Load C library in Julia

It is very convenient to load C or C++ library in julia. It is no more than just run a ccall function. There is no need to write any additional C code to wrap it up as long as the C library is sharable. Here we use the Yeppp package as an illustration.

First download the C++ library Yeppp! and make sure that libyeppp.so (or .dylib or .dll) file is available on the system library search path or in the current directory.

Then the following julia function will call adding function in Yeppp!.

function add!(res::Array{Float64}, x::Array{Float64}, y::Array{Float64})
    assert(length(x) == length(y))
    n = length(x)
    const status = ccall( (:yepCore_Add_V64fV64f_V64f, libyeppp), Cint, (Ptr{Float64}, Ptr{Float64}, Ptr{Float64}, Culong), x, y, res, n)
    status != 0 && error("yepCore_Add_V64fV64f_V64f: error: ", status)
    res
end

where :yepCore_Add_V64fV64f_V64f is the function name and libyeppp is the library name.

Since a similar definition is applicable for many other functions in Yeppp!, I wrote a macro to map many more into julia.

macro yepppfunsAA_A(fname, libname, BT)
    errorname = libname * ": error: "
    quote 
        global $(fname)
        function $(fname)(res::Array{$(BT)}, x::Array{$(BT)}, y::Array{$(BT)})
            n = length(x)
            assert(n == length(y) == length(res))
            
            const status = ccall( ($(libname), libyeppp), Cint, (Ptr{$(BT)}, Ptr{$(BT)}, Ptr{$(BT)}, Culong), x, y, res, n)
            status != 0 && error($(errorname), status)
            res
        end
    end 
end 
@yepppfunsAA_A add! "yepCore_Add_V64fV64f_V64f" Float64
@yepppfunsAA_A multiply! "yepCore_Multiply_V64fV64f_V64f" Float64

This will automatically define add!, multiply! and more julia functions.