[Lazarus] "Running" a library

Mark Morgan Lloyd markMLl.lazarus at telemetry.co.uk
Sat Sep 15 13:12:50 CEST 2012

Sven Barth wrote:
> On 15.09.2012 12:36, Mark Morgan Lloyd wrote:
>> Sven Barth wrote:
>>>> /Why/ can't a library be run standalone? We're already at the position
>>>> that an executable can decide whether it's been invoked from a shell or
>>>> the GUI and behave as appropriate, so why can't it decide whether it's
>>>> being run as a program or being initialised as a library?
>>> A library and an application have different entry point signatures.
>>> Take Windows for example. There the entry point for applications is
>>> "procedure EntryPoint; stdcall;" while for DLLs it is "procedure
>>> EntryPoint(aHinstance: PtrInt; aDLLReason: Word; aDLLParam: Pointer);
>>> stdcall;". Additionally the entry point of a DLL is called multiple
>>> times (once the process loads the library, every time a new thread is
>>> created and destroyed and once the process unloads the library) while
>>> the entry point of an application is only called once. Also AFAIK
>>> Windows does not let you run binaries that are flagged as "DLL".
>>> That's the reason why there is a program called "rundll32" ;) [though
>>> it expects an exported function with a certain signature...]
>> Assuming for a moment that a binary can be built that is basically an
>> executable but also exports library-style entry points, and which could
>> be loaded into memory using DynLib or whatever: could the initialisation
>> function be told to return fast and cleanly if it detected that it
>> wasn't being run as a program? In that case, the caller could use DynLib
>> to load it and then invoke a different entry point explicitly to handle
>> initialisation.
> You are able to define exports from a program. And you can also load the 
> program binary using LoadLibrary from a library that is already loaded 
> from that program (because the entry point is not called then),

I'm already calling from a loaded library back into the calling program 
using that technique. What happens if a library tries to load a 
/different/ program: is the main entry point called and are the 
subsidiary entry points available?

> but you 
> can not build an application that can be used also as a dynamic library, 
> because as I already wrote the entry point signatures are different and 
> thus if you would e.g. use the DLL's signature for the program your code 
> would expect parameters (aHinstance, aDLLReason and aDLLParam) where the 
> OS did not pass you anything ( => stack corruption).

Although the called code might be able to determine that the parameters 
aren't valid and react accordingly, and being stdcall (unless my 
understanding is outdated) it's the caller that restores the stack state.

Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]

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