nee@cf_su14.Salomon.Com (Robert Nee) (12/20/90)
I am having a problem implementing a Hook function in windows. I feel
I must be doing something wrong. I have two programs that both install
WH_GETMESSAGE hooks. Either will work fine by itself but if I run
both it seems that the second program (it doesn't matter which one)
prevents the first's hook function from getting called. Both hook
functions are in DLLs and have a structure similar to the following:
void FAR PASCAL Hook (int iCode, WORD wParam, DWORD lParam)
{
if (iCode >= 0) {
/* Check if the message is something I'm interested in
}
else {
DefHookProc (iCode, wParam, lParam, NextHook)
}
where NextHook is a pointer to a pointer to the next hook. Even if this
nexthook parameter is wrong it shouldn't matter because each program
installs only one hook. The documentation claims that this NextHook is
only for multiple hooks in one program. Is this correct?
This routine also only passes control to DefHookProc if it doesn't
proccess the message itself. Is this correct? Do you need to ALWAYS
call DefHookProc at the end of your routine?
SetWindowsHook returns the Proc Instance of the previous hook function
installed. Do I need to call that function manually when I am done?
(the sample above does not do that)
I am certain I am installing these functions properly. And they are in
DLLs as recomended. One final question; If the application is not really
designed for real mode can I place the hook function in my main program,
or more specifically a non-discardable segment of my program without any
side effects?
Any help on this subject would be appreciated. I am working on several
utilities that I hope many users will find useful. Any assistance would
furthur that effort.
Thank you.
Robert Needjd@rice-krispies.ai.mit.edu (David D'Souza) (12/24/90)
In article <143@cf_su20.cf_su10.Sbi.COM> nee@cf_su14.Salomon.Com (Robert Nee) writes: >I am having a problem implementing a Hook function in windows. I feel >I must be doing something wrong. I have two programs that both install >WH_GETMESSAGE hooks. Either will work fine by itself but if I run >both it seems that the second program (it doesn't matter which one) >prevents the first's hook function from getting called. Both hook >functions are in DLLs and have a structure similar to the following: You should always call DefHookProc if you want hook functions down the chain to be called. This may not have been documented too clearly but this should solve your problem. >where NextHook is a pointer to a pointer to the next hook. Even if this >nexthook parameter is wrong it shouldn't matter because each program >installs only one hook. The documentation claims that this NextHook is >only for multiple hooks in one program. Is this correct? Nope, next hook is a pointer (actually, better to treat it as a magic cookie) to the next hook in the chain regardless of where it is. >SetWindowsHook returns the Proc Instance of the previous hook function >installed. Do I need to call that function manually when I am done? >(the sample above does not do that) No, don't do this... For the magic cookie reasons stated above. >I am certain I am installing these functions properly. And they are in >DLLs as recomended. One final question; If the application is not really >designed for real mode can I place the hook function in my main program, >or more specifically a non-discardable segment of my program without any >side effects? No, don't place the hook function in your app. Very very bad idea. First, most apps are written SS==DS. Your hook function is pretty much always called SS!=DS. Bad things happen if you runn SS==DS runtime libraries in a SS!=DS environment etc... Also, there are some "future compatibility" issues so Microsoft is telling you this for your own good... --Dave