{
    TCHAR szFile[_MAX_PATH];
    GUID guidSignature;

} S_COPY_PACKET;

I prepare to send data with WM_COPYDATA like this:

CString strFile = GetFileToOpenFromFileExplorerPath();

S_COPY_PACKET sCopyDataPacket;
_tcscpy_s(sCopyDataPacket.szFile, _MAX_PATH, strFile);
sCopyDataPacket.guidSignature = CopyData_Signature;
COPYDATASTRUCT cds;
cds.dwData = COPYDATA_TYPE_MSA;
cds.cbData = sizeof(sCopyDataPacket);
cds.lpData = &sCopyDataPacket;

DWORD_PTR dwResult;
if (SendMessageTimeout(hOtherInstance, WM_COPYDATA,
                NULL, (LPARAM)(LPVOID)&cds, SMTO_BLOCK, 2000, &dwResult) != 0)
{
}

And at the other end:

BOOL CMeetingScheduleAssistantDlg::OnCopyData(CWnd* pWnd, COPYDATASTRUCT* pCopyDataStruct)
{
    if (pCopyDataStruct->dwData == COPYDATA_TYPE_MSA)
    {
        S_COPY_PACKET* pCopyPacket = (S_COPY_PACKET*)(pCopyDataStruct->lpData);
        if (pCopyPacket->guidSignature != CopyData_Signature)
        {
            // Bad GUID
            return FALSE;
        }

        if (GetLastActivePopup() != this) // Popup windows!
        {
            // TODO: Tell user?
            return FALSE;
        }

        theApp.SetFileToOpenFromFileExplorer(pCopyPacket->szFile);

        OpenFileFromFileExplorer();

        return TRUE;
    }

    return FALSE;
}

It works fine. I just wanted to know if it would be acceptable to change my structure like this:

typedef struct tagCOPY_PACKET
{
    LPCTSTR lpszFile;
    GUID guidSignature;

} S_COPY_PACKET;

And then use:

S_COPY_PACKET sCopyDataPacket;
sCopyDataPacket.lpszFile = strFile.GetBufferSetLength(_MAX_PATH);
sCopyDataPacket.guidSignature = CopyData_Signature;
COPYDATASTRUCT cds;
cds.dwData = COPYDATA_TYPE_MSA;
cds.cbData = sizeof(sCopyDataPacket);
cds.lpData = &sCopyDataPacket;
...

And, once the message has been posted and processed:

strFile.ReleaseBuffer();

Is it safe to use the LPCTSTR approach with WM_COPYDATA and if so, why?

From the receiving app point of view there is absolutely no difference between these two: cds.lpData = &sCopyDataPacket; and cds.lpData = strFile.GetString();. They both point to the local data copy. So yes, it's absolutely acceptable, unless you're not trying to use cds.lpData pointer in the receiving app after SendMessage returns. — Jovibor
@Jovibor I get what you are saying. But I need to send this structure that contains the GUID and the CString contents. — Andrew Truckle
Your S_COPY_PACKET sCopyDataPacket; and CString strFile are both local data objects, right? So, what's the difference what pointer you provide in cds.lpData = to? Well, here the suggestion - try both cases and see result yourself. — Jovibor
@Jovibor The receiving application cannot read data from the memory of the sending application. The rest does not matter. — Martin Prikryl
@MartinPrikryl But it can read S_COPY_PACKET sCopyDataPacket; ? — Jovibor

j6t j6t · Accepted Answer · 2020-05-11T08:32:51

The documentation of WM_COPYDATA says:

The data being passed must not contain pointers or other references to objects not accessible to the application receiving the data.

Therefore, when CMeetingScheduleAssistantDlg lives in a different executable, you cannot pass an LPCTSTR value. If it is in the same executable, chances are that it works.

Is it safe to use LPCTSTR in a structure with the WM_COPYDATA message?

2 Answers