typedef struct a_init_pre_filter {
   a_text_source          *in_text_source;   /* IN */
   a_text_source          *out_text_source;  /* OUT */
   const char             *desired_charset;  /* IN */
   char                   *actual_charset;   /* OUT */
   short                  is_binary;         /* IN */
} a_init_pre_filter;