dword (UInt32) buffers?

c0ol · September 22, 2004, 03:09 AM

I am reading file data in using a file stream. I am reading this data into a Byte array. I would like to access this data in terms of UInt32s. In c++ i would simply typecast the buffer, but in c# this does not seem to be the solution. How can I access this array in terms of UInt32s without copying the data into a UInt32 array with Buffer.BlockCopy(); this method would be far too slow for my needs.

MyndFyre · September 22, 2004, 05:02 AM

Code Select


// compile with /unsafe
private unsafe void doSomethingWithUint32s(byte[] bytes) {
  int nCurPos = 0;
  fixed (uint *pnUint = bytes)
  {
     // use old-fashioned pointer manipulation here.
     while (bytes.Length - nCurPost >= 4)
     {
       // do your thing regarding pnUint.
       pnUint += sizeof(uint); // same as += 4;
       nCurPos += 4;
     }
  }
}

c0ol · September 22, 2004, 12:49 PM

mm pointers in c#, interesting

K · September 22, 2004, 04:24 PM

You could also intialize a System.IO.MemoryStream containing the bytes and read it from there.

BTW, won't that code complain here:

Code Select


fixed(uint* pnUint = bytes)

that bytes is not a pointer to a uint?

I seem to recall having that problem; the next thing I tried was:

Code Select


fixed(uint* p = (uint*)bytes)

and it complained that you cannot have a typecast insided a fixed() statement, so I ended up doing:

Code Select


fixed(byte* bTmp = bytes)
{
    uint* p = (uint*)bTmp;
}

Or maybe I'm crazy

c0ol · September 22, 2004, 05:04 PM

well thanks guys, i wasnt even aware you could do pointers in c# lol. This helps alot!

Skywing · September 22, 2004, 05:07 PM

Quote from: c0ol on September 22, 2004, 05:04 PM
well thanks guys, i wasnt even aware you could do pointers in c# lol. This helps alot!

This isn't always desireable, though. Your images will be marked "unsafe" / "unverified", so if you are planning to make something that runs in, say, browsers (e.g. like a Java applet) then you would probably not want to use pointers (as "untrusted" code cannot do "unsafe" things, IIRC).

MyndFyre · September 22, 2004, 05:49 PM

That's a good point K, heh, sorry I put that up while at work w/o a compiler available.

Also, you could just use BitConvert.ToUInt32(bytes, i); and increment i by four every iteration.

Valhalla Legends Archive

dword (UInt32) buffers?

c0ol

MyndFyre

c0ol

K

c0ol

Skywing

MyndFyre