This is a great idea for a work-around. Fixed, thanks!
On Friday, 10 July 2020 at 00:34:58 UTC+1 [email protected] wrote:
> Just write a wrapper on the C side (in the import "C" code) that takes a
> uintptr and casts it to void*.
>
> func wrapper(a uintptr, b, c, d, e ...) {
> op_open_callbacks((void*)a, b, c, d, e)
> }
>
> Then you can call C.wrapper and pass it s.id directly.
>
>
> On Thursday, July 9, 2020 at 4:17:08 PM UTC-7, Hraban Luyat wrote:
>>
>> Because the C function I'm calling takes a "void *" param. Can you pass a
>> regular Go int as a "void *" argument somehow without going through
>> unsafe.Pointer?
>>
>> Note I don't directly call go_readcallback; I call op_open_callbacks (not
>> under my control) and pass it the go_readcallback as a callback (hence the
>> name). It does some setup, and only calls go_readcallback when data comes
>> in. With that void * argument that it was passed during this initial call,
>> here.
>> On Thursday, 9 July 2020 at 20:11:24 UTC+1 Tamás Gulácsi wrote:
>>
>>> As far as I see, you pass c.id to go_readcallback, which gets the
>>> stream from streamsMap.
>>>
>>> Why unsafe.Pointer and uintptr? Why not just a simple int or uint32 ?
>>> For map key, they're indifferent, and the unsafe.Pointer is totally
>>> uneeded here.
>>>
>>>
>>>
>>> 2020. július 9., csütörtök 18:58:52 UTC+2 időpontban Hraban Luyat a
>>> következőt írta:
>>>>
>>>> Hi all,
>>>>
>>>> In package github.com/hraban/opus (currently 57179df), if I run go
>>>> test -race I get an error:
>>>>
>>>> fatal error: checkptr: unsafe pointer arithmetic
>>>>
>>>> The offending line is:
>>>>
>>>> https://github.com/hraban/opus/blob/v2/stream.go#L108:
>>>>
>>>> oggfile := C.op_open_callbacks(
>>>> unsafe.Pointer(s.id), // <-- this guy
>>>> &C.callbacks,
>>>> nil,
>>>> 0,
>>>> &errno)
>>>>
>>>> Background: I need to pass a reference to a go struct to that C call,
>>>> so when it calls my callback function, that callback function can find the
>>>> Go struct. The C function expects a void *, but I have a
>>>> map[uintptr]myType
>>>> locally with a fresh int for every new struct. I cast that uintptr to an
>>>> unsafe.Pointer, pass that to the C call, and once the callback is called,
>>>> I
>>>> cast it back to a uintptr, and use that to fetch the relevant struct from
>>>> the map. That's what the "s.id" thing is.
>>>>
>>>> If I understand correctly, since 1.14 (?) go test -race includes
>>>> -d=checkptr, and checkptr does not like arbitrary uintptrs to be case to
>>>> unsafe.Pointer. However, as far as I understand, that error is spurious in
>>>> my specific case, because nobody ever dereferences the pointer at all, or
>>>> does any arithmetic on it; it's only cast straight back to a uintptr.
>>>>
>>>> Question 1: Is my map[uintptr]myType rigmarole even necessary? It came
>>>> about from my understanding of
>>>> https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md,
>>>>
>>>> but maybe I misunderstood.
>>>>
>>>> Question 2: if it is necessary, is this "arbitrary uintptr in void *"
>>>> valid? As far as I understand the C standard, it is, but again maybe I
>>>> misunderstood.
>>>>
>>>> Question 3: If it is valid, what can / should I do about go test -race?
>>>>
>>>> Related:
>>>> https://groups.google.com/g/golang-nuts/c/FAOZoQzodAI/m/IF_xKit2DXYJ ,
>>>> seems like a similar issue, and the reply suggests the approach is valid.
