Pass 2D array from Go to C using runtime.Pinner

Question content

I am trying to pass a 2D array from Go to some C functionvoid foo(in **float, out *double). Since I want a wrapper for this C function, I want the Go function to have a definition like func FooWrapper([][]float32) []float64. The simplest but not very efficient implementation is to allocate all memory via C listed below:

<code>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
}
</code>

Is such an implementation safe? Can I pass a pointer to the result data? As far as I know, this pointer will be pinned because it is passed to the C function.

But I want to allocate less memory and just reuse Go memory, I have learned about runtime.Pinner which keeps the pointer pinned until runtime.Pinner.Unpin() is called . I tried writing another implementation using pinner:

<code>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
}
</code>

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?)

Am I wrong to use runtime.Pinner (as far as I know I can pin slice data)? Or there is another bug in this code. Is there some implementation of passing 3d (4d etc.) arrays to C functions besides allocating and copying all the data to C memory?

Solution

I found the answer to my problem. Using malloc is dangerous because when I try to write a Go pointer to C memory, the GC can identify uninitialized memory as "bad" memory, so if I change malloc to calloc there will be no problem. Of course this is not the best solution, it is better to allocate memory in Go as it is faster and safer.

All discussionshere一个>.

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