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

问题

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?

回答1:

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.

回答2:

No. LPCTSTR is a pointer. So the recipient application will receive a pointer to a memory location of the sender application. It would cause the recipient application to crash due to a memory violation, had it tried to access the data.

来源：https://stackoverflow.com/questions/61725198/is-it-safe-to-use-lpctstr-in-a-structure-with-the-wm-copydata-message

标签

c++

winapi

mfc

wm-copydata