Awesome! Thanks again for your kindly help. :) And I updated the code, it
works!
在 2017年8月8日星期二 UTC+8下午3:51:10，Konstantin Khomoutov写道：
>
> On Mon, Aug 07, 2017 at 08:20:01PM -0700, jianzh...@gmail.com
> <javascript:> wrote:
>
> [...]
> > > > > > golevelmatrix := [][]int{{1}, {3, 3}, {3, 3, 2}}
> > > > > > levelmatrix := make([][]C.int, len(golevelmatrix))
> > > > > > for i, _ := range golevelmatrix {
> > > > > > levelmatrix[i] = make([]C.int, len(golevelmatrix[i]))
> > > > > > for j, _ := range golevelmatrix[i] {
> > > > > > levelmatrix[i][j] = C.int(golevelmatrix[i][j])
> > > > > > }
> > > > > > }
> > > [...]
> > >
> > > I'd like to reiterate more precisely: your client is expecting a
> > > multi-dimensional array which, in C, would be a contiguous region of
> > > memory. The Go's [][]C.int is a slice of individual []C.int slices.
> > > Hence when you pass &levelmatrix[0][0] to your C client, it receives
> the
> > > address of the first element of the first slice, and the only valid
> > > region of memory to access via that pointer is that element and all
> the
> > > element following it up to (but not including) the length of the first
> > > slice.
> [...]
> > But, if I can not change the C client, how to implement it? the C client
> as
> > the following. As described on the above, I have to change the ` int**
> > _levelmatrix` argument, right?
> > bool test_settopologyresource(bsagomodule* _self, char* _nodename,
> > int _resourceindex, char** _nameunits, int** _levelmatrix) {
> > printf("\n set topology resource-> node:%s, resource index:%d",
> _nodename
> > , _resourceindex);
> > bool result = settopologyresource(_self->client, _nodename,
> > _resourceindex, _nameunits, _levelmatrix);
> > return result;
> > }
>
> Ah, I see.
>
> So, do I understand correctly that your C side is actually expecting
> an "array of arrays" (or "jagged array" as some call them)?
> That would explain the int** type your function expects.
>
> If yes, this is doable without changing the C side -- by C.malloc()'ing
> each member array and storing the pointer to it in the main array:
>
> package main
>
> import "unsafe"
>
> /*
> extern int crunch_matrix(int **a);
> */
> import "C"
>
> func main() {
> golevelmatrix := [][]int{{1}, {3, 3}, {3, 3, 2}}
>
> matrix := make([]*C.int, len(golevelmatrix))
> for i, row := range golevelmatrix {
> p := (*C.int)(C.malloc(C.size_t(C.sizeof_int *
> len(row))))
> matrix[i] = p
>
> pa := (*[1 << 30]C.int)(unsafe.Pointer(p))
> for j, v := range row {
> (*pa)[j] = C.int(v)
> }
> }
> C.crunch_matrix(&matrix[0])
> }
>
> Here, for each row of the source matrix we allocate an array of C.int of
> the source row's size, and place a pointer to it into the destination
> slice.
>
> To fill the allocated row array with the source values without jumping
> through the hoops of direct pointer arithmetics, we type-convert the
> value of "p" (which has type *C.int) to the type *[1 << 30]C.int which
> is a pointer to a very long array of C.int (read more on this in the
> section "Turning C arrays into Go slices" in [1]).
>
> Note two problems with this approach.
>
> First, I made no effort to free the memory obtained by calls to
> C.malloc(). That's mostly for brewity, but see below.
>
> Second, as the code stands now, as soon as we allocate an array for the
> target row, the information of its size is lost. We can safely pass the
> pointer to it to C.free (because C.malloc stores a special block of
> information along with each block it allocates which contains the length
> of that block) but I fail to see how your C side is aware of the length
> of each row array. On the Go side, the matrix is a slice of slices, and
> since a Go slice is fully self-aware, that's OK but in C, an array is
> merely a pointer, and it carries no information of the length of the
> memory block pointed to by that pointer.
>
> Hence, unless I fail to spot where you encode this information in
> the arguments supplied to test_settopologyresource(), this might
> indicate a problem with your design of your C side: you might need to
> either use a custom type for the rows of your C matrix (a struct
> containing a pointer and a size, for instance) or operate on full square
> matrices as in my first example.
>
> 1. https://github.com/golang/go/wiki/cgo
>
>
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