wide function pointer type

[Martin Uecker]

Am Sonntag, den 10.10.2021, 11:17 -0700 schrieb Daniel Colascione:
> On October 10, 2021 11:05:07 Martin Uecker <ma.uecker@gmail.com> wrote:
> 
> > Am Sonntag, den 10.10.2021, 10:49 -0700 schrieb Daniel Colascione:
> > > On October 10, 2021 10:44:32 Martin Uecker via Libffi-discuss
> > > <libffi-discuss@sourceware.org> wrote:
> > > 
> > > > Am Sonntag, den 10.10.2021, 10:01 -0700 schrieb Kaz Kylheku (libffi):
> > > > > On 2021-10-10 04:32, Martin Uecker via Libffi-discuss wrote:
> > > > > > Hi all,
> > > > > > 
> > > > > > I will propose a wide function pointer type (actually
> > > > > > a wide function type) to WG14 for C23 as a common
> > > > > > type for callbacks, closures, which now require an
> > > > > > additional void pointer argument in C APIs. This
> > > > > > is intended to be compatible with ABIs with now
> > > > > > use a static chain register.
> > > > > 
> > > > > Opposed. There is nothing wrong with separate arguments
> > > > > for function pointer and context.
> > > > 
> > > > Noted.  Your argument sbelow all boil down to the point
> > > > that there are cases where it might not be the ideal
> > > > choice. But nobody forces anyone to use it.
> > > "I like it" is not by itself a rationale for including something in a core
> > > language standard. This is not something that should be in C. Why,
> > > precisely, can't you just define a struct with two fields and make that
> > > your closure type?
> > 
> > Sure. There are at least five reasons:
> > 
> > - one now does has to do it manually which is
> > inconvenient because it has to be done for each type,
> > although this a common pattern which is trivial
> > to automate.
> > 
> > - APIs are often unsafe because they have to use
> > void pointer to be generic. When the void pointer
> > can be packaged into the wide pointer this problem
> > goes away.
> > 
> > - APIs are often different for no good reason.
> > Having a standard approach would make this more
> > canonical, hence simpler and less error prone,
> > and also more compatible - requiring less adapter
> > code.
> > 
> > - There are APIs where a pointer to a
> > function is expected but there is no data pointer.
> > Then other languages have a problem interfacing
> > with such APIs.   (or why does libffi generate
> > trampolines?)
> > 
> > - Finally, C might get lambda expressions
> > which are far more useful with such a pointer type.
> > (for nested function you could avoid executable
> > stack).
> > 
> > 
> > All five are good reasons to put a standard
> > approach in the core language in my opinion.
> > 
> > 
> > But I am actually not interested in this discussion
> > and do not have time for it.
> > 
> > I am more interested in constructive technical
> > feedback.
> 
> Your proposal doesn't actually solve any of these problems though, and as 
> we agree, users can already trivially make their own closure types.

I wonder why you think it does not address
these problems?

> What *would* be interesting is a new standard library facility for making 
> libffi-style closures and a standard struct type for packaging the things.

> *That's* something that could be justified for inclusion in a standard 
> library, since it's not something that programs can make on their own 
> without the kind of platform specific glue that libffi provides. 

I also would love to have this, but see below...

> Basically, 
> the interface would look something like this:
> 
> typedef opaque_mumble stdclosure_t;
> 
> // Returns a callable function pointer or NULL on error with errno set
> void* stdclosure_init(stdclosure_t* closure, void* fnptr, void* data);

A void* might not be big enough for a callable function
pointer an all architectures supported by C so this
needs to return something like

void (*)();


(for POSIX this is not a problem)

> void stsclosure_destroy(stdclosure_t* closure);
> 
> The function pointer returned by stdclosure_init would, when called, call 
> FNPTR with an extra initial parameter (or trailing parameter or something) 
> of type void* and value DATA and all the remaining parameters with which it 
> was called.
> 
> This facility would actually probably have to be more complicated in real 
> life due to varying platform ABIs, but you get the idea. This is something 
> that could belong in the standard because it'd provide a generic 
> abstraction for something every platform can do but that every platform has 
> to do in a slightly different way.

One problem could be that this requires generating
trampolines at run-time (and it is not clear we
can do this everywhere where C is supported) or
having a fixed set of preallocated trampolines.

The other limitation (which the new type would
avoid) is the need for dynamic resource allocation,
which is sometimes not desirable.

But I would love to see such a proposal
brought forward.

Martin