Bison reads a specification in Bison syntax (described as "machine-readable BNF"[3]), warns about any parsing ambiguities, and generates a parser that reads sequences of tokens and decides whether the sequence conforms to the syntax specified by the grammar.

[1] Later, in 1989, Robert Corbett released another parser generator named Berkeley Yacc.

[6] Bison is free software and is available under the GNU General Public License, with an exception (discussed below) allowing its generated code to be used without triggering the copyleft requirements of the licence.

To aid the user in understanding conflicts more intuitively, Bison can instead automatically generate counterexamples.

However, distributing only the input carries the minor inconvenience that the recipients must have a compatible copy of Bison installed so that they can generate the necessary C code when compiling the project.

And distributing only the C code in output, creates the problem of making it very difficult for the recipients to modify the parser since this code was written neither by a human nor for humans - its purpose is to be fed directly into a C compiler.

Some licenses, such as the GPL, require that the source code be in "the preferred form of the work for making modifications to it".

In a language such as C, "int *ptr" denotes the definition of a pointer, not a product: it would be wrong to name this "*" "TOKEN_MULTIPLY".

[23] The data type used for communication, YYSTYPE, is set using Bison %union declaration.

Since in this sample we use the reentrant version of both flex and yacc we are forced to provide parameters for the yylex function, when called from yyparse.