How do I pass an array of C.double's to a Cgo function? - go

Question

I'm just getting started with CGo and I'm trying to send data to a C library that performs statistical computations on arrays of floats/doubles. What I'm trying to figure out right now is how to send an array of floats, or C.double's, to a CGo function that has a signature like this:
double pop_mean(int numPoints, double a[])
I've figured out how to get in the C.int in there, but I'm having trouble figuring out how to send in an array of doubles.
I haven't yet seen any blog posts or SO Questions about this exact thing, so I thought I'd ask.
The following is my best effort so far.
// Get a basic function to work, while passing in an ARRAY arr := make([]C.double, 0)
arr = append(arr, C.double(10.0))
arr = append(arr, C.double(20.0))
arr = append(arr, C.double(30.0))
var fixedArray [3]C.double = arr[:]
// ptr := C.CBytes(arr)
// defer C.free(unsafe.Pointer(ptr))
coolMean := C.pop_mean(3, &fixedArray)
fmt.Println("pop_mean (10, 20, 30): ", coolMean)
And this is the error I'm getting:
./main.go:64:6: cannot use arr[:] (type []_Ctype_double) as type [3]_Ctype_double in assignment
./main.go:69:35: cannot use &fixedArray (type *[3]_Ctype_double) as type *_Ctype_double in argument to _Cfunc_pop_mean
How should I be passing an array of C.double to the code?

Answer 1

When an array name is passed to a function, what is passed is the
location of the initial element. Within the called function, this
argument is a local variable, and so an array name parameter is a
pointer, that is, a variable containing an address.
C Programming Language, 2nd Edition
Slice types
A slice is a descriptor for a contiguous segment of an underlying
array and provides access to a numbered sequence of elements from that
array.
Like arrays, slices are indexable and have a length. The length of a
slice s can be discovered by the built-in function len; unlike with
arrays it may change during execution. The elements can be addressed
by integer indices 0 through len(s)-1. The slice index of a given
element may be less than the index of the same element in the
underlying array.
A slice, once initialized, is always associated with an underlying
array that holds its elements.
The Go Programming Language Specification
Reference: Go Command cgo
For a slice a, the arguments to the pop_mean(int numPoints, double a[]) C function are len(a), the length of the slice underlying array, and &a[0], the address of the first element of the slice underlying array.
In Go, we often hide details in a function. For example, a popMean function,
package main
import (
"fmt"
)
/*
double pop_mean(int numPoints, double a[]) {
if (a == NULL || numPoints == 0) {
return 0;
}
double mean = 0;
for (int i = 0; i < numPoints; i++) {
mean+=a[i];
}
return mean / numPoints;
}
*/
import "C"
func popMean(a []float64) float64 {
// This is the general case, which includes the special cases
// of zero-value (a == nil and len(a) == 0)
// and zero-length (len(a) == 0) slices.
if len(a) == 0 {
return 0
}
return float64(C.pop_mean(C.int(len(a)), (*C.double)(&a[0])))
}
func main() {
a := make([]float64, 10)
for i := range a {
a[i] = float64(i + 1)
}
// slice
fmt.Println(len(a), a)
pm := popMean(a)
fmt.Println(pm)
// subslice
b := a[1:4]
fmt.Println(len(b), b)
pm = popMean(b)
fmt.Println(pm)
// zero length
c := a[:0]
fmt.Println(len(c), c)
pm = popMean(c)
fmt.Println(pm)
// zero value (nil)
var z []float64
fmt.Println(len(z), z, z == nil)
pm = popMean(z)
fmt.Println(pm)
}
Output:
10 [1 2 3 4 5 6 7 8 9 10]
5.5
3 [2 3 4]
3
0 []
0
0 [] true
0

Answer 2

I figured out that you have to send a pointer to the first value in the array, rather than sending a pointer to the first element of the slice, or to the slice itself.
AND I also ran into the problem where I had created a new variable that was assigned the value of the first item in the slice and later created a pointer to that variable (which was no longer a part of the original array), instead of creating a pointer to the first item in the array (like I wanted).
Below is the working code, with comments to help avoid the problem in the paragraph above.
// Get a basic function to work, while passing in an ARRAY
// Create a dummy array of (10,20,30), the mean of which is 20.
arr := make([]C.double, 0)
arr = append(arr, C.double(10.0))
arr = append(arr, C.double(20.0))
arr = append(arr, C.double(30.0))
firstValue := &(arr[0]) // this notation seems to be pretty important... Re-use this!
// if you don't make it a pointer right away, then you make a whole new object in a different location, so the contiguous-ness of the array is jeopardized.
// Because we have IMMEDIATELY made a pointer to the original value,the first value in the array, we have preserved the contiguous-ness of the array.
fmt.Println("array length: ", len(arr))
var arrayLength C.int
arrayLength = C.int(len(arr))
// arrayLength = C.int(2)
fmt.Println("array length we are using: ", arrayLength)
arrayMean := C.pop_mean(arrayLength, firstValue)
fmt.Println("pop_mean (10, 20, 30): ", arrayMean)
This produces the following result:
array length: 3
array length we are using: 3
pop_mean (10, 20, 30): 20
Or if we uncomment the line that changes the arrayLength to be 2, we get this result:
array length: 3
array length we are using: 2
pop_mean (10, 20, 30): 15