al_ustr_vappendf - Allegro 5 API

       Like  al_ustr_appendf(3alleg5) but you pass the variable argument list directly, instead of the arguments
       themselves.  See al_ustr_newf(3alleg5) about the “%s” and “%c” specifiers.

       Returns true on success, false on error.

       al_ustr_vappendf - Allegro 5 API

al_ustr_appendf(3alleg5), al_ustr_append(3alleg5)

Allegro reference manual                                                               al_ustr_vappendf(3alleg5)

              #include <allegro5/allegro.h>

              bool al_ustr_vappendf(ALLEGRO_USTR *us, const char *fmt, va_list ap)