CGo Passing Multidimensional Array

[pet...@ollins.email]

My question is similar to the original question asked by this stackoverflow post.  Basically how would I be able to pass in a multiple dimensional byte ([][]byte) array such that it could be read by my C code.

The Go code compiles and runs, but crashes within the C code which just reads the array like below.  My C code is working, I'm just having trouble passing the pointer correctly.

Passing in a [][]byte multiple dimensional array into this C psuedocode function

void getMatrix(unsigned char **matrix) {
        for (j = 0; j < srcs; j++) {
                for (i = 0; i < len; i++) {
                     s = src[j][i];
                }
        }
}

with this Go psuedocode

import "C"
import "unsafe"

func Hello() {

sources := make([][]byte, k)

for i := range sources {

sources[i] = make([]byte, sourceLength)

}

C.getMatrix((**C.uchar)(unsafe.Pointer(&sources[0])))

}

The problem I am having is casting the Go [][]byte array correctly to the C unsigned char** correctly. Any ideas? Can this be done? Would a single dimensional array be better?

[egon]

First take a look at how slices are stored internally: 

http://blog.golang.org/go-slices-usage-and-internals

http://research.swtch.com/godata

https://code.google.com/p/go-wiki/wiki/cgo

It should give you an idea why it doesn't work.

Yes, a single dimensional array would be better.

Essentially write your C function as:

getMatrix(unsigned char *matrix, int ncols, int nrows)

And keep the Go matrix in a single array []byte.

Also ensure that array doesn't get prematurely garbage collected.

sources := make([]byte, 100)

data := unsafe.Pointer(&sources[0])

C.getMatrix((*C.uchar)(data), 10, 10)

_ = data

I can't remember the exact rules how to properly pass pointers to C, but it probably is covered in some thread/page.

+ egon

[Stephen Gutekanst]

Perhaps without understanding why, the reason that you write:

unsafe.Pointer(&sources[0])

Is because slices have a slice header, you can see an effective slice header definition in Go's reflect package: http://golang.org/pkg/reflect/#SliceHeader

When you get a pointer to the first element in a slice, you're grabbing a pointer to the start of the slice's data.

Now, in your C code you are treating slices as if they are arrays: they are not. [][]byte and **uchar are two totally separate things.

A single dimensional array would simplify the problem, yes. Mainly you just need to acknowledge the slice header in your C code.

- Stephen

[peterGo]

Peter,

A multidimensional array is not the same as a multidimensional slice. Construct the multidimensional slice so that it uses a multidimensional array as its underlying array. For example,

Playground: http://play.golang.org/p/hbrh1iF1w_

Then you can write

    C.getMatrix((**C.uchar)(unsafe.Pointer(CMatrixPtr(matrix))))

Peter