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Re: A new protocol and tool for PNG file attachments

Eric Farmer <erfarmer@gmail.com>
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Very cool, and as usual, I love how lean your implementation is. I
recalled seeing something similar a few weeks ago:

https://github.com/Fusion/pngsource

that I first saw discussed on Hacker News:

https://news.ycombinator.com/item?id=29657072

I didn't dig too deeply through the implementation to see what was
happening under its hood. It seemed like a lot more to unpack-- yours
is much easier to read and understand.

I have a down-in-the-weeds question: what motivates the `void *`
buffer type in `crc32_update`, when it's `unsigned char *` in other
places? (I thought there might be some raw structs getting plugged in
somewhere, but I didn't see any, it looks like all of the actual
buffers being provided are also `unsigned char` arrays?)

Re: A new protocol and tool for PNG file attachments

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<20220105021623.6wq5zlciqfgtdlrp@nullprogram.com>
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<CAAuXPZoLFAn1o-c1+yKrVL1qQtBS5U_WWW5UJseUJjjgYXWLCw@mail.gmail.com> (view parent)
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Thanks, Eric! That very project being shared around was what got the gears 
turning for my project. I was inspired by the headline, skimmed the PNG 
specification to brush up on the format, went to sleep, and by morning I 
had a few ideas for how to tackle it. I Initially considered compatibility 
with pngsource (i.e. the draw.io format), but saw the XML and noped out.

> what motivates the `void *` buffer type in `crc32_update`

That's just the result of a copy-paste incompletely integrated into the 
new project. :-)

https://github.com/skeeto/scratch/blob/master/checksums/crc32.h

The original is C89 — no stdint.h, no for loop variable declaration — and, 
being agnostic of the surrounding context, accepts a generic block of 
memory, which as you pointed out could be a struct. I like to avoid even 
"char *" when accepting raw memory since it's opinionated about the sign 
of its input.

As I worked, I massaged it into the context of pngattach, adapting it to 
C99 and fixed-width integers. Since I use "unsigned char" consistently 
throughout the rest of the program as an alias for raw PNG bytes, I 
probably should have changed that aspect of its interface as well.

Side note: In the past when processing string data ("char *") as though 
unsigned bytes, such as to decode UTF-8 or do table lookups, I've cast 
"char *" to "unsigned char *". However, I now use "&0xff" on the read 
value to force it to the equivalent unsigned value without the awkward 
pointer cast. Every compiler I've tested handles this perfectly (turns it 
into a zero-extension), so there's no penalty. Technically this requires a 
two's complement representation, but practically speaking, none of my 
programs will ever see a computer that isn't so.
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