question about returning by reference + `PackedArray`

[samuelpmish]

Thank you for developing wll-interface, it's been incredibly useful in my personal projects. My question is:

Is there a way to return a wll::tensor by-reference if the dimensions are not known a priori?

Right now, I'm splitting up any C++ routines that have multiple returns into:

• one function that calculates the outputs and writes them to some global variable
• "getters" that access that global and return each output individually

Returning multiple things by-reference would be cleaner and thread-safe, but when I tried it, my C++ functions that resized the by-reference output wll::tensors caused the kernel to crash.

[njpipeorgan]

I think it might be possible if the arguments passed though librarylink is not a proxy. I need to check that.

By the way, how did you "resize the by-reference output wll::tensor's"? I would be best if you can show some actual code.

[samuelpmish]

Passing by-reference works when I don't interfere with the tensor dimensions in C++

// bindings.cpp
#include "wll_interface.h"

void set_to_two(wll::tensor<double, 2> & foo) {
  for (std::size_t i = 0; i < foo.dimension(0); i++) {
    for (std::size_t j = 0; j < foo.dimension(1); j++) {
      foo(i,j) = 2;
    }
  }
}
DEFINE_WLL_FUNCTION(set_to_two);

setToTwo = LibraryFunctionLoad[mylib, "wll_set_to_two", {{Real, 2, "Shared"}}, "Void"];
------------------------------------------------------------------------------------------
arr = ToPackedArray[Table[i + j + 0.5, {i, 1, 2}, {j, 1, 2}]];
> {{2.5, 3.5}, {3.5, 4.5}}
------------------------------------------------------------------------------------------
PackedArrayQ[arr]
> True
------------------------------------------------------------------------------------------
setToTwo[arr]
arr
> {{2., 2.}, {2., 2.}}
(* works! *)

---

But when I do something like (e.g. if I can't figure out the tensor dimensions until running the C++ function)

// bindings.cpp
#include "wll_interface.h"

void set_to_two(wll::tensor<double, 2> & foo) {
  foo = wll::tensor<double, 2>({1, 3}); // <--------
  for (std::size_t i = 0; i < foo.dimension(0); i++) {
    for (std::size_t j = 0; j < foo.dimension(1); j++) {
      foo(i,j) = 2;
    }
  }
}
DEFINE_WLL_FUNCTION(set_to_two);

Then, on the mathematica side of things I get this on my linux machine

------------------------------------------------------------------------------------------
setToTwo[arr]
arr

(...) LibraryFunction::dimerr: An error caused by inconsistent dimensions or exceeding array bounds was encountered evaluating the function wll_set_to_two.

LibraryFunctionError["LIBRARY_DIMENSION_ERROR", 3]

> {{2.5, 3.5}, {3.5, 4.5}}

and just a kernel crash on my mac when invoking setToTwo[arr].

[njpipeorgan]

The error occured on this line

foo = wll::tensor<double, 2>({1, 3});

foo is a tensor with shape 2x2 and its memory shared between mathematica and c++, and you are try to assign to it another temporary tensor with shape 1x3 and its memory managed by c++. In this case, wll-interface will try to copy the data from one tensor to the other, but since they have different dimensions, it will throw LIBRARY_DIMENSION_ERROR.

You can actually get the error message after getting this error with

exception = LibraryFunctionLoad[(*libpath*), "wll_exception_msg", {}, "UTF8String"];
exception[]

I guess in terms of designing operator=, to copy the data for this assignment is the safest behavior. There could be a function like tensor1.clone_from(tensor2) that copy/move both the data and the attributes from tensor2.

[samuelpmish]

Does this mean the shape of PackedArrays passed into C++ functions is immutable, or that the wll::tensor container just isn't set up to support resizing?