[Fontconfig] Re: incompatible changes in cache format

Mike FABIAN mfabian at suse.de
Mon Feb 20 07:47:10 PST 2006

Patrick Lam <plam at MIT.EDU> さんは書きました:

> Ideally we wouldn't change the cache format once fontconfig becomes
> stable.  But I guess that's not a very satisfactory answer, because we
> might have to change the cache format more than we might like.  One
> option is to increment the cache version number (fonts.cache-3, etc).
>> fcint.h contains a magic number
>>     #define FC_CACHE_MAGIC 0xFC02FC02
>> which appears to be intended for that purpose.
> The magic number just helps determine that it's a fontconfig cache, but
> I suppose it could be changed.  The other thing is that there's a line
> for determining architectures (by recording sizeof various things); this
> line could also have a version number incorporated into it.
> The question, I guess, is how often we're going to have these
> incompatible changes.  If it's not very often, then we increment the
> filename number.  If it's going to be more often than that, we can
> change the magic number.  I'm not really sure which is best.

Dirk Müller <dmueller at suse.de> told me that incrementing the magic
number might cause a problem if some 3rd party software vendors
distribute applications which are statically linked against older
versions of fontconfig.

When calling such statically linked applications and "normal"
applications alternately, the cache files will always be updated
because the magic number is always wrong.

Dirk had the idea that this could be avoided by having a version
number in the architecture signature. You also wrote above that this
is a possibility.

Having the version number in the architecture signature might avoid
the problem with applications statically linked against fontconfig
because those would get their own architecture section.

I don't know whether applications linked statically against fontconfig
exist, i.e. I don't know whether this problem is important in practice
or not.

Mike FABIAN   <mfabian at suse.de>   http://www.suse.de/~mfabian

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