Re: wide function pointer type

[Martin Uecker <ma.uecker@gmail.com>]

Am Mittwoch, den 20.10.2021, 11:10 +0200 schrieb Florian Weimer:
> * Martin Uecker:
> 
> > One (of several) use cases is for language interoperability.
> > 
> > A common problem is to pass a function of a high-level
> > language as a callback to an C API.  This now often
> > requires special boiler plate code for each case and
> > there is no automatic way to do this. The fundamental
> > problem is that the C type can not express that a data
> > pointer belongs to a function pointer.
> > 
> > void foo(
> >   void (cb1)(void* data, int a), void* data1,
> >   void* other_data);  
> > 
> > Here a human (and maybe also a machine) could guess
> > that data1 belongs to cb1 but not other_data.  But
> > it is not clear and in more complicated cases
> > even less so.
> > 
> > void foo(void (_Wide cb1)(int a), void* other_data);
> > 
> > 
> > With the new type that would be unambiguous (and 
> > wrappers could then often be created automatically).
> > At least for the C API one would also expect ABI
> > stability.
> 
> I don't expect a lot of uptake for this.  One problem is the suggested
> aggregate representation of wide function pointers.  It's not exactly
> common for non-C implementations to implement those aspects of the
> platform C ABI.  

Can you explain what you mean by this?  Usually non-C
implementations build on C ABI, so if the C ABI is
extended the extension is then available to them.

I would expect a lot of update (in the long run) because
there is now a 1:1 mapping for a function pointer
type in other languages to the function type in C.

> For many targets, even C and C++ compilers do not
> always agree completely on the finer points. 

Unfortunately, e.g. atomics. But the goal and intention
is interoperability. 

> Passing code and data pointer in separate arguments is
> much more robust, and safely within the
> interoperable ABI subset.

It is not my experience that the void pointer based
APIs are robust and safe.

> Perhaps a set of standard attributes to describe the explicit closure
> argument would be useful, though.

I also thought about this (and this could be added
too) but this does not solve all problems.

Martin