format function attribute syntax 1 __attribute__ (( 2+ , 2.0 format 2.0 __format__ 2.1 ( 2.2 printf 2.2 scanf 2.2 strftime 2.2 strfmon 2.2 __printf__ 2.2 __scanf__ 2.2 __strftime__ 2.2 __strfmon__ 2.3 , string_index , first_to_check ) 2.3.1 ))