SAMPLE: Code Demonstrates Multiple Instance DLL

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

SUMMARY

A dynamic-link library (DLL) in the Windows environment has a single data segment that is used by each task (application instance) that links to the DLL. However, there are many situations in which a DLL must maintain data for each individual task. This article discusses the method used by the MULTINST sample application to create a multiple instance DLL. The DLL creates a separate data segment for each task that links to it.

Download MULTINST.EXE, a self-extracting file, from the Microsoft Software Library (MSL) on the following services:

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download MULTINST.EXE (size: 26551 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get MULTINST.EXE (size: 26551 bytes) 
  

MORE INFORMATION

Because each task in the system has a separate stack segment, the value of the stack segment can be used as a unique identifier. MULTINST stores data for each task in a block of memory obtained using GlobalAlloc(). Each time a task calls the DLL, use the value of its stack segment as an index into the table of selectors for the global memory blocks. Load the corresponding selector into the DS register. After this is done, all static data and the local heap correspond to the calling task.

When a new task calls into the DLL for the first time, the DLL allocates a new block of memory to hold data for the task. It copies initial values for static variables into the data block, and initializes a local heap.

When a task that uses the DLL shuts down, it calls the UnregisterTask() function in the DLL, which frees the data block associated with the task and removes the task from the DLL's task list. Failing to call UnregisterTask() might cause serious problems because Windows could reassign a stack segment value to a new task. If this task called the DLL, it would receive the data values associated with the previous task, which might be completely inappropriate.

Place the following six lines of code at the beginning of each function exported by the DLL to load the correct data segment:

   if ((wDS = LoadInstanceData()) == 0) // Get DS for this instance
      return;                           // If DS==0 then out of memory

   _asm {
        mov     ax,wDS
        mov     DS,ax
        }

AddTask(): Allocate data for the task, initialize the data, and add the

           task to the task list.

            Used to demonstrate that the local heap will grow.

                    initialize the task list.

           size of the data segment and the heap.

                    for the task that called the DLL.

              the segment address (selector) for the data for the task.
              If the task is not in the list, this function returns zero.

WEP(): Required by a DLL.