packages

FPC/Lazarus unfortunately are a bit ambiguous with respect to the term packages. Some meanings:

1. A set of units in FPC that are treated together. Both using the older fpcmake as the newer fppkg packagemanager.
2. A set of units in Lazarus (with designtime parts that require registration into the DLL). See Lazarus Packages
3. Delphi packages, which are like DLLs.

This article is meant as a brainstorm session about the latter, one of the missing pieces of the Delphi compability.

A Delphi package is essentially a DLL with some extra's.

• One of those extra's are two procedures, a pascal level initializer ("Initialize") and a module finalizer ("Finalize"). I'm not sure if they are library procedures that traverse compiler generated tables, or if they are wholly compiled generated.
• If your system uses packages,
  • the RTL and other units the packages use, must also be in a package.
  • A unit can exist only once in one package (see Swart reference below), and probably the EXE too . On load of a package this is checked, but possibly only when packages are dynamically loaded by the program. This combined with the previous condition avoids problems like multiple definitions of the VMTs. Note also that this means a list of units per package must be accessable to the main program on load (?)
  • Since they are DLLs, due to the Windows DLL symbol resolving, packages can probably survive some minor patching (allowing implementations to be fixed), but in general it must be pretty much the same packages as the program was compiled with.
• Packages can use higher level functions (like ansistrings and classes) over packages borders. This means they use the same memory manager. It is not know if this automatically is changed to COM compatible memmanager sharemem (e.g. in the rtl BPL creation) or if this is the normal suballocator.
• Like DLLs, packages can be statically linked (so that its presence is required for startup), and programs can load additional packages using loadpackage (unloadpackage?). Since a package can only depends on existing packages, and not on the exe (RTL also in package remember!), this means that dynloaded packages can be crafted AFTER .exe generation, allowing for plugin systems.
• packages are loaded by a procedure called "safeloadlibrary" which is a loadlibrary wrapper that saves the FPU statusword and disables some windows errorwindows on failure to load. (?!?)
• relocation, for this a little thought experiment using the following realistic Delphi scenario: Assume packages A and B both import packages RTL, C, D, but are compiled separately and used in one .EXE program. This means that the compiler can't guarantee all BPL's are already on a unique baseaddress and addresspace (since A and B could be compiled without the other present). -> they are relocatable, though probably DLL loading will resolve this transparantly mostly.

References

• Swart on pkgs