# Transfer Float64 matrix to double \*\*arg in C

**URL:** <https://discourse.julialang.org/t/transfer-float64-matrix-to-double-arg-in-c/1463>\
**Category:** General Usage\
**Created:** [January 13, 2017, 2:53pm UTC](https://discourse.julialang.org/t/transfer-float64-matrix-to-double-arg-in-c/1463 "2017-01-13T14:53:50Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![robertfeldt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/robertfeldt/32/239_2.png) [@robertfeldt](https://discourse.julialang.org/u/robertfeldt)\
**Post date:** [January 13, 2017, 2:53pm UTC](https://discourse.julialang.org/t/transfer-float64-matrix-to-double-arg-in-c/1463/1 "2017-01-13T14:53:51Z")

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I’ve been trying to transfer a Julia matrix of Float64 to a C function that has a “double \*\*arg” but it does not work as intended so help would be appreciated.

C function I want to call (simplified body just to get idea across):

```
double emd2(int n_x, double *weight_x, int n_y, double *weight_y, double **cost) {
    double sum = 0.0;
    int i, j;

    for(i = 0; i < n_x; i++) {
        sum = sum + weight_x[i];
    }
    for(i = 0; i < n_y; i++) {
        sum = sum + weight_y[i];
    }
    for(j = 0; j < n_y; j++) {
        for(i = 0; i < n_x; i++) {
            sum = sum + cost[j][i];
        }
    }
    return sum;
}

```

then I compile this into a lib called libemd and define a low-level function to call it like so:

```
libemd_emd2(n_x, weight_x, n_y, weight_y, cost) =
    ccall((:emd2,libemd), Cdouble,
          (Cint, Ptr{Cdouble}, Cint, Ptr{Cdouble}, Ptr{Ptr{Cdouble}}),
          n_x, pointer(weight_x), n_y, pointer(weight_y), pointer(cost))

```

but when I then try to use it:

```
w1 = [0.4, 0.2, 0.2, 0.1, 0.1]
w2 = [0.6, 0.2, 0.2]
cost = ones(Float64, 5, 3)
cost_ref = [Ref(cost,i) for i=1:size(cost,1):length(cost)]
libemd_emd2(length(w1), w1, length(w2), w2, cost_ref)

```

The result is some large number rather than the sums of the values of the matrix and the two arrays. Seems it is reading some random place in memory so I guess I set up the pointers the wrong way or something. I based my solution on this stackoverflow entry: [Calling a C function from Julia and passing a 2D array as a pointer of pointers as argument - Stack Overflow](http://stackoverflow.com/questions/33003174/calling-a-c-function-from-julia-and-passing-a-2d-array-as-a-pointer-of-pointers)

What am I missing here? Thanks for any advice.

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [January 13, 2017, 2:59pm UTC](https://discourse.julialang.org/t/transfer-float64-matrix-to-double-arg-in-c/1463/2 "2017-01-13T14:59:41Z")

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This doesn’t really answer your question, but if the C code is something you can edit I would rewrite it to use `double *cost` as an `n_x` by `n_y` column-major array. Just change `double **cost` to `double *cost` and change `cost[j][i]` to `cost[j*n_x + i]`. Not only will this be faster and easier to call from Julia, but it will probably also be faster in C as well because it avoids the double pointer indirection.

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<div class="post-metadata">

**Author:** ![robertfeldt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/robertfeldt/32/239_2.png) [@robertfeldt](https://discourse.julialang.org/u/robertfeldt)\
**Post date:** [January 13, 2017, 3:04pm UTC](https://discourse.julialang.org/t/transfer-float64-matrix-to-double-arg-in-c/1463/3 "2017-01-13T15:04:54Z")

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Thanks for the quick reply Steven. I do have access to the C file and it is under an open-source license so I guess I can write a small script to automatically do this conversion, yes. It would simplify and speed up since I will also not need to transpose the matrix (which might be a big one). Only downside is if the original author evolves the code and my script might not be “matched up”… But I’ll see if this is a viable way forward. Thanks!

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**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [January 13, 2017, 3:41pm UTC](https://discourse.julialang.org/t/transfer-float64-matrix-to-double-arg-in-c/1463/4 "2017-01-13T15:41:17Z")

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Note that if you can change the C code, you should change it to a multi-dimensional C array and you don’t need to compute the index yourself.

See either solution at [Redirecting to Google Groups](https://groups.google.com/forum/#!topic/julia-users/EK9oNzzaoAk) or [Calling a c-function that returns a 2-dimensional array - #8 by yuyichao](https://discourse.julialang.org/t/calling-a-c-function-that-returns-a-2-dimensional-array/855/8)

Be careful about column/row ordering…

Somehow this question comes up much more often than I though…

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<div class="post-metadata">

**Author:** ![robertfeldt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/robertfeldt/32/239_2.png) [@robertfeldt](https://discourse.julialang.org/u/robertfeldt)\
**Post date:** [January 13, 2017, 4:06pm UTC](https://discourse.julialang.org/t/transfer-float64-matrix-to-double-arg-in-c/1463/5 "2017-01-13T16:06:12Z")

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Thanks to both of you. I went with Steven’s proposal since I find it easier to get the column/row ordering right this way. It works now, thanks. If this is a common question maybe some comments about multi-dimensional arrays should be added to the Julia docs for interfacing with C?