Monotouch floating point pointer throws NullReferenceException when not 4-byte aligned

问题

I'm facing a problem I just can't understand.

While playing with unsafe pointers in C# with Monotouch, I get a NullReferenceException on device (ARM), but I can't explain why, let's see some code

var rand = new Random();
var buffer = new byte[2 * 1024 * 1024];
rand.NextBytes(buffer);

fixed (byte* ptr = buffer) {
    var ptr2 = ptr + 982515;

    //This works
    var bfr = new byte[8];
    for (int i = 0; i < 8; i++)
        bfr[i] = ptr2[i];
    var v = BitConverter.ToDouble(bfr, 0);

    //This throws a NullReferenceException on device
    var v2 = *(double*)ptr2;

    Console.WriteLine("v: {0}; v2: {1}", v, v2);
}

It only crashes on device. Anything to do with ARM structured alignment ?

Edit

After some research I ended with this:

A floating point value can be read only from a 4-bytes aligned address on ARM

static void Main(string[] args) {
    Test(982512); //Works
    Test(982516); //Works
    Test(982515); //Crash on device only
}

unsafe static void Test(int offset) {
    var rand = new Random();
    var buffer = new byte[2 * 1024 * 1024];
    rand.NextBytes(buffer);

    fixed (byte* ptr = buffer) {
        var ptr2 = ptr + offset;

        //Always works
        var bfr = new byte[8];
        for (int i = 0; i < 8; i++)
            bfr[i] = ptr2[i];
        var v = BitConverter.ToDouble(bfr, 0);

        //Throws a NullReferenceException on device if offset is not 4-byte aligned
        var v2 = *(double*)ptr2;

        Console.WriteLine("v: {0}; v2: {1}", v, v2);
    }
}

Any idea on how to bypass this ?

回答1:

On ARM devices, dereferencing a floating-point value (Single, Double) is only possible at a 4-bytes aligned address.

http://www.aleph1.co.uk/chapter-10-arm-structured-alignment-faq

So the solution is something like this:

static double ReadDouble(byte* ptr, int offset) {
    var ptr2 = ptr + offset;
    if ((int)ptr2 % 4 == 0)
        return *(double*)ptr2;
    else {
        var bfr = new byte[8];
        for (int i = 0; i < 8; i++)
            bfr[i] = ptr2[i];
        var v = BitConverter.ToDouble(bfr, 0);
    }
}

来源：https://stackoverflow.com/questions/28436327/monotouch-floating-point-pointer-throws-nullreferenceexception-when-not-4-byte-a

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