[Python-Dev] Converting crc32 functions to use unsigned (original) (raw)

Tim Peters tim.peters at gmail.com
Wed May 31 03:24:05 CEST 2006

[Bob Ippolito]

It seems that we should convert the crc32 functions in binascii, zlib, etc. to deal with unsigned integers. Currently it seems that 32- bit and 64-bit platforms are going to have different results for these functions.

binascii.crc32 very deliberately intends to return the same signed integer values on all platforms. That's what this section at the end is trying to do:

#if SIZEOF_LONG > 4 /* Extend the sign bit. This is one way to ensure the result is the * same across platforms. The other way would be to return an * unbounded unsigned long, but the evidence suggests that lots of * code outside this treats the result as if it were a signed 4-byte * integer. */ result |= -(result & (1L << 31)); #endif return PyInt_FromLong(result);

I wouldn't try to fight that, unless that code is buggy (it looks correct to me). I don't remember "the evidence" now, but Barry & I found "lots of code" relying on it at the time ;-)

Should we do the same as the struct module, and do DeprecationWarning when the input value is < 0?

If the result is going to change from sometimes-negative to always-positive, then FutureWarning is most appropriate.

... None of the unit tests seem to exercise values where 32-bit and 64- bit platforms would have differing results, but that's easy enough to fix...

As above, it shouldn't be possible to concoct a case where binascii.crc32's result differ across boxes. That function also intends to treat negative inputs the same way across boxes.

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