from scanf(3):
The a modifier is not available if the program is compiled with
gcc -std=c99 or gcc -D_ISOC99_SOURCE (unless _GNU_SOURCE
is also specified), in which case the a is interpreted
as a specifier for floating-point numbers (see above).

Since version 2.7, glibc also provides the m modifier for the
same purpose as the a modifier. The m modifier has the following
advantages:
* It may also be applied to %c conversion specifiers (e.g., %3mc).
* It avoids ambiguity with respect to the %a floating-point
  conversion specifier (and is unaffected by gcc -std=c99 etc.)
* It is specified in the upcoming revision of the POSIX.1 standard.