Using FontConfig to pick a default font.

fontconfig-example.c

/* WARNING -- THE CODE IN THIS FILE DOESN'T DO ANY ERROR HANDLING! */

static const char *TXDI_DEFAULT_FAMILY_NAMES[] = {
   "monospace",
   "sans",
   "serif"
};

TXD_API TxdFont *txd_opendefaultfont(int family, int style, float height, float slant, float outline) {
   FcPattern *pat, *match;
   FcChar8 *path;
   TxdFont *font = NULL;
   FcResult result;

   TXDI_ASSERT(family >= 0 && family < TXD_COUNT_DEFAULT);
   TXDI_ASSERT(height > 0.0f);

   pat = FcPatternCreate();
   FcPatternAddString(pat, "family", (const FcChar8*)TXDI_DEFAULT_FAMILY_NAMES[family]);
   if (style & TXD_STYLE_BOLD) { FcPatternAddInteger(pat, "weight", FC_WEIGHT_BOLD); }
   if (style & TXD_STYLE_ITALIC) { FcPatternAddInteger(pat, "slant", FC_SLANT_ITALIC); }

   FcPatternAddDouble(pat, "dpi", 72.0); /* 72 dpi = 1 pixel per 'point' */
   FcPatternAddDouble(pat, "scale", 1.0);
   FcPatternAddDouble(pat, "size", height);

   FcDefaultSubstitute(pat); /* fill in other expected pattern fields */
   FcConfigSubstitute(0, pat, FcMatchPattern); /* apply any system/user config rules */
   match = FcFontMatch(0, pat, &result); /* find 'best' matching font */

   if (result != FcResultMatch || !match) {
      /* FIXME: better error reporting/handling here...
       * want to minimise the situations where opendefaultfont gives you *nothing* */
      return NULL;
   }

   FcPatternGetString(match, "file", 0, &path);

   font = txd_openfont((const char*)path, height, slant, outline);

   FcPatternDestroy(match);
   FcPatternDestroy(pat);

   return font;
}

/* On my system, this finds:

mono:               /usr/share/fonts/TTF/DejaVuSansMono.ttf
mono, bold:         /usr/share/fonts/TTF/DejaVuSansMono-Bold.ttf
mono, italic:       /usr/share/fonts/TTF/DejaVuSansMono-Oblique.ttf
mono, bold-italic:  /usr/share/fonts/TTF/DejaVuSansMono-BoldOblique.ttf
sans:               /usr/share/fonts/TTF/DejaVuSans.ttf
sans, bold:         /usr/share/fonts/TTF/DejaVuSans-Bold.ttf
sans, italic:       /usr/share/fonts/TTF/DejaVuSans-Oblique.ttf
sans, bold-italic:  /usr/share/fonts/TTF/DejaVuSans-BoldOblique.ttf
serif:              /usr/share/fonts/TTF/DejaVuSerif.ttf
serif, bold:        /usr/share/fonts/TTF/DejaVuSerif-Bold.ttf
serif, italic:      /usr/share/fonts/TTF/DejaVuSerif-Italic.ttf
serif, bold-italic: /usr/share/fonts/TTF/DejaVuSerif-BoldItalic.ttf

Note that the sans-serif fonts have 'oblique' styles, while the serif font has 'italic' styles.

 */