[fpc-devel] Portability Standards

Michael Van Canneyt michael at freepascal.org
Sat Jan 1 18:54:03 CET 2005

On Sat, 1 Jan 2005, DrDiettrich wrote:

> In the meantime I downloaded the Abbrevia package from SourceForge, and
> came across several unpleasent constructs. Please let me introduce my
> preferred programming model for portable code.
> 1) Target Dependencies
> I don't accept any OS or machine specific conditional compilation in
> code, except in dedicated units. In Abbrevia the same fixes occur in
> various places in the code, instead of in a dedicated OS specific
> procedure. Such a procedure exists, but unfortunately only in the
> unpatched form :-(
> Also compiler specific conditionals should be used only where absolutely
> necessary (local options, syntax differences...).

Agreed 100%.
In general, a component suite should have all os-dependent code in a single
unit, presenting the rest of the suite with a uniform API.

> 2) Code Checks
> It should be possible to check code for compilation on other platforms.
> This requires that target specific code can be compiled on any host,
> somehow. Consequently it must be possible to override the compiler
> generated symbols that describe the environment.

Why, you can simply cross-compile ?

> 3) Standard Units
> Portable code shall use standard procedures and units for target
> specific actions. This requires the definition, implementation, and
> consequential use of one or more standard units for:
> - Machine specific data types and procedures (endianness...)

Compiler defines the endianness.
System unit has 'guaranteed' types.

> - OS specific data types and procedures

? This is not portable.

> - C and POSIX specific data types (for code ported from GNU/C)
> Then it should be clear which portable data types and procedures should
> be taken from which unit. It also should be easier to determine which
> procedures need special considerations in portable code, from the
> defined data types and procedures in these units.
> It's not necessary that everybody follows these principles, but I will
> do so in my own published portable code. Otherwise I can continue to
> write my code for Delphi, and let it to somebody else to port that code
> to e.g. FPC.
> In the simplest case these units will have an unconditional interface
> part, the target specific data type declarations and the implementation
> part can be included conditionally from the appropriate locations. For
> testing purposes fake units or code sections can contain the OS specific
> procedure declarations, with empty bodies. This way I could verify that
> my changes to the main stream code would not break the code for other
> environments, due to the use of not yet ported constants or data types
> etc.
> This model has one disadvantage, in so far as complex procedures with
> small embedded platform specific code could no more be checked, because
> it often was impossible to determine the impact of changes to local
> variables etc. in the conditionally excluded code. It also was and is
> impossible to automatically verify that the conditional sections really
> are implemented for every target!
> In another implementation of a target independent library I defined
> portable classes with standardized methods. These virtual methods were
> almost abstract in the base class, so that they had to be overridden in
> every derived class. In the initialization section of the unit the
> appropriate class was created and assigned to the according variable,
> based on the actual or desired target environment. Of course this
> approach has the disadvantage of increasing runtime (how much?), due to
> the virtual method calls. Nonetheless I think that in many cases the
> difference from the traditional method will not be noticeable, due to
> the (unknown, long?) runtime spent in the embedded OS calls.
> The best compromise might be a mix of both methods. The interface can be
> fully procedural, and objects with virtual methods are used only in the
> implementation part, and only when this approach makes sense. Perhaps
> somebody finds another way to achieve the same goal, with less
> disadvantages?

I don't think you should worry about virtuals. For archiving purposes,
it's the compression/decompression algorithms that will take most time,
and tha actual reading/writing from stream.

> I'm willing to demonstrate my ideas in a redesign and extension of
> Abbrevia, so that we have a concreter base for further discussions. But
> before starting with that work I would like to hear some encouraging <g>
> opinions or suggestions.

I think you can do this. I will be pleased to help where I can.
But send a proposal before you start, I wouldn't want to you
end up rewriting half your code after a discussion. ;-)


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