Re: [ft-devel] thread and non-thread safe issues

[Werner LEMBERG]

> I'm currently using freetype to create a simple demo of my further
> project, and I've noticed that I cannot use FTC_ImageCache_Lookup
> and FTC_Manager related functions at same time in different threads.
> It is documented that we can't operate ft_library or some other
> struct simultaneously, but I didn't find a conclusion or some kind
> of multithread programming guideline document that figures out all
> thread safety issues.  Can you please give a note on that?

We have the following in the CHANGES file for version 2.6.

    - Behdad  Esfahbod contributed  code  for improved  thread-safety,
      which results in the following model.

      * An `FT_Face' object can only be safely used from one thread at
        a time.

      * An `FT_Library'  object can  now be used  without modification
        from multiple threads at the same time.

      * `FT_Face' creation and destruction  with the same `FT_Library'
        object can only be done from one thread at a time.

      One can use a single  `FT_Library' object across threads as long
      as a mutex lock is used around `FT_New_Face' and `FT_Done_Face'.
      Any calls to `FT_Load_Glyph' and similar API are safe and do not
      need the lock  to be held as  long as the same  `FT_Face' is not
      used from multiple threads at the same time.

Note that I have no experience in thread safety code.  However, since
the cache code is essentially based on FT_Face objects, I believe that
you have to use separate caches for different threads.  Maybe this can
be improved, but unfortunately I'm the wrong guy to do this.


    Werner