Sharing memory in .dll

[Ondrej Majerech]

In my textbook, there`s that I can share data in .dll like this:

  // some code before
#pragma data_seg ("shared")
  //some initialized, global variables
int a = 0;
#pragma data_seg ()
#pragma comment (linker, "/SECTION:shared,RWS")

but that doesn`t work, when I run the app twice, the data AREN`T shared, between theese two instances. Can anyone help?

Thanks,

Oxygenium.

[Patrick Ogay]

There was a disussion on pragma in GCC...
The conclusion was, that pragma make incompatible code and GCC doesn't support (m)any pragmas.

I guess you have to use the shared memory calls...

Patrick

[Nobody/Anonymous]

Shared memory calls? How can I do that?

[Nobody/Anonymous]

Same issue here.  I'm trying to code a keyboard hook and all the examples (MSVC) I can find use the pragmas to define a shared dataseg.  Someone please post a work-around if minGW can't do it.

Thanks in advance.

[Ondrej Majerech]

That's nice, but you haven't told me how to share that data?

Please help,

Oxygenium.

[Nobody/Anonymous]

Ok, I figured it out on my own. Character building, even if a waste of time.

Here is an example of a keyboard hook that works when compiled by minGW. It includes the steps of how to implement shared memory within a DLL:

#include <windows.h>
#include <stdio.h>

#include <memory.h>

static LPVOID lpvMem = NULL; // pointer to shared memory

typedef struct sharedstruct{
  UINT uMsg;
  HWND hWndMain;
  HHOOK hKeyHook;
} shareddataareatype;

HINSTANCE hInstance = 0;
HOOKPROC lpfnHookProc = 0;
#include "hookmain.h"

int writetrace(char *buf,char *mode){
  FILE *fd=fopen("tracedll.txt",mode);
//  char buf[100];
//  sprintf(buf,"setting module handle to %d\n", hInstance);
  fputs(buf,fd);
  fclose(fd);
}

//int DllClass::instances;
LRESULT __stdcall KeyboardFunc (int nCode, WPARAM wParam, LPARAM lParam) {
   char szDebug[100];
   shareddataareatype *shared=(shareddataareatype*)lpvMem;

   // Check for exception cases...
   char buf[100];

   if (nCode < 0)
      return (CallNextHookEx (shared->hKeyHook, nCode, wParam, lParam));
   if (nCode == HC_NOREMOVE)
      return (CallNextHookEx (shared->hKeyHook, nCode, wParam, lParam));

   // pass it to hook caller
   int up=(HIWORD (lParam) & 0xC000);
   if(!up) {
     GetKeyNameText(lParam, szDebug,sizeof(szDebug));
     sprintf(buf,"KeyboardFunc(%d) ncode=%d wparam=%d, lparam=%d (%d,%d) - %s up:%d\n",
        (int)shared,nCode,wParam,lParam,shared->hWndMain,shared->hKeyHook,szDebug);
     writetrace(buf,"a");
     sprintf(buf,"send to %d, msg:%d, %d,%d\n",shared->hWndMain, shared->uMsg, wParam, lParam);
     writetrace(buf,"a");
     PostMessage (shared->hWndMain, shared->uMsg, wParam, lParam);
   }

   // pass it on through to who it belongs to
   return (CallNextHookEx (shared->hKeyHook, nCode, wParam, lParam));
}

extern "C" EXPORT BOOL __stdcall InstallKeyboardHook (HWND hWnd, UINT uMyMsg, DWORD threadId)
{
//   hWndMain = hWnd ;
//   uMsg     = uMyMsg;
   shareddataareatype *shared=(shareddataareatype*)lpvMem;
   char buf[100];
   HHOOK hKeyHook;

   shared->hWndMain=hWnd;
   shared->uMsg=uMyMsg;
   lpfnHookProc = (HOOKPROC) KeyboardFunc ;
   sprintf(buf,"InstallKeyboardHook hInstance=%d\n", hInstance);
   writetrace(buf,"a");
   hKeyHook = SetWindowsHookEx (WH_KEYBOARD, lpfnHookProc, hInstance, threadId);
   if (hKeyHook) {
     writetrace("SetWindowsHookEx worked\n","a");
     shared->hKeyHook=hKeyHook;
     return TRUE ;
   }
   else {
     sprintf(buf,"SetWindowsHookEx failed err=%d\n", GetLastError());
     writetrace(buf,"a");
      return FALSE ;
   }
}

extern "C" BOOL APIENTRY DllMain(
  HINSTANCE hModule     /* Library instance handle. */ ,
  DWORD reason        /* Reason this function is being called. */ ,
  LPVOID reserved     /* Not used. */ )
{
  HANDLE hMapObject = NULL;  // handle to file mapping
  BOOL fInit, fIgnore;

  char buf[100];
    switch (reason)
    {
      case DLL_PROCESS_ATTACH:
        hInstance = hModule; // the DLLs instance handle.
        sprintf(buf,"DLL_PROCESS_ATTACH hInstance=%d\n",hInstance);
        writetrace(buf,"a");
        hMapObject = CreateFileMapping(
                INVALID_HANDLE_VALUE, // use paging file
                NULL,                 // default security attributes
                PAGE_READWRITE,       // read/write access
                0,                    // size: high 32-bits
                sizeof(shareddataareatype), // size: low 32-bits
                "HookShareDataFile");       // name of map object
        if (hMapObject == NULL)
            return FALSE;

        // The first process to attach initializes memory.
        fInit = (GetLastError() != ERROR_ALREADY_EXISTS);

        // Get a pointer to the file-mapped shared memory.
        lpvMem = MapViewOfFile(
            hMapObject,     // object to map view of
            FILE_MAP_WRITE, // read/write access
            0,              // high offset:  map from
            0,              // low offset:   beginning
            0);             // default: map entire file
        if (lpvMem == NULL)
          return FALSE;

        // Initialize memory if this is the first process.

//        if (fInit)
//           memset(lpvMem, '\0', SHMEMSIZE);
        break;

      case DLL_PROCESS_DETACH:
        break;
      case DLL_THREAD_ATTACH:
        break;
      case DLL_THREAD_DETACH:
        // Unmap shared memory from the process's address space.
        fIgnore = UnmapViewOfFile(lpvMem);
        // Close the process's handle to the file-mapping object.
        fIgnore = CloseHandle(hMapObject);
        break;
    }

    /* Returns TRUE on success, FALSE on failure */
    return TRUE;
}

[Nobody/Anonymous]

apparently you can't post anything more than a code snippet here.  You can find an example of how to implement shared memory under Win32 at:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/using_shared_memory_in_a_dynamic_link_library.asp