harbour-core/doc/windll.txt

# Windows DLLs with Harbour code

Programs created with Clipper or Harbour are traditionally a
monolithic executable containing all executable code. This includes
the Virtual Machine (VM) and the Runtime Library (RTL) as well as
your own code. Running under Windows with Harbour, you
can now also create and use Windows DLLs that contain PRG code.

Harbour supports Windows DLLs in 3 ways.

  1. Self-contained DLLs containing functions from any platform.
     (These are not what we call a "harbour.dll", although they may
     be named that. The DLL entry points are different.)
     These have the VM/RTL inside them and can be used by any other
     Windows program. You can create a library for static linking,
     or use GetProcAddress() as in any standard Windows DLL.
     Calling Harbour/.prg functions directly is limited to
     those that take no parameters unless you include C functions
     in the DLL that take parameters and then call the PRG-level
     code.

     To do static linking, do this to create the library:
        hbmk2 -hbimplib harbour.dll

  2. PCode executables using a harbour.dll

     A harbour.dll is designed to be called from a Harbour app.
     A pcode executable is a small Harbour executable that does not
     contain the VM/RTL. To execute its functions, it must load and access
     a harbour.dll.
     If you want dynamic linking, then use this to execute a Harbour
     dynamically loaded pcode DLL function or procedure:
        hb_LibDo( <cFuncName>[, <params...>] ) --> [<uResult>]

     This lets you have all your common code in a DLL, and have lots
     of small executables that use it. Realize however that, even though
     this may be a nice way to manage your code, each executable may
     load its own image of the harbour.dll into memory at runtime.
     In terms of Windows memory, there may not be a benefit to using pcode
     executables over monolithic executables. But it may be a worthwhile
     maintenance benefit to have lots of replaceable small exes.

  3. PCode DLLs used from traditional executables
     A pcode DLL does not contain the VM/RTL.
     It is a library of Harbour-compiled PRG code that uses the VM/RTL
     of the executable that calls it. This has the benefit of having
     replaceable modules in DLLs that don't necessarily require updating
     your executable.


The following is clipped from a msg by Antonio Linares to the Harbour
developer list explaining some of the details:

Please notice that there are three different Windows DLL entry points:

   * src/vm/maindll.c     Windows self-contained DLL entry point

     To produce Harbour code, as DLLs, that may be used from other
     programming languages applications (as VB, Delphi, C++, etc...)

   * src/vm/maindllh.c    Windows Harbour DLL entry point (harbour.dll)

     To produce harbour.dll, to be just used from small pcode Harbour
     executables.

   * src/vm/maindllp.c    Windows pcode DLL entry point and VM/RTL routing functions

     To produce small pcode DLLs, to be used just from Harbour executable
     apps. maindllp.c is the entry point for the Harbour pcode DLLs. pcode
     DLLs are quite small DLLs, that just contain pcode and/or C (using
     Extend API) functions.

mainwin.c is the entry point for Windows executables, not for DLLs.

You may use maindll.c, maindllh.c or maindllp.c based on your needs.

If you are looking to build a harbour.dll, then you must use maindllh.c.