DLL: Microsoft Dynamically Linked Libraries

• DLL -- Dynamically Linked Library
• Used to store object code and resources in dynamically loaded files under Win32.
• A DLL exports a flat API, a one-dimensional list of function calls and data. You can refer to them by index or name. This isn't high level enough for component software, a problem that higher level frameworks like COM and OLE address. OLE defines Type Libraries that provide metadata describing the contents of the DLL, and functions to automatically register objects and services in the system registry. There are two interface definition languages that describe the contents of DLL's to COM and OLE: MIDL and ODL.
• Self Registering DLLs have a string resource that declares that they are self-registering, which means they have a couple of functions to call that register and unregister their classes in the windows registry: DLLRegisterServer and DLLUnregisterServer.
• OLE allows object servers to be completely implemented inside of a DLL. This type of server is called an "in-proc server". MFC does not completely support in-proc servers for all the features of visual editing, mainly because OLE does not provide a way for a server to hook into the container's main message loop. MFC requires access to the container application's message loop to handle accelerator keys and idle-time processing. If you are writing an OLE Automation server and your server has no user interface, you can make your server an in-proc server and put it completely into a DLL.
• Multithreading Issues
  • A DLL can keep track of thread specific data using TlsAlloc and TlsGetValue (part of the Win32 thread local storage API).
  • DLLs notified of threads attaching and detaching in their DLLMain function, so they can keep track of all threads in the process and allocate local storage as necessary.
  • Windows uses the "apartment model" of multithreading, where only the thread that created an object is allowed to send messages to it. All other messages from other threads are serialized via the Windows message queue, that the thread must service. This makes it possible to "kind of" multithread code that is not thread safe, since only one thread will ever be executing the code of a particular object. You can explicitly program around this threading model if it doesn't meet your needs, but it is the default behavior.
• Extension DLL
  • Extend the Microsoft Foundation Classes.
  • If you need to pass an MFC or MFC-derived object pointer to or from an MFC DLL, the DLL should be an Extension DLL. The member functions associated with the passed object exist in the module where the object was created. Since these functions are properly exported when using the Shared DLL version of MFC, you can freely pass MFC or MFC-derived object pointers between an application and the Extension DLLs it loads.
  • An extension DLL creates a CDynLinkLibrary object to export CRuntimeClass objects or resources to the application.