FcFontSetSort man page
FcFontSetSort — Add to a font set
#include <fontconfig/fontconfig.h> FcFontSetSort (FcConfig *config, FcFontSet **sets, intnsets, FcPattern *pattern, FcBool trim, FcCharSet **csp, FcResult *result);
Returns the list of fonts from sets sorted by closeness to pattern. If trim is FcTrue, elements in the list which don't include Unicode coverage not provided by earlier elements in the list are elided. The union of Unicode coverage of all of the fonts is returned in csp, if csp is not NULL. This function should be called only after FcConfigSubstitute and FcDefaultSubstitute have been called for p; otherwise the results will not be correct.
The returned FcFontSet references FcPattern structures which may be shared by the return value from multiple FcFontSort calls, applications cannot modify these patterns. Instead, they should be passed, along with pattern to FcFontRenderPrepare which combines them into a complete pattern.
The FcFontSet returned by FcFontSetSort is destroyed by calling FcFontSetDestroy.