Determining the presence of a font / font fallback

[Urs Liska <ul@xxxxxxxxxxxxxxx>]

Dear list users,

this is my first post to this list, and also I'm completely new to using fontconfig from a programming language. But I have tried to get my head around the function references, and in the project I'm working at there are other people who are experienced with font issues and who couldn't help me either. So I dare to ask here directly.
I apologize in advance for the (not unlikely) case that I'm simply not seeing the obvious.

I am working on a patch for GNU LilyPond (http://lilypond.org) that will make the use of (music and text) fonts significantly easier and more powerful. However, to achieve what I want I need to reliably determine if a font (that is given by its font name) is visible to fontconfig on the user's system. And all of my approaches so far have limitations that are inacceptable (and seem unnecessary). I'll concentrate on one approach that seems the most promising in my eyes.

LilyPond provides a function (ly:font-config-get-font-file <name>) in its C++ part which returns the full path and file name to the font, and the path to a substitution font if the requested font is not found.
However, the substitution font is different on the users' systems, so my initial approach (checking if the returned font is the one *I* was getting as substitution) wasn't successful. However, I can see a way to handle that  by modifying the function's implementation.

The existing function is defined as follows:
http://git.savannah.gnu.org/cgit/lilypond.git/tree/lily/font-config-scheme.cc, line 101ff.

LY_DEFINE (ly_font_config_get_font_file, "ly:font-config-get-font-file", 1, 0, 0,
           (SCM name),
           "Get the file for font @var{name}.")
{
  LY_ASSERT_TYPE (scm_is_string, name, 1);

  FcPattern *pat = FcPatternCreate ();
  FcValue val;

  val.type = FcTypeString;
  val.u.s = (const FcChar8 *)ly_scm2string (name).c_str (); // FC_SLANT_ITALIC;
  FcPatternAdd (pat, FC_FAMILY, val, FcFalse);

  FcResult result;
  SCM scm_result = SCM_BOOL_F;

  FcConfigSubstitute (NULL, pat, FcMatchFont);
  FcDefaultSubstitute (pat);

  pat = FcFontMatch (NULL, pat, &result);
  FcChar8 *str = 0;
  if (FcPatternGetString (pat, FC_FILE, 0, &str) == FcResultMatch)
    scm_result = scm_from_utf8_string ((char const *)str);

  FcPatternDestroy (pat);

  return scm_result;
}

I hope the LilyPond/Guile specific stuff doesn't step in the way of looking at the code.
Despite looking at the documentation of the different Fc... functions and some example code, e.g. on StackOverflow I don't really see how to properly modify that function. My goal would be to set it so *no* replacement font is returned when the requested font isn't found. Basically str should only hold the path and the `if` _expression_ should only evaluate to true when the font is found.
The function as a whole would then either return the string or 'false' (which is no problem with the Guile/Scheme wrappers).

Can you advise me how to achieve that? AFAICS it should be a simple matter of setting the configuration properly. Or is the return of a default font built-in at a deeper level? (I mean running fc-match in the terminal also returns a default for non-present fonts.)
In that case I'd try another approach that *seemed* promising but also had some issues: returning the family name and comparing that to the originally given family name.

Thanks in advance
Urs

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