Send array from Julia to Rust (using "ccall")

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Can someone explain? How to send any array from julia to rust using "ccall" function. There is no problem with sending usial variables or constants.

I have Julia code:

A = Array{Float64,1}(undef, 2)
print("A is ",A, "\n")
ccall((:recvstruct, "target/debug/liblib"),Float64, (Ref{Array{Float64,1}}, Int32,), A, sizeof(A))

And Rustcode

#[no_mangle]
pub extern fn recvstruct(vec: &mut Vec<f64> , len: usize){
    println!("reiceved?: {:?}", len);
    println!("reiceved?: {:?}", vec);
}

Output is:

A is [0.0, 0.0]
Hello from cargo

reiceved?: 16

and weird endless array:

, 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000069329110091509, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291559949425, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291100915405, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291559949425, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291100915405, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291559949425, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291100915563, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291559949425, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291100915563, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291559949425, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291100915563, 0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000693291559949425, 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000069329110091572, 0.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Julia installed: Version 1.5.2 Rust (cargo): cargo 1.49.0 (d00d64df9 2020-12-05)

send rust julia arrays
1 Answers

Julia side

In (argtype1, ...) of ccall declare the type of first argument of recvstruct as a pointer to a memory address with Ptr{T}.

direclty from ccall docs:

Each argvalue to the ccall will be converted to the corresponding argtype, by automatic insertion of calls to unsafe_convert(argtype, cconvert(argtype, argvalue))

Pass the number of array elements and not the size in bytes of the array:

a = [2.0, 3.0]
result = ccall((:recvstruct, "target/debug/liblib"), Float64, (Ptr{Float64}, UInt,), a, length(a))
println("got: $result")

Rust side

Declare the first argument of recvstruct as a memory pointer to an array of float64.

Moreover use slice::from_raw_parts for getting the slice of values.

For example:

#[no_mangle]
pub extern "C" fn recvstruct(array_ptr: *const f64, len: usize) -> f64 {
    _recvstruct(unsafe {
        std::slice::from_raw_parts(array_ptr as *const f64, len)
    }, len)
}

fn _recvstruct(vec: &[f64] , len: usize) -> f64 {
    match len {
        2 => vec[0] * vec[1],
        _ => 0.0
    }
}

Note that from_raw_parts is marked as unsafe. The Safety section of the manual page contains all the details to be considered.

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