C#•7mo ago

Can Memory<T> be created from a pointer?

Can Memory<T> be created with a pointer and a length? If not, how come?

9 Replies

you would need to use https://github.com/dotnet/runtime/blob/main/src/libraries/Common/src/System/Memory/PointerMemoryManager.cs (or something that does the same thing) just copy it into your project, and use myMemoryManager.Memory

nathanAjacobsOP•7mo ago

Ty this will work! I was going crazy looking for a something in the BCL that does this.

Servator•7mo ago

AI Answer:

unsafe 
{
    byte* ptr = stackalloc byte[100];
    System.Runtime.InteropServices.MemoryMarshal.CreateFromPinnedArray(new Span<byte>(ptr, 100));
}

unsafe 
{
    byte* ptr = stackalloc byte[100];
    System.Runtime.InteropServices.MemoryMarshal.CreateFromPinnedArray(new Span<byte>(ptr, 100));
}

reflectronic•7mo ago

CreateFromPinnedArray takes an array, not a span, so that does not work but, there was an attempt

Servator•7mo ago

GPT-4:

        unsafe
        {
            fixed (int* ptr = array)
            {
                // Create a Span<int> from pointer and length
                Span<int> span = new Span<int>(ptr, length);
                
                // Use the span to initialize memory
                for (int i = 0; i < span.Length; i++)
                    span[i] = i;

                // Optionally convert to Memory<int>
                Memory<int> memory = MemoryMarshal.CreateFromPinnedArray(array, 0, length);

                // Example usage of Memory<int>
                foreach (var item in memory.Span)
                    Console.WriteLine(item);
            }
        }

        unsafe
        {
            fixed (int* ptr = array)
            {
                // Create a Span<int> from pointer and length
                Span<int> span = new Span<int>(ptr, length);
                
                // Use the span to initialize memory
                for (int i = 0; i < span.Length; i++)
                    span[i] = i;

                // Optionally convert to Memory<int>
                Memory<int> memory = MemoryMarshal.CreateFromPinnedArray(array, 0, length);

                // Example usage of Memory<int>
                foreach (var item in memory.Span)
                    Console.WriteLine(item);
            }
        }

Still same issue i think damn

nathanAjacobsOP•7mo ago

Yeah I tried asking copilot earlier and got a lot of stuff that was incorrect

Servator•7mo ago

My last attempt:

        unsafe
        {
            var ptr = stackalloc byte[100];
            var byteArray = new byte[100];
            Marshal.Copy((IntPtr)ptr, byteArray, 0, 100);
            var memory = new Memory<byte>(byteArray);
        }

        unsafe
        {
            var ptr = stackalloc byte[100];
            var byteArray = new byte[100];
            Marshal.Copy((IntPtr)ptr, byteArray, 0, 100);
            var memory = new Memory<byte>(byteArray);
        }

public unsafe Memory<T> CreateMemoryFromPointer<T>(T* ptr, int length) where T : unmanaged
{
    int size = sizeof(T);
    byte* bytePtr = (byte*)ptr;

    byte[] byteArray = new byte[length * size];

    fixed (byte* destination = byteArray)
    {
        System.Buffer.MemoryCopy(bytePtr, destination, length * size, length * size);
    }

    return new Memory<T>(byteArray, 0, length);
}

public unsafe Memory<T> CreateMemoryFromPointer<T>(T* ptr, int length) where T : unmanaged
{
    int size = sizeof(T);
    byte* bytePtr = (byte*)ptr;

    byte[] byteArray = new byte[length * size];

    fixed (byte* destination = byteArray)
    {
        System.Buffer.MemoryCopy(bytePtr, destination, length * size, length * size);
    }

    return new Memory<T>(byteArray, 0, length);
}

Looks like you could use something like this. I didn't test it tho.

unsafe 
{
    int* ptr = stackalloc int[100];
    var memory = CreateMemoryFromPointer(ptr, 100);
}

unsafe 
{
    int* ptr = stackalloc int[100];
    var memory = CreateMemoryFromPointer(ptr, 100);
}

then this should work according to ai

nathanAjacobsOP•7mo ago

This allocates a managed array and has to perform a copy. The MemoryManager<T> is the better solution

FusedQyou•7mo ago

Please don't post AI answers, especially because they are often wrong like is the case here

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Can Memory<T> be created from a pointer?