Question

I have some Thread count pCount and I have some float[] array. I want to get a pointer to the array and then based on pCount create that many threads and fill the array with data.

fixed (float* pointer = array)
{
    IntPtr fPtr = new IntPtr(pointer);

    for (int i = 0; i < pCount; i++)
    {
        Thread t = new Thread(() => ThreadMethod(fPtr, blockWidth, blockHeight, xIndex)));
        t.Start();
    }
}


private unsafe void ThreadMethod(IntPtr p, int blockWidth, int blockHeight, int startX)
{
    Random RandomGenerator = new Random();
    for (int x = startX; x < startX + blockWidth * blockHeight; x++)
    {
        ((float*)p)[x] = ((float)(RandomGenerator.NextDouble()) - 0.5f) * 2.0f;
    }
}

So if the array was 1000×1000 and I have 4 threads I want thread 1 to fill data from 0 – 250 then thread 2 from 250 – 500, thread 3 from 500 – 750 and thread 4 from 750 – 1000.

But the way I have described up there does not work. Can anyone help?

Answer 1

There’s no need to use pointer arithmetic to access array in C#. Here’s a simplified example of how it can be done:

public void ParalellizeArrayFill(int threadCount, float[] array)
{
    if (array == null || array.Length == 0)
        throw new ArgumentException("Array cannot be empty");

    if (threadCount <= 1)
        throw new ArgumentException("Thread count should be bigger than 1");

    int itemsPerThread = array.Length / threadCount;
    for (int i = 0; i < threadCount; i++)
    {
        Thread thread = new Thread( (state) => FillArray(array, i*itemsPerThread, itemsPerThread));
        thread.Start();
    }  
}

private void FillArray(float[] array, int startIndex, int count)
{
    for (int i = startIndex; i < startIndex + count; i++)
    {
        // init value
        array[i] = value; 
    }
}

There are a few caveats to be aware of. First of all, your division may not divide equally (500/3 for example), so you have to handle this case. Also, you don’t have to use pointer arithmetic since array is already passed by reference and can be accessed by index.