GNU bison is a free software parser generator computer program written for the GNU project, and available for virtually all common operating systems. It is mostly compatible with Yacc, and offers several improvements over the earlier program. It is commonly used in conjunction with Flex_lexical_analyser.

GNU bison is not as portable as YACC as you need other GNU tools like GNU_Compiler_Collection to compile it, though not to use it. It is not fully compatible with YACC, and its generated code may not be completely compatible with ANSI C.

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