Using BroadcastSystemMessage to ping another application

Question

I have a server application from a vendor that provides various services I need to use.  The vendor also provided a sample client application written in Borland C++ Builder.  I'm trying to implement the same message passing scheme they use in their client ina CVI program, but I can't get past the first step.
The Borland app first uses BroadcastSystemMessage to ping their server application.  Then it waits for a response from the server in the form of a WM_COPYDATA windows message.  The original Borland code for pinging the server looks something like this:
   UINT ping_id = RegisterWindowMessage("client_ping");
   DWORD dwApps = BSM_APPLICATIONS;
   BroadcastSystemMessage( BSF_NOHANG, &dwApps, ping_id, (WPARAM) m_client_handle, 0 );
The m_client_handle is the windows handle of the client application.  The server uses that handle to send it's reponse directly back to the client. My CVI implementation of the ping looks like this:
    GetPanelAttribute (panelHandle, ATTR_SYSTEM_WINDOW_HANDLE, (int *)&ClientWindowHandle);
    int PingId = RegisterWindowMessage("client_ping");
    DWORD dwApps = BSM_APPLICATIONS;
    int err = BroadcastSystemMessage(BSF_NOHANG, &dwApps, PingId, (WPARAM) ClientWindowHandle, 0);
BroadcastSystemMessage returns a 1 indicating that everything worked fine.
The next half of the ping is to catch the return message from the server.  The Borland app does this by implementing the call OnCopyData for it's main form.  As best as I can determine, this is essentially a predefined message handler for WM_COPYDATA.  I implemented this in CVI with the following code:
int main (int argc, char *argv[])
    // this is really the same call as before (i.e., I only get the handle once)
    GetPanelAttribute (panelHandle, ATTR_SYSTEM_WINDOW_HANDLE, (int *)&ClientWindowHandle);
    InstallWinMsgCallback (panelHandle, WM_COPYDATA, ProcessWmCopydataMessage, VAL_MODE_INTERCEPT, &panelHandle, &ClientWindowHandle);
int CVICALLBACK ProcessWmCopydataMessage (int panelHandle, int message, unsigned int* wParam, unsigned int* lParam, void* callbackData)
    PCOPYDATASTRUCT ServerInfo;
    if (message == WM_COPYDATA)
        // process returned message
The problem is that I never get a call to PrcoessWmCopydataMessage.  Am I doing something wrong in my setup?  In particular, am I retrieving the correct handle for my application and am I sending it correctly in the broadcast message?  Is there any spy tool that allows you to watch all windows messages and reponses so I can determine if the server is doing anything?
I tired writing my own dummy server just to verify that BroadcastSystemMessage works the way I expect.  The only problem is that I can't figure out how to catch the broadcast message in my dummy server app.  How do I install a callback that will catch broadcast messages?
One last thing: at first I couldn't get BroadcastServerMessage to compile.  It said it was not declared.  After a quick search on the forum, I added the following line before my inclusion of the windows header:
    #define _WIN32_WINDOWS 0x0410
It compiles now, but I'm concerned that by faking my Windows version I may have messed up something else.  I running Windows 7 BTW.
Tony G.

Answer 1

Hey Tony -
The reason you get the compile error is because CVI does not define _WIN32_WINNT in the version of CVI you are using.  We expect the programmer to define it based on their own needs.  BroadcastSystemMessage is conditionally compiled so that it will not be defined if _WIN32_WINNT is not defined:
WinUser.h
#if defined(_WIN32_WINNT)
WINUSERAPI
long
WINAPI
BroadcastSystemMessageA(
The consequence of defining it to be 0x0410 is that you are stating your program is compatible with version NT 4.0 of Windows and later.  This means that you can use all functionality of the Windows OS that was available in Windows NT 4.0, but none of the newer functionality will be available to you.  If you try to run your program on a version of Windows older than Windows NT 4.0, the program will likely not run (this should not be an issue...)
Note that 0x0410 is not actually a defined Windows version.  From a recent version of the Interface to the Win32 API, you can read from sdkddkver.h:
#define _WIN32_WINNT_NT4     0x0400
#define _WIN32_WINNT_WIN2K   0x0500
This means that you are stating your program uses no Windows fuctionality more recent than Windows NT 4.0.  Here's some more good information on using these macros.
NickB
National Instruments