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

[Ulrich Eckhardt]

Am 11.06.2013 16:33, schrieb Werner LEMBERG:
> > The documentation for FT_Init_FreeType says "For multi-threading
> > applications each thread should have its own FT_Library object." and
> > I'm wondering how to understand this sentence.  Is this meant as
> > "don't access FT_Library objects concurrently from different
> > threads" or does every thread have to create its own library
> > instance and I can't move objects between threads?  I assume the
> > former, but I'd like to clarify that first.
>
> Actually, it's the latter.  Calls to FreeType are only thread-safe if
> you have one FT_Library object per thread.
>
> > When rendering text, I would
> > 1. Lock the library mutex.
> > 2. If the faces is not cached, load it now and store it in the cache.
> > 3. Then, lock the face mutex and release the library mutex.
> > 4. Load the glyphs and render their outlines.
> > 5. Release the face mutex.
>
> I must admit that I'm bad at multi-threading, but I think this doesn't
> work in general.  For example, here's a comment from one of the
> auto-hinter's (internal) header files, `afloader.h', describing the
> AF_ModuleRec structure:
>
>    The autofitter module's (global) data structure to communicate with
>    actual fonts.  If necessary, `local' data like the current face, the
>    current face's auto-hint data, or the current glyph's parameters
>    relevant to auto-hinting are `swapped in'.  Cf. functions like
>    `af_loader_reset' and `af_loader_load_g'.
>
> In other words, functions related to FT_Face (temporarily) manipulate
> data in the various FreeType font driver, hinting, and renderer
> modules, and those modules are contained in FT_Library.

Okay, so I'll definitely drop the two-level approach. Just to confirm, 
the "autofitter module's (global) data structure" is not really global 
but attached to the library, or? If it is attached to the library, two 
library instances are completely separate, and using them from different 
threads even when using the autofitter will work.


Then, does anything there use thread-local storage or write to global 
data structures? The reason I'm asking is that I don't understand why I 
can't use one library from different threads, even if I can guarantee 
that I'm not using it from different threads *at the same time*. But 
maybe we're just misunderstanding each other.


> > Also, what is still not completely clear to me is when I can use
> > FT_Done_FreeType().  For the faces, it seems that they are
> > reference-counted, but what about the library itself?  Will an early
> > call pull the library from under my feet?
>
> Ah, this is indeed missing in the docs.  While `FT_Done_FreeType'
> simply calls `FT_Done_Library', which *is* reference-counted, it also
> calls `FT_Done_Memory', which is not.
>
> 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.


In any case, many thanks for your help!

Uli

(Please keep me on CC when replying)

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