On Thu, May 16, 2024 at 12:57 AM Pavan <sudarshan...@gmail.com> wrote:
>
> Thanks . Yes it works when C API takes uintptr_t. In my case, the C API
> expects void * which i can not change, so I was trying to make it work with
> the variant example stated above.
In C, unlike Go, you can convert between void* and uintptr_t. When
using cgo, one approach is a little wrapper function in the cgo
comment that takes a uintptr_t and calls the real function with a
conversion to void*.
Ian
> On Thursday, May 16, 2024 at 5:28:55 AM UTC+5:30 Ian Lance Taylor wrote:
>>
>> On Tue, May 14, 2024 at 10:37 PM Pavan <sudars...@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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