>>>>
>>>> Thanks all and kind regards
>>>>
>>>> Hraban
>>>>
>>>>
>>>> Full go test -race output:
>>>>
>>>> fatal error: checkptr: unsafe pointer arithmetic
>>>>
>>>> goroutine 56 [running]:
>>>> runtime.throw(0x42354f9, 0x23)
>>>> /usr/local/opt/go/libexec/src/runtime/panic.go:1116 +0x72
>>>> fp=0xc000048ce0 sp=0xc000048cb0 pc=0x4034c22
>>>> runtime.checkptrArithmetic(0x40, 0x0, 0x0, 0x0)
>>>> /usr/local/opt/go/libexec/src/runtime/checkptr.go:26 +0xce
>>>> fp=0xc000048d10 sp=0xc000048ce0 pc=0x400859e
>>>> _/Users/hraban/code/personal/opus.(*Stream).Init.func1(0xc0001525c0,
>>>> 0xc000128758, 0x1)
>>>> /Users/hraban/code/personal/opus/stream.go:108 +0x6a
>>>> fp=0xc000048d68 sp=0xc000048d10 pc=0x419ddba
>>>> _/Users/hraban/code/personal/opus.(*Stream).Init(0xc0001525c0,
>>>> 0x42630c0, 0xc0001323e0, 0x0, 0x0)
>>>> /Users/hraban/code/personal/opus/stream.go:112 +0x277
>>>> fp=0xc000048e38 sp=0xc000048d68 pc=0x419a9c7
>>>> _/Users/hraban/code/personal/opus.NewStream(0x42630c0, 0xc0001323e0,
>>>> 0x0, 0x0, 0x0)
>>>> /Users/hraban/code/personal/opus/stream.go:75 +0xa0 fp=0xc000048e88
>>>> sp=0xc000048e38 pc=0x419a6b0
>>>> _/Users/hraban/code/personal/opus.TestStreamIllegal(0xc0002fcb40)
>>>> /Users/hraban/code/personal/opus/stream_test.go:20 +0xc0
>>>> fp=0xc000048ed0 sp=0xc000048e88 pc=0x4194320
>>>> testing.tRunner(0xc0002fcb40, 0x4238c08)
>>>> /usr/local/opt/go/libexec/src/testing/testing.go:991 +0x1ec
>>>> fp=0xc000048fd0 sp=0xc000048ed0 pc=0x41270ec
>>>> runtime.goexit()
>>>> /usr/local/opt/go/libexec/src/runtime/asm_amd64.s:1373 +0x1
>>>> fp=0xc000048fd8 sp=0xc000048fd0 pc=0x4066851
>>>> created by testing.(*T).Run
>>>> /usr/local/opt/go/libexec/src/testing/testing.go:1042 +0x661
>>>>
>>>> goroutine 1 [chan receive]:
>>>> testing.(*T).Run(0xc000156000, 0x4230a45, 0x11, 0x4238c08, 0x1)
>>>> /usr/local/opt/go/libexec/src/testing/testing.go:1043 +0x699
>>>> testing.runTests.func1(0xc000156000)
>>>> /usr/local/opt/go/libexec/src/testing/testing.go:1284 +0xa7
>>>> testing.tRunner(0xc000156000, 0xc00013bd50)
>>>> /usr/local/opt/go/libexec/src/testing/testing.go:991 +0x1ec
>>>> testing.runTests(0xc000132040, 0x437c680, 0x1d, 0x1d, 0x0)
>>>> /usr/local/opt/go/libexec/src/testing/testing.go:1282 +0x528
>>>> testing.(*M).Run(0xc000154000, 0x0)
>>>> /usr/local/opt/go/libexec/src/testing/testing.go:1199 +0x300
>>>> main.main()
>>>> _testmain.go:100 +0x224
>>>> exit status 2
>>>> FAIL _/Users/hraban/code/personal/opus 0.231s
>>>>
>>>
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