Shared data in DLL

ChrisBoy · June 4th, 2003, 08:36 AM

I am employing a system-wide CBT hook that needs to respond to certain Windows messages and to inform a single instance of a 'Handler'. The hook works fine globally but of course each instance of the DLL (i.e. for each process it gets injected into) has its own 'Handler'.

I have tried to make the Handler object 'shared' through the following code:

#pragma comment(linker, "/SECTION:.SHARED,RWS")
#pragma data_seg(".SHARED")
HHOOK hHookCbt = 0;
Handler handler;
#pragma data_seg()

This doesn't seem to work though (I believe that objects may not be sharable, only primitive data types?) so perhaps file mapping might be the answer.

Does anybody have any ideas and/or suggestions on how to go about this please?

Thanks,

Chris.

josuegomes · June 5th, 2003, 08:58 AM

What you mean by not working?
Could you post some compilable code?

JosuÃ© Andrade Gomes

Kvido · March 30th, 2006, 08:06 AM

According to
http://msdn2.microsoft.com/en-US/library/h90dkhs0(VS.80).aspx

the data in shared section have to be initialized. It means STATICALLY assigned in source code, the compiler inserts this initialization directly to the compiled DLL. So you cannot share objects this way...

-----------------
http://www.ubytujnas.cz

ianw · April 18th, 2006, 11:29 AM

You can share objects in a shared section, but you have to be sneaky to do it by using placement new:

Code:

#pragma comment(linker, "/SECTION:.SHARED,RWS")
#pragma data_seg(".SHARED")
HHOOK hHookCbt = 0;
static BYTE buffer[sizeof(Handler)] = {0};
#pragma data_seg()
// each process gets its own copy of the pointer:
static Handler* pHandler = NULL;
...
// In the first process to load the dll, create the single shared handler using placement new:
pHandler = new (buffer) Handler;
// subsequent processes just cast the pointer:
pHandler = reinterpret_cast<Handler*>(buffer);

The destructor won't get called automatically using this mechanism, so if you need it to run, call it explicitly when the last process exits:

Code:

pHandler->~Handler();

Ian