Exchange data structures (arrays) between Go (golang) and C++

Question

I'm trying to connect a C++ library to an appserver written in Go. The goal is that both the C++ library and the appserver are working on a common data structure, which means that:

The Go appserver can access an array created by the C++ library.
The C++ library can work on an array created by the Go appserver.

I was playing a little bit around with cgo and connecting C++ and everything worked so far... However, when it comes to exchanging pointers to data structures I'm lost. What I tried so far:

//c++ library header: xyz.h

#include <stdlib.h>

class CppLib {
 public:
  CppLib(unsigned int input);

  int * CreateArray();
};

//C++ library implementation: xyz.cpp
#include "xyz.h"

CppLib::CppLib(unsigned int input) {
    _input = input;
  }

int * CppLib::CreateArray() {
    int values = 5;
    int * myPointer = new int [values];
    for (unsigned i = 0; i < values; ++i) {
        myPointer[i] = i;
    }
    return myPointer;
}

The interface implementation looks like:

//interface.h

int * CCreateArray();

//interface.cc
#include "../lib/xyz.h"

extern "C" {

  int * CCreateArray() {
    CppLib lib(1);
    return lib.CreateArray();
  }
}

Finally the go implementation looks like:

package cgo_lib

// #cgo CFLAGS: -I../lib
// #cgo LDFLAGS: -L../lib -linterfacelib
// #include "interface.h"
import "C"

func GoCreateArray() *int {
    return *int(C.CCreateArray())
}

When compiling I receive the following error:

# cgo_lib
../cgo_lib/cgo_lib.go:13: cannot convert _Cfunc_CCreateArray() (type *C.int) to type int
../cgo_lib/cgo_lib.go:13: invalid indirect of int(_Cfunc_CCreateArray()) (type int)

So my question is: How to exchange pointers to data structures between C++ and Go. Above I just described the way from C++ to GO, but I'm also interested in the other way round.

Thanks a lot for your help in advance.

You can't return a pointer to an item allocated on the stack. This is completely invalid. CreateArray needs to specifically allocate that array using new. — tadman
– tadman, Commented Feb 17, 2015 at 16:34
Hi tadman, thanks for the comment. Yes thats true... thanks for pointing that out. But even using new the core issue of the question remains, because also the errors are still there. — Energetics
– Energetics, Commented Feb 17, 2015 at 16:43
Does casting with (*int)(C.CCreateArray()) work better? Not as familiar with Go. I think you're inadvertently casting to int and then trying to de-reference that somehow. I'd also try skipping the explicit cast and seeing if Go can infer that. — tadman
– tadman, Commented Feb 17, 2015 at 16:47
Unfortunately not. I still receive: cannot convert _Cfunc_CCreateArray() (type *C.int) to type *int — Energetics
– Energetics, Commented Feb 17, 2015 at 16:49
Thanks for pointing this out tadman. In this case the best option would be to create an own free method that releases the memory using delete []. — Energetics
– Energetics, Commented Feb 17, 2015 at 20:47

thwd · Accepted Answer · 2015-02-17 17:44:46Z

1

This here:

return *int(C.CCreateArray())

can be written as

return *((int)(C.CCreateArray()))

You're derreferencing an int, which results in:

invalid indirect of int(_Cfunc_CCreateArray()) (type int)

And inside that statement, observing only this part:

(int)(C.CCreateArray())

You're trying to convert an *C.int to an int, which won't work cause the first is a pointer and the second not.

Beyond that and the memory-management trouble that @tadman mentions in the comments of your question, Go does not represent arrays as pointers to their first element as C does.

If you want to share ints you'll have to be specific about their width: an int in Go has an arch-dependent width, as well as a C int.

Read more about CGO: http://golang.org/cmd/cgo/

Once you have fixed all these problems, you'll have to proceed something like this:

s := make([]int32, 0, 0)

h := (*reflect.SliceHeader)((unsafe.Pointer)(&s))

h.Data, h.Len, h.Cap = P, L, L // see below

s = *(*[]int32)(h)

where P is a Go uintptr to your array, which you created in C++ and L is the length of the array as a Go int

answered Feb 17, 2015 at 17:44

thwd

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3 Comments

Energetics Over a year ago

Hi thwd, thanks for the answer. I could make it work exactly as suggested in your post as well as in the source you were linking. The only question that remains is: When using the method above, one receives an array of C.int values. Would it be advisable to convert them to the respective go uint?

thwd Over a year ago

Yes, make them Go types, regardless if signed or unsigned. If you don't then you'll have to import "C" all over your Go code.

Energetics Over a year ago

Just two minor corrections: The second line should look like -- h := *(*reflect.SliceHeader)((unsafe.Pointer)(&s)) -- and the forth line should be -- s = *(*[]int32)(unsafe.Pointer(&h))

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