Julia Error: @ccallable: argument types must be concrete

Question

I am new to Julia and trying to compile a function. My code is something like this and I get an error saying "@ccallable: argument types must be concrete" when I run the code. I also found this is because of the initial value I set to the function. Could anyone help me to solve this problem?

# this doesn't work
Base.@ccallable function test_function(a::Vector{Float64}, b::Float64, c=0.0::Float64)::Vector{Int64}
    c = b + c
    return a
end

# this works
Base.@ccallable function test_function(a::Vector{Float64}, b::Float64, c::Float64)::Vector{Int64}
    c = b + c
    return a
end

Answer 1

The function needs to be C-compatible and C does not have optional arguments.

Also, you probably don't want to take Julia Arrays from C, you want to take pointers, see the example at https://github.com/simonbyrne/libcg/blob/25e859ef587f3d00a0a8b6f304a7494f222874d3/CG/src/CG.jl#L27-L38.

Answer 2

For the error itself, the problem is c=0.0::Float64 doesn't annotate the type of c, it asserts that the type of the default value 0.0 is Float64. You can actually check methods(test_function) to confirm this. To annotate c, write this: c::Float64=0, though that's not actually C-compatible.

Bigger picture, I'm confused about what you mean by "compile a function." The code looks like you're using PackageCompiler and making a Julia function callable from C. If you only want to compile a Julia function instead, Julia just does it upon every first call with a unique set of concrete argument types.