Cgo: How to return double array from C to Go

Question

I got a C function like this

double* c_func(int n_rows) {
  double result[n_rows];
  for (int i = 0; i < n_rows; ++i) {
    result[i] = (double)i;
  }
  return result;
}

And I use this Go function to process the C double:

// convert C double pointer to float64 slice ...
func doubleToFloats(in *C.double, length int) []float64 {
    out := make([]float64, length, length)

    start := unsafe.Pointer(in)
    size := unsafe.Sizeof(C.double(0))
    for i := 0; i < length; i++ {
        val := *(*C.double)(unsafe.Pointer(uintptr(start) + size*uintptr(i)))
        out[i] = float64(val)
    }
    return out
}

This sometimes works but sometimes not. When it not works, it return something like this:

[0 1 2 3 4 5 6 7 8 9 10 2.53e-321 3.32018606e-316 4.24664374149e-312 4.24399158193e-312 1.1383e-320 3.31882387e-316 3.71924634e-316 3.31885594e-316 3.71924634e-316 5e-324 0 4.6950308e-316 4.24664374149e-312 3.7175681e-316 3.3200616e-316]

which looks like somewhat memory issue to me...

I'm not sure if this is the correct way to handle returned double array from C in Go. If yes, how to fix the problem (which occurs ocassionally). If no, what is the correct way to handle returned double array from C?

Thanks.

score 0 · Answer 1 · answered Mar 19 '16 at 07:09

0

In C, that pointer you return is going to be stale. You would need to allocate the table like double *result = calloc(sizeof(double), nrows) --- which would require also have a way to free the memory.

answered Mar 19 '16 at 07:09

B. Nadolson

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score -1 · Accepted Answer · answered Mar 19 '16 at 14:46

OK, so I figured out a simple way to achieve this.

We first use calloc to allocate memory for the array:

double* c_func(int n_rows) {
    double* result;
    result = calloc(n_rows, sizeof(double));
    for (int i = 0; i < n_rows; ++i) {
        result[i] = (double)i;
    }
    return result;
}

and after that, we simply convert the data into proper type in Go. The trick is to use C.free to free the memory allocated from C side.

// convert C double pointer to float64 slice ...
func doubleToFloats(in *C.double, size int) []float64 {
    defer C.free(unsafe.Pointer(in))
    out := (*[1 << 30]float64)(unsafe.Pointer(in))[:size:size]
    return out
}

Cgo: How to return double array from C to Go

2 Answers2