Ignoring more efficient ways to pass memory to C for a moment,
superficially I do think your code using Pinner should work. Do you have a
full reproducer? It's hard to tell from just looking at your code if your
code is the problem, or its just enough to trigger some other cgo issue
elsewhere in your codebase. There's a slim chance this is a bug in the
Pinner implementation, but that would be surprising to me. The Pinner
trivially keeps all pointers passed to it live by just holding onto their
references.
Also, have you tried running your code with GODEBUG=cgocheck=1 and/or
building your code with GOEXPERIMENT=cgocheck2? That might help identify
what the issue is.
On Friday, January 26, 2024 at 7:05:45 AM UTC-5 Tamás Gulácsi wrote:
> To convert a Go slice to C array is easy with unsafe.Slice.
>
> The problem is that the multi-dimensional C array is an array of pointers
> (*(*float64))
> which cannot travel between the C/Go barrier.
>
> In your example, you flatten your slice, and recreate the pointers in that
> one big slice.
> You could go on with that example, but from the Go side:
>
> 1. have a "flat" []float64 wihich is N*M
> 2. have the [][]float64 as subslices of that one big flattened []float64:
> flat[i*M:i*(M+1)]
> 3. send that flat slice to the C side with unsafe.Slice
> 4. have the C side implement the same subslicing: create a []*float64
> array and have each point to the corredt &flat[i*M].
> 5. profit
>
>
> Denis a következőt írta (2024. január 26., péntek, 1:18:59 UTC+1):
>
>> I am trying to pass 2d array from Go to some C function void foo(in
>> **float, out *double). Since I want to have wrapper for this C function,
>> I'd like that Go function has definition like func
>> FooWrapper([][]float32) []float64. The easiest but not efficient
>> implementation is allocating all memory through C that listed below:
>>
>> func FooWrapper(values [][]float32) []float64 {
>> totalObj := len(values)
>> totalParams := len(values[0])
>> results := make([]float64, totalObj)
>>
>> ptrArrLength := uintptr(totalObj) * cPointerSize
>> paramArrLength := uintptr(totalParams) * cFloatSize
>>
>> ptr := C.malloc(C.size_t(ptrArrLength +
>> paramArrLength*uintptr(totalObj)))
>> defer C.free(ptr)
>>
>> ptrSlice := unsafe.Slice((**C.float)(ptr), totalObj)
>> for i, obj := range values {
>> paramArrPtr := (*C.float)(unsafe.Add(ptr,
>> ptrArrLength+uintptr(i)*paramArrLength))
>> ptrSlice[i] = paramArrPtr
>>
>> paramSlice := unsafe.Slice(paramArrPtr, totalParams)
>> for j, param := range obj {
>> paramSlice[j] = (C.float)(param)
>> }
>> }
>>
>> C.foo((**C.float)(ptr), (*C.double)(&results[0]))
>>
>> return results
>> }
>>
>> Is that safe implementation? Can I pass pointer of result data? As far
>> as I know, this pointer will be pinned because it passed to C function.
>>
>> But I want to allocate less memory just reusing Go memory, I've learned
>> about runtime.Pinner that make pointer pinned until
>> runtime.Pinner.Unpin() invocation. I tried to write another
>> implementation using pinner:
>>
>> func FooWrapper(values [][]float32) []float64 {
>> length := len(values)
>> results := make([]float64, length)
>>
>> pinner := runtime.Pinner{}
>> defer pinner.Unpin()
>>
>> arr := (**C.float)(C.malloc(C.size_t(uintptr(length) * cPointerSize)))
>> defer C.free(unsafe.Pointer(arr))
>> slice := unsafe.Slice(arr, length)
>> for i, v := range values {
>> pinner.Pin(&v[0])
>> slice[i] = (*C.float)(&v[0])
>> }
>>
>> C.foo(arr, (*C.double)(&results[0]))
>>
>> return results
>> }
>>
>> But, unfortunately, this code doesn't work
>>
>> runtime: pointer 0xc016ecbfc0 to unused region of span
>> span.base()=0xc016eca000 span.limit=0xc016ecbfa0 span.state=1
>> fatal error: found bad pointer in Go heap (incorrect use of unsafe or
>> cgo?)
>>
>> Do I use runtime.Pinner wrong (as far as I know, I can pin slice data)?
>> Or there is another error in this code. Are there some implementations for
>> passing 3d (4d and so on) array to C function except for allocatiing and
>> copying all data to C memory?
>>
>
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