Well, for one thing, we aren't sure (or at least I'm not sure) that the Op's application uses files in increments of 16 bits. Maybe this is download to an 8-bit uP?

I don't understand the point you're making here. To me, the OPer seems to want a 16-bit | more precisely, I guess it's an unsigned 16-bit sum of bytes | unsigned bytes ('C' data elements in terms of pack) from a subsection of a file. The OS or underlying hardware doesn't seem to matter.

The OP's spec appears to be a bizarre thing where only a "window" of the binary file is check-summed. I've never seen anything like that before.

I once worked with an application that used data files that had several subsections that each had a simple 32-bit checksum of bytes. We had to generate these files, and subsequently extract and verify the subsections for use. And yes, we recognized that a simple 32-bit checksum offered little "security", and no, we had no option to change anything, so I have some sympathy for what Chris01234 may be facing.

I am not at all sure that performance is an issue here at all! I think the goal should be clear, understandable code and then work on performance optimization later.

I agree, and that's the sort of solution I tried to offer here, with Chris01234's byte-by-byte code being altered as little as possible to achieve what seemed to me a significant increase in clarity – and no pack/unpack need apply! But if you're going to introduce a block-processing approach, the use of unpack's '%' checksumming feature is, to me, ideally simple and clear, and it's also documented; see unpack and Doing Sums in perlpacktut. If this simple, clear code also performs well, so much the better.

I don't know anything about Chris01234's personal odyssey among the Perlish islands of sysread and pack/unpack and the demons he or she met there, but I'll bet it was "interesting", and now all Chris01234 wants to do is get back home and start coding C# again.

Give a man a fish:  <%-{-{-{-<

In reply to Re^5: Accessing individual bytes of a binary file as numerical values by AnomalousMonk
in thread Accessing individual bytes of a binary file as numerical values by Chris01234

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