how to call functions taking a variable number of arguments

[zoggy]

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I have to bind a printf-like function, i.e. a function taking a format string and a list of additional parameters depending on the format string.

Is there a way to do this in ctypes ?

[dbuenzli]

Related to #103.

[yallop]

This is likely to be tricky for reasons that are not really specific to ctypes. What type would you like the function to have on the OCaml side?

[zoggy]

The same as the printf functions, using formats :-)

[yallop]

It's possible in theory, at least if the API you're binding exposes something analogous to vprintf. However, it's likely to be quite a bit of work. The implementation of the format type used in OCaml's printf is not entirely trivial:

https://github.com/ocaml/ocaml/blob/trunk/stdlib/camlinternalFormatBasics.ml

[zoggy]

Indeed, I thought about doing the same but this would be quite heavy.

[dbuenzli]

I don't know if that may work for your but in tsdl what I did is to use OCaml's pretty printing machinery and simply use the variadic function with a "%s" formatter, see API and implementation.

[zoggy]

Nice but in my case, I really need to pass arguments to the openmaple printf functions, because it handles maple objects in formats to turn them into strings :(

[artob]

Some mechanism for invoking (simple) variadic functions would indeed be great. As per #103, I, too, need to call ioctl(2) and there does not seem to be any way to do it portably from OCaml other than writing my own project-specific C stubs. (Thankfully, since everyone has had to do this for ioctl(2) for decades now, there is at least no shortage of example code out there...)

[dsheets]

Hi, @bendiken, I believe that if you only require static arities on the OCaml side, using Ctypes's cstubs support (0.4.0+ iirc) should generate code that works without ABI restriction. If you require the variadicity in OCaml, too, there is a type encoding problem which may need to be resolved.

Out of curiosity, what is the purpose of your ioctl(2) calls? I am interested in libraries for binding systems APIs to OCaml.

[artob]

@dsheets Thanks—fixed arity (e.g., defining ioctl0, ioctl1, etc, depending on the required number of arguments) would be just fine; I'll have a look at cstubs. ioctl(2) is needed for interfacing to many device drivers; in the immediate matter, V4L2, but generally speaking it's an essential facility for interfacing with hardware (and hypervisors, as it may be).

[artob]

@dsheets FYI, I decided it would take me longer to figure out Cstubs usage than to just write the pesky stubs, so in this instance I ended up adding basic ioctl(2) support to ExtUnix: https://github.com/ygrek/extunix/pull/11. Will revisit once Cstubs has more documentation.