指针是否由于垃圾回收而改变了Go中的值？

Question

I'm a relative newbie to go. I have been working a lot with cgo and building an interface to a C api. The C api uses a registered callback and a void* cookie parameter to the callback.

Here is some code exemplifying my problem:

/*
void cb(void *cookie) {
   callGo(cookie);
}

go_register_cb(void *ptr) {
   register_cb(&cb, ptr);
}
*/
import "C"

and

import "C"

//export callGo
callGo(ptr unsafe.Pointer) {
    x := (*MyStruct)(ptr)
    x.doSomething()
}

func (x *MyStruct) Start() {
    C.go_register_cb(unsafe.Pointer(x))
}

This code causes a runtime error about Go pointers to Go pointers.

I think I understand the basic implications of keeping Go pointer inside C-land, which is that the C-land reference will not be accounted for in the go garbage collector so C may end up using a stale reference.

The specific requirement that I cannot pass a Go pointer to a data-structure that itself contains a Go pointer is more puzzling. The only thing that makes sense to me is that pointers in Go may change value (that is, the address they point to) due to garbage collection. Is this the case?

Answer 1

These rules for passing pointer between Go code and C code are dynamically checked at runtime. So when you break them the error occur.

And your guess is right. Addresses are allowed to change due to garbage collection (Pointers are updated).

Note here from golang.org:

Even if a uintptr holds the address of some object, the garbage collector will not update that uintptr's value if the object moves, nor will that uintptr keep the object from being reclaimed.

So moving object due to gc is possible.