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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