POSIX (The GNU Awk User’s Guide)

The POSIX Command Language and Utilities standard for awk (1992) introduced the following changes into the language:

The use of -W for implementation-specific options (see Command-Line Options)
The use of CONVFMT for controlling the conversion of numbers to strings (see Conversion of Strings and Numbers)
The concept of a numeric string and tighter comparison rules to go with it (see Variable Typing and Comparison Expressions)
The use of predefined variables as function parameter names is forbidden (see Function Definition Syntax)
More complete documentation of many of the previously undocumented features of the language

In 2012, a number of extensions that had been commonly available for many years were finally added to POSIX. They are:

The fflush() built-in function for flushing buffered output (see Input/Output Functions)
The nextfile statement (see The nextfile Statement)
The ability to delete all of an array at once with ‘delete array’ (see The delete Statement)

See Common Extensions Summary for a list of common extensions not permitted by the POSIX standard.