The C Programming Language, 2nd Edition, by Kernighan and Ritchie

The Meanings of the Solution Category Numbers For Exercise Solutions As Used In This Wiki:

0	ANSI/ISO C89 compliant. The example only uses the subset of C already covered at the point in the book at which the exercise appears.
1	ANSI/ISO C89 compliant. The example uses aspects of C which may not have been covered at the point in the book at which the exercise appears.
2	ANSI/ISO C99 compliant. The example only uses the subset of C already covered at the point in the book at which the exercise appears. It is compliant with the C99 standard (e.g. doesn't use implicit int ).
3	ANSI/ISO C99 compliant. The example uses some aspects of C which exist only in the C99 release of the language.