wjt | wjt is a slider widget for X | Widget library

I'm getting an IStream for a file using SHCreateStreamOnFileEx, but its Read() method appears to misbehave on extremely large files when the new position of the seek pointer is 2 ** 32 bytes or further into the file.

ISequentialStream::Read's documentation says:

This method adjusts the seek pointer by the actual number of bytes read.

This is the same behaviour as read(2) and fread(3) on all platforms I'm aware of.

But with these streams, this isn't the actual behaviour I see in some cases:

• Seek(2 ** 32 - 2, SEEK_SET, &pos), Read(buf, 1, &bytesRead), Seek(0, MOVE_CUR, &pos) → bytesRead == 1 and pos == 2 ** 32 - 1, as expected.
• Seek(2 ** 32 - 1, SEEK_SET, &pos), Read(buf, 1, &bytesRead), Seek(0, MOVE_CUR, &pos) → bytesRead == 1, but pos == (2 ** 32 - 1) + 4096, which is incorrect. This means that any subsequent reads (without another Seek to fix the cursor position) read the wrong data, and my application doesn't work!

Am I “holding it wrong”? Is there some flag I need to set to make this class behave properly? Or is this a bug in Shlwapi.dll?

The code below reproduces this problem for me. (Set OFFSET = WORKS to see the successful case.)