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

How to create an array or a slice from an array unsafe.Pointer in Go?

Question:

Given an unsafe pointer to an array, how can we create an array or a slice from it without incurring the cost of memory copy?

Answer:

Creating a Slice Using Reflect:

The recommended approach is to use the reflect package to create a slice header that points to the same underlying data as the unsafe pointer.

// Create a slice header
sh := &reflect.SliceHeader{
    Data: p, // Unsafe pointer to the array
    Len:  size,
    Cap:  size,
}

// Use the slice header to create a slice
data := *(*[]byte)(unsafe.Pointer(sh))

Creating an Array Using Reflect:

To create an array (pointer to elements in contiguous memory) from an unsafe pointer, we can first create a slice and then take its address:

// Create a slice
data := *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
    Data: p,
    Len:  size,
    Cap:  size,
}))

// Get the address of the slice (pointer to the first element)
arr := (*[size]byte)(unsafe.Pointer(&data[0]))

Caveats and Warnings:

• Ensure that the lifetime of the original array is managed to prevent unexpected memory access.
• Avoid using uintptr variables as references to objects, as garbage collection may occur unexpectedly.
• Use runtime.KeepAlive() if necessary to prevent the original array from being garbage collected before it is referenced by the slice or array created from the unsafe pointer.

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