newlib ML を探してみたら、どうやら %a は対応済みらしい。
2007-05-11 Eric Blake <ebb9@byu.net> Minimize printf/scanf size on platforms that don't need C99. * acconfig.h (_WANT_IO_C99_FORMATS): New macro. * newlib.hin (_WANT_IO_C99_FORMATS): Likewise. * configure.in (newlib-io-c99-formats): New configure option. (_WANT_IO_C99_FORMATS): Define appropriately. * configure.host (*-linux*, cygwin): Default c99-formats to yes. * libc/stdio/vfprintf.c (_VFPRINTF_R) [!_WANT_IO_C99_FORMATS]: Cripple ' flag; hh, z, j, t sizes; a, A, F, C, S specifiers. * libc/stdio/vfscanf.c (_VFSCANF_R) [!_WANT_IO_C99_FORMATS]: Likewise. * configure: Regenerate.
じゃあ、なんで %a を認識しないんだ?