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PCRE, and how to not write an API.

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.