Windows DLLs with Harbour code

There are a lot of infomation that can be gleaned if one chose to go through the \doc folder of any official harbour installation. In this post, I have chosen to re-publish the following:-


Windows DLLs with Harbour code

Programs created with Clipper or Harbour are traditionally a
monolithic EXE 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 .lib 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 .lib:
        implib harbour.lib harbour.dll
     For the Borland C platform, use that library and import32.lib
     and cw32.lib from Borland, and you are ready to go.


  2) PCode EXEs using a Harbour.dll

     A Harbour.dll is designed to be called from a Harbour app.
     A pcode EXE 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_DllDo( [,] ) --> []

     This lets you have all your common code in a DLL, and have lots
     of small EXEs that use it.  Realize however that, even though this
     may be a nice way to manage your code, each EXE 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
     EXEs over monolithic EXEs. But it may be a worthwhile maintenance
     benefit to have lots of replaceable small exes.

  3) PCode DLLs used from traditional EXEs
     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 EXE that calls it.  This has the benefit of having
     replaceable modules in DLLs that don't necessarily require updating
     your EXE.


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
   * maindllh.c    Windows Harbour DLL entry point  (harbour.dll)
   * maindllp.c    Windows pcode DLL entry point and VM/RTL routing functions

   >    * 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...)

   >    * maindllh.c    Windows Harbour DLL entry point  (harbour.dll)
   To produce Harbour.dll, to be just used from small pcode Harbour EXEs

   >    * maindllp.c    Windows pcode DLL entry point and VM/RTL routing
   To produce small pcode DLLs, to be used just from Harbour EXE 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 EXEs, 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