On Tue, May 14, 2024 at 10:37 PM Pavan <sudarshan...@gmail.com> wrote:
>
> I need to pass value of struct type, which has C strings and functions to C
> function as void* argument. C layer would provide this struct type data back
> to go function. Below is the sample program. It panics if the handle used is
> returned from getH function. It works fine if the handle is global. Can the
> handle be dynamically returned from getH and make it work. Please correct if
> i missed something..
>
>
>
> package main
>
> /*
> struct dt {
> void *context;
> };
> typedef struct dt dt;
>
> extern void MyGoPrint(void *context);
> static inline void myprint(struct dt *a1) {
> MyGoPrint(a1->context);
> }
> */
> import "C"
> import (
> "context"
> "runtime/cgo"
> "unsafe"
> )
>
> //export MyGoPrint
> func MyGoPrint(context unsafe.Pointer) {
> h := *(*cgo.Handle)(context)
> val := h.Value().(accessTokenCB)
> println(val.id)
> h.Delete()
> }
>
> type At struct {
> Tok string
> }
>
> type accessTokenCB struct {
> ctx context.Context
> callback func(context.Context, *At) error
> id uint64
> ctoken *C.char
> cprivateKey *C.char
> }
>
> func getH() cgo.Handle {
> cb := func(ctx context.Context, tok *At) error {
> tok.Tok = "123"
> return nil
> }
> val := accessTokenCB{callback: cb, ctx: context.Background(), id: 32, ctoken:
> nil, cprivateKey: nil}
> h := cgo.NewHandle(val)
> return h
>
> }
>
> var h cgo.Handle
>
> func main() {
> var h cgo.Handle // commenting this line runs the program successfully else
> it panics. (cgo argument has Go pointer to unpinned Go pointer)
> h = getH()
> var poolCt C.dt
> poolCt.context = unsafe.Pointer(&h)
> C.myprint(&poolCt)
> // Output: 32
> }
You aren't following the pattern shown at
https://pkg.go.dev/runtime/cgo#Handle. Pass the cgo.Handle value to C
code, not the address of the cgo.Handle value. Catch the cgo.Handle
value as a uintptr_t in C. The point of using cgo.Handle is to avoid
difficulties passing pointers between Go and C. When you pass a
pointer to a cgo.Handle, you just get those difficulties back again.
Ian
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