How to Efficiently Create a Go Array or Slice from an unsafe.Pointer?

How to efficiently initialize an array or slice from an array unsafe.Pointer in Go

In the world of Go, you may encounter scenarios where you need to work with data structures represented as unsafe.Pointer. For instance, you might have a pointer to an array like:

p := uintptr(unsafe.Pointer(&array))
size := 5

However, directly accessing array is not feasible, as the pointer is merely a representation of its address. This leaves you with the challenge of creating an array or slice using the known pointer, size, and data type without resorting to memory copies.

Unsafe Approach to Array/Slice Creation

Initially, you may consider the following approach:

data := make([]byte, size)
stepSize := unsafe.Sizeof(data[0])
for i := 0; i < size; i++ {
    data[i] = *(*byte)(unsafe.Pointer(p))
    p += stepSize
}

This solution copies data from the original array to the new slice, which can be inefficient, especially for large data sets.

Efficient Approach using reflect.SliceHeader

A more efficient alternative is to harness the power of Go's reflection package and the reflect.SliceHeader type. By directly manipulating the fields of reflect.SliceHeader, you can create an array or slice that references the same underlying data as your pointer.

Step 1: Obtain reflect.SliceHeader descriptor

Declare a slice variable and cast its []byte type to *reflect.SliceHeader using unsafe conversions:

var data []byte
sh := (*reflect.SliceHeader)(unsafe.Pointer(&data))

Step 2: Modify SliceHeader fields

Assign the pointer, size, and optionally, the capacity to the fields of reflect.SliceHeader:

sh.Data = p
sh.Len = size
sh.Cap = size

Step 3: Convert back to the original data type

Finally, cast the modified reflect.SliceHeader back to your desired data type using unsafe conversions:

data = *(*[]byte)(unsafe.Pointer(sh))

This efficient approach allows you to create an array or slice that shares the same underlying data as your pointer without incurring the cost of unnecessary memory copies.

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