Re: Map C bit-field to Gambas datatype

[Benoît Minisini <benoit.minisini@xxxxxxxxxxxxxxxx>]

Le 14/03/2024 à 21:45, T Lee Davidson a écrit :
> On 3/14/24 16:12, Jussi Lahtinen wrote:
> > I suppose for usage in external functions there would be nothing 
> > special, just create structure with matching variables. But if you 
> > want the memory behaviour in Gambas, then you must implement it by 
> > yourself.
> >
> > Jussi
>
> Thank you for the response, Jussi.
>
> So then, for this structure 
> [https://mongoose.ws/documentation/#struct-mg_connection] (which would 
> be received by a Gambas Sub as an event handler/callback in this example 
> [https://mongoose.ws/documentation/tutorials/http-server/#a-minimal-static-server], I could simply declare the unsigned integers as Integers or even Bytes?
>
> Or, should I declare the connection parameter not as a structure but as 
> a pointer and just pass it along as is, and IF I need to access it's 
> fields use pointer arithmetic based on the SizeOf the datatypes?

A bit field is one or more bits in memory. It's the C compiler that 
decides which bit of which byte of the structure is associated with the 
field name.

So you must count the number of bits of -successive- bit fields and 
divide by 8 to know how many bytes it will take.

BUT... The C compiler can freely decides to reorder the bit fields to 
minimize the number of possible unused bits in the structure.

There is a compiler directive ("packed" in gcc) that tells it to put the 
fields in memory exactly in the order they are declared in the structure.

Unless this directive is used (which is usually the case), and as what 
the compiler actually does is, afaik, not declared anywhere in the 
executable, the Gambas interpreter cannot know at runtime where a bit 
field is located in memory.

You will tell me that it is the same thing for normal fields (bytes, 
integers...). But there is a library named "libffi" (ffi means "foreign 
function interface") that provide a way to create C structure 
declaration and access them in a standard way. Alas, this library does 
not support bit fields.

So you have to check at runtime how the structure is organized in 
memory, and then access the bit fields with the Gambas bit manipulation 
function (BTst, BSet...). You must aware that this structure 
organisation can change if you change the library version, the compiler, 
the system...

Regards,

--
Benoît Minisini.