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 ))