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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