drhozgt6007
2018-11-30 00:16
浏览 126
已采纳

如何将C.double数组传递给Cgo函数?

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?

  • 写回答
  • 好问题 提建议
  • 关注问题
  • 收藏
  • 邀请回答

2条回答 默认 最新

  • duanmuyao0463 2018-11-30 02:06
    已采纳

    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
    
    已采纳该答案
    评论
    解决 无用
    打赏 举报
  • douxing8939 2018-11-30 00:16

    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
    
    评论
    解决 无用
    打赏 举报

相关推荐 更多相似问题