Question

I’m attempting to write a Python C extension that reads packed binary data (it is stored as structs of structs) and then parses it out into Python objects. Everything works as expected on a 32 bit machine (the binary files are always written on 32bit architecture), but not on a 64 bit box. Is there a ‘preferred’ way of doing this?

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It would be a lot of code to post but as an example:

struct {     WORD    version;     BOOL    upgrade;     time_t  time1;             time_t  time2; } apparms;  File *fp; fp = fopen(filePath, 'r+b'); fread(&apparms, sizeof(apparms), 1, fp); return Py_BuildValue('{s:i,s:l,s:l}',   'sysVersion',apparms.version,   'powerFailTime', apparms.time1,   'normKitExpDate', apparms.time2  ); 

Now on a 32 bit system this works great, but on a 64 bit my time_t sizes are different (32bit vs 64 bit longs).

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Damn, you people are fast.

Patrick, I originally started using the struct package but found it just way to slow for my needs. Plus I was looking for an excuse to write a Python Extension.

I know this is a stupid question but what types do I need to watch out for?

Thanks.

Answer 1

Explicitly specify that your data types (e.g. integers) are 32-bit. Otherwise if you have two integers next to each other when you read them they will be read as one 64-bit integer.

When you are dealing with cross-platform issues, the two main things to watch out for are:

1. Bitness. If your packed data is written with 32-bit ints, then all of your code must explicitly specify 32-bit ints when reading and writing.
2. Byte order. If you move your code from Intel chips to PPC or SPARC, your byte order will be wrong. You will have to import your data and then byte-flip it so that it matches up with the current architecture. Otherwise 12 (0x0000000C) will be read as 201326592 (0x0C000000).

Hopefully this helps.