[Python-Dev] xreadlines : readlines :: xrange : range

[Guido van Rossum]

> So it's probably a wash.  In that case, do we want to maintain two hacks for
> this?  I can't use the FLOCKFILE/etc approach on Windows, while "the
> Windows" approach probably works everywhere (although its speed relies on
> the platform factoring out at least the locking/unlocking in fgets).

I'm much more confident about the getc_unlocked() approach than about
fgets() -- with the latter we need much more faith in the C library
implementers.  (E.g. that fgets() never writes beyond the null bytes
it promises, and that it locks/unlocks only once.)  Also, you're
relying on blindingly fast memchr() and memset() implementations.

> Both methods lack a refinement I would like to see, but can't achieve in
> "the Windows way":  ensure that consistency is on no worse than a per-line
> basis.  [Example omitted]

The only portable way to ensure this that I can see, is to have a
separate mutex in the Python file object.  Since this is hardly a
common thing to do, I think it's better to let the application manage
that lock if they need it.

(Then why are we bothering with flockfile(), you may ask?  Because
otherwise, accidental multithreaded reading from the same file could
cause core dumps.)

--Guido van Rossum (home page: http://www.python.org/~guido/)