cgo指针值已更改

Question

I made bindings to a C api (bullet physics engine) using cgo, some functions make use of data pointers. The idea is that I can attach a pointer to an object and retrieve it later when the physics engine invokes a callback. My problem is that when i get the value back, it change and I didn't do it. It seems that no source code is explicitelly changing the value.

CollisionObject: source, header, The go codes that interracts with that class

heres how i send the values, the reconversion to *int and int is fine, the correct numbers are printed:

num := x*amounty*amountz + y*amountz + z + 1
ptr := unsafe.Pointer(&num)
fmt.Printf("created %v %v
", ptr, *(*int)(ptr))
rb := sphere.RigidBody(ptr, 1)

But when I get it back from a raytest the value changed:

ptr := hit.GetUserPointer()
log.Printf("we got back: %v %v", ptr, *(*int)(ptr))

the pointer value itself didnt change, i can look up and see that there was a pointer pointing to this location, but the value its pointing at is different.

Now i'm wondering if maybe go didn't clean the value (garbage collected) since it wouldn't be used anymore and replaced this memory location with something else.

example output (with junk values removed):

created: 0xc2080006e0 40
2014/11/07 17:10:01  we got back: 0xc2080006e0 4921947622888946315

ANY pointer (hehe) is appreciated :)

Answer 1

Go's garbage collector doesn't know about the pointers held by C or C++ code, so there is nothing to keep the num variable alive.

You can work around this by storing a second copy of the pointer in a Go variable. One way is to use a global variable with a type like map[*C.some_c_type]*int or similar, and store &num there too. Remember to protect the map with a mutex so things behave correctly when you have concurrent access.

In order not to leak, you will need to manually delete &num from the map when the underlying C code is no longer holding a reference to it. If the C library provides the ability to set a destroy notify function when storing the user pointer, this will be easy: just export a Go function to C and use it as the notify function. If it doesn't, but the Go binding knows when the the pointer will be finished with (e.g. if the RigidBody variable is always freed via the Go API, you can do the clean up there.