Re: [ft] Clarification about FT_Init_FreeType in a multithreaded environment

[Ulrich Eckhardt]

Am 12.06.2013 19:02, schrieb Werner LEMBERG:
> If you can guarantee that you don't render multiple glyphs at the same
> time in parallel, one FT_Library object for all threads certainly
> works.  But this is something which I won't call thread-safe (please
> correct me if I'm mixing up definitions).

I would call that "thread-safe when used correctly". ;) Since rendering 
changes internal state of the library object (raster pool), it must not 
be done concurrently.


> > > So a program with multiple threads should use `FT_New_Library' and
> > > `FT_Done_Library' (instead of `FT_Init_FreeType' and
> > > `FT_Done_FreeType'), providing its own FT_Memory object.
> >
> > Okay...?  I must admit that I don't fully understand this part of
> > your answer yet, probably because I don't understand FreeType's
> > structure enough.  It seems that this also influences my other
> > approaches above, I'll have to verify that.
>
> `With multiple threads' implies parallel access of FreeType.  I would
> be glad if you could formulate this better!

Maybe I can help out here. The sentence that confused me was:

   For multi-threading applications each thread should have
   its own FT_Library object.

which I would replace with:

   In multi-threaded applications, you must make sure that
   you don't access the same FT_Library object or any of
   its children in parallel.

Actually, this paragraph in CHANGES sums it up, too:

    - thread synchronisation  has been dropped, for  the simple reason
      that the library  is already re-entrant, and that  if you really
      need  two  threads accessing  the  same  FT_Library, you  should
      really synchronize access to it yourself with a simple mutex.

I added the reference to the children because the FT_Library object is 
only one handle into a whole tree of objects (see diagram at the end of 
http://www.freetype.org/freetype2/docs/design/design-4.html), but other 
objects (faces, glyphs) are also part of the library and must not be 
used in parallel either (right?).


Thank you for your help!

Uli


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