al_ustr_newf - Allegro 5 API

       Create a new string using a printf-style format string.

       Notes:

       The  “%s”  specifier takes C string arguments, not ALLEGRO_USTRs.  Therefore to pass an ALLEGRO_USTR as a
       parameter you must use al_cstr(3alleg5), and it must be  NUL  terminated.   If  the  string  contains  an
       embedded NUL byte everything from that byte onwards will be ignored.

       The  “%c”  specifier outputs a single byte, not the UTF-8 encoding of a code point.  Therefore it is only
       usable for ASCII characters (value <= 127) or if you really mean to output byte values from 128–255.   To
       insert  the  UTF-8 encoding of a code point, encode it into a memory buffer using al_utf8_encode(3alleg5)
       then use the “%s” specifier.  Remember to NUL terminate the buffer.

       al_ustr_newf - Allegro 5 API

al_ustr_new(3alleg5), al_ustr_appendf(3alleg5)

Allegro reference manual                                                                   al_ustr_newf(3alleg5)

              #include <allegro5/allegro.h>

              ALLEGRO_USTR *al_ustr_newf(const char *fmt, ...)