The runtime and checks does not know what's inside an uintptr, so converting an uintptr to a(n unsafe.)Pointer triggers the warnings. Only the special specified cases are legal.
TL;DR; Do not use uintptr. (More elaborate: do not use an uintptr to store and retrieve a pointer - just store the pointer directly). Tamas snmed a következőt írta (2021. július 9., péntek, 11:26:48 UTC+2): > Yes uintptr is the address the C pointer is pointing to , right? So C > doesn't move any memory and therefore the address is still valid until > deleted in C code. > If I take the value of uintptr variable, convert it back with > unsafe.Pointer(uintptr) and pass it to a C function, does C not interpret > that as a C pointer pointing to the same address as uintptr holds? > Therefore the conversion should be alright? > > Sorry for not getting it... > > Sandro > > Tamás Gulácsi schrieb am Freitag, 9. Juli 2021 um 11:12:22 UTC+2: > >> An uintptr *is not a pointer*, so 2. is false. >> NOTHING at Go's side holds anything regarding MyAppHwnd, and it doesn't >> seem to be a pointer at all (just a number), >> so converting that to an unsafe.Pointer is unsafe. >> >> The rules about uintptr are there for a reason! >> Only the allowed use cases are guaranteed to have the desired effect, by >> preventing any memory movement >> int that block of code. >> >> Tamas >> snmed a következőt írta (2021. július 9., péntek, 8:21:37 UTC+2): >> >>> Thx Ian to pointing me to the documentation I read it, but still unsure >>> regarding my example. Probably rule number 4 could apply, so let me go >>> through my example and correct me if I'm wrong. >>> >>> 1. C.CreateApp() creates memory in C code and returns it as C void >>> pointer >>> 2. Go function CreateApp() returns that as MyAppHwnd (uintptr ) so >>> MyAppHwnd contains a valid C memory address >>> 3. app variable holds still a valid C memory address as long as it is >>> not deleted in C code >>> 4. So calling ShowApp() it should be legit to convert MyAppHwnd back to >>> an unsafe.Pointer and cast it to C.MyAppPtr. It is still a valid memory >>> address in C and therefore should be interpreted as pointer in C, right? >>> >>> So therefore the warning can be ignored or do I miss something important? >>> >>> Cheers >>> Sandro >>> >>> >>> Ian Lance Taylor schrieb am Freitag, 9. Juli 2021 um 05:37:16 UTC+2: >>> >>>> On Thu, Jul 8, 2021 at 10:09 AM snmed <sandro....@gmail.com> wrote: >>>> > >>>> > Thanks for your reply. I came across a similar solution today, I made >>>> a struct which is visible outside the package and use a private field >>>> myApp >>>> C.MyAppPtr to hide the C type. >>>> > I still wondering why the uintptr version works but shows a warning >>>> "possible misuse of unsafe.pointer", CGO documentation is not very clear >>>> to >>>> me about the intricacies of type conversions between Go and C. >>>> >>>> There is a very limited number of cases in which it is OK to convert a >>>> pointer to uintptr. Those cases are described at >>>> https://golang.org/pkg/unsafe. >>>> >>>> Ian >>>> >>> -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/b57a16e1-e6be-4ab8-bda7-67043d5dd79an%40googlegroups.com.
