al_fungetc - Allegro 5 API

       Ungets a single byte from a file.  Pushed-back bytes are not written to the file, only made available for
       subsequent reads, in reverse order.

       The  number  of  pushbacks  depends  on  the  backend.  The standard I/O backend only guarantees a single
       pushback; this depends on the libc implementation.

       For backends that follow the standard behavior, the pushback buffer will be cleared after any seeking  or
       writing;  also  calls to al_fseek(3alleg5) and al_ftell(3alleg5) are relative to the number of pushbacks.
       If  a  pushback  causes  the  position  to  become  negative,  the  behavior  of  al_fseek(3alleg5)   and
       al_ftell(3alleg5) are undefined.

       al_fungetc - Allegro 5 API

al_fgetc(3alleg5), al_get_errno(3alleg5)

Allegro reference manual                                                                     al_fungetc(3alleg5)

              #include <allegro5/allegro.h>

              int al_fungetc(ALLEGRO_FILE *f, int c)