Cfunctions with local types on v0.7

ffi

#1

I am trying to get cfunction to work with local functions and types

I think I am having some GC segfaults because of this. @rdeits said he was playing around with this and functions can be interpolated into the macro:

julia> function local_cfunction(f)
         @cfunction($f, Float64, (Int,))
       end
local_cfunction (generic function with 1 method)

julia> local_cfunction(x -> x + 1.0)
Base.CFunction(Ptr{Nothing} @0x00007f41f46d9f80, getfield(Main, Symbol("##3#4"))(), Ptr{Nothing} @0x0000000000000000, Ptr{Nothing} @0x0000000000000000)

but the types cannot, and so those need to be handled via an @generated function:

julia> @generated function local_cfunction(f, ret, arg)
         quote
           @cfunction($(Expr(:$, :f)), $ret, ($arg,))
         end
       end
local_cfunction (generic function with 2 methods)

julia> local_cfunction(x -> x + 1.0, Float64, Int)
Base.CFunction(Ptr{Nothing} @0x00007f41df522f40, getfield(Main, Symbol("##6#7"))(), Ptr{Nothing} @0x0000000000000000, Ptr{Nothing} @0x0000000000000000)

It seems like that is a quite complicated solution though. Is there something simpler or is this @generated helper function required everywhere?


#2

@tkoolen has also been taking a crack at this, x-ref: https://github.com/yuyichao/FunctionWrappers.jl/pull/7


#3

I don’t quite understand the need for runtime local types. The only point of a @cfunction is to use it as a callback with an external C library, in which case the set of allowed argument types should be known statically.


#4

They are known statically. These types can be inferred by the compiler since everything is type-stable, and so the arguments are defined as things like typeof(t) or Ref{typeof(userfun)}) since it’s a generic implementation. I don’t know what’s different between v0.6 cfunction and v0.7 @cfunction which makes this incompatible, and this incompatibility doesn’t seem to be mentioned in the NEWS or depwarn.

I can work around it in this specific case though.


#5

Yes, but knowing statically does not mean knowing locally. The usecase is writing helper functions that requires cfunction and the issue is that not allowing local type breaks the transparency wrapper functions.


#6

Notice that the generated function should be

@generated function local_cfunction(f, ret, arg)
   quote
     @cfunction($(Expr(:$, :f)), $ret, ($arg,)).ptr
   end
 end

i.e. it should return the .ptr to match v0.6


#7

I tried a bunch of different combinations:

# Good but depwarns
CVodeInit(mem, cfunction(cvodefun, Cint, (realtype, N_Vector, N_Vector, Ref{typeof(userfun)})), t[1], convert(N_Vector, y0nv))

# Segfaults
CVodeInit(mem, local_cfunction(cvodefun, Cint, (realtype, N_Vector, N_Vector, Ref{typeof(userfun)})), t[1], convert(N_Vector, y0nv))

# Doesn't compile because of local type
CVodeInit(mem, @cfunction(cvodefun, Cint, (realtype, N_Vector, N_Vector, Ref{typeof(userfun)})), t[1], convert(N_Vector, y0nv))

# Errors due to local type
funtype = Ref{typeof(userfun)}
CVodeInit(mem, @cfunction(cvodefun, Cint, (realtype, N_Vector, N_Vector, funtype)), t[1], convert(N_Vector, y0nv))

# Depwarn
funtype = Ref{typeof(userfun)}
CVodeInit(mem, cfunction(cvodefun, Cint, (realtype, N_Vector, N_Vector, funtype)), t[1], convert(N_Vector, y0nv))

So what exactly is the deprecation turning the cfunction call into? It’s not the same as the macro since the depwarn route works while the macro fails to compile, while the local_cfunction route causes segfaults. If I can just match what the deprecation route is doing then I’d be set.


#8

Following the depwarn logic, it seems that the way to handle this is:

 @noinline function old_cfunction(f, r, a)
   ccall(:jl_function_ptr, Ptr{Cvoid}, (Any, Any, Any), f, r, a)
 end
CVodeInit(mem, old_cfunction(cvodefun, Cint, Tuple{realtype, N_Vector, N_Vector, funtype}), t[1], convert(N_Vector, y0nv))

The Tuple{...} is from https://github.com/JuliaLang/julia/pull/23066 which shows that the previous way to doing the types is a performance trap. But done like this the old_cfunction should be fine and doesn’t throw any depwarns, so that’s at least a viable solution. This really feels like a big regression from v0.6 though.


#9

I don’t think that’s right. The CFunction type is meant to keep all the necessary things from being garbage collected, so by just returning the ptr member and letting everything else go out of scope you’re circumventing that mechanism and running into segfaults. The CFunction docs (on master) state that

it should be passed to ccall as a Ptr{Cvoid}, and will be converted automatically at
the call site to the appropriate type.

I wonder if this could be made safer by overloading Base.getproperty for CFunction to throw an error for naive field accesses (and then provide e.g. .ptr_unsafe as a way for internal use).


#10

This should be written as (x::T) where {T} -> cfunction(f, Ref{T}, (Ref{T},)). This ensures that lowering can see that everything is type-stable. (Basically just the same rules as ccall)


#11

Note that if you’re C library provide the ability to thread though a void* environment pointer (as all good libraries should, and as it looks like this one does), you should typically use that instead of allocating a new cfunction pointer object. The qsort example in the manual goes into more detail about how to do this.