PCRE, and how to not write an API.
Posted Tue, 03 Jun 2008
From the pcreapi(3) manpage:
The first two-thirds of the vector is used to pass back captured sub- strings, each substring using a pair of integers. The remaining third of the vector is used as workspace by pcre_exec() while matching capturing subpatterns, and is not available for passing back information. The length passed in ovecsize should always be a multiple of three. If it is not, it is rounded down.The 'vector' in question is used by pcre to store offset information for captured groups. It's a good and simple way to figure out where each capture starts and ends.
What doesn't make sense is the portion I put in bold. Why wouldn't pcre_exec simply allocate that scratch space itself? This does not make sense to me. In the mean time, I'm left wondering why I am allocating parts of an array I am told are unusable. I hope there's a good reason. Perhaps some unknown efficiency is gained from doing it this way.
In this case though, I doubt it -- I would guess they're just recursing and reusing that vector as-is, to store the opaque "capturing subpatterns" data. Definite bad code smell off that, if so.