CGO; C function has int pointer argument, how to pass correct type?

Question

Currently I'm using Cgo to call C functions from Go. I'm trying to recreate the 'Read a Photo' example in Go.

One on the C functions, however, expects a int* len argument (bonus question; is that the same as int *len?). As I read this, this is a pointer to an integer. The function in question is ccv_write of the libccv library. It's full signature is:

int ccv_write(ccv_dense_matrix_t* mat, char* out, int* len, int type, void* conf)

The relevant code snippet is as follows:

type Image struct {
    image *C.ccv_dense_matrix_t
}

func main() {
    image := new(Image)

    /* ... snip ... */

    dst := C.CString("black_and_white_painting.jpg")
    defer C.free(unsafe.Pointer(dst))
    x := 0 // <- perhaps var x int ?
    C.ccv_write(image.image, dst, (*C.int)(&x), C.CCV_IO_PNG_FILE, 0)
}

The above example generates the following compile time error: cannot convert &x (type *int) to type *_Ctype_int

Any thoughts on how to pass the correct argument?

Answer 1

The issue here is you have two different types. You have an int which is defined by your Go compiler and C.int which is defined by your C compiler. Depending on the compilers and architecture these may be the same size. However, it is very possible that int will be 64 bits in size and C.int would be 32 bits.

In order for the compiler to enforce the type safety, it requires you to do an explicit conversion or define the type at initialization.

In your example, you do: x := 0. This is the same as x := int(0). The int is implied. Instead, you can do x := C.int(0).

However, I think the "cleaner" method would be to do what jnml recommended: var x C.int. This does exactly the same thing but just looks nicer.

---

Later, you will most likely want to convert that C.int back to an int. This is easy to do with a conversion.

y := int(x)

This will convert x to an int and then copy that value into the newly initialized variable y.

---

Bonus Anwser: int* len is indeed the same as int *len

Answer 2

This seems to work for me:

package main

/*

void foo(int *len) { *len = 1234; }

*/
import "C"

func main() {
        var x C.int
        C.foo(&x)
        println(x)
}

Answer 3

You will need an unsafe.Pointer cast.

C.ccv_write(image.image, dst, (*C.int)(unsafe.Pointer(&x)), C.CCV_IO_PNG_FILE, 0)