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4.3 The Lexical Analyzer Function yylex

The lexical analyzer function, yylex, recognizes tokens from the input stream and returns them to the parser. Bison does not create this function automatically; you must write it so that yyparse can call it. The function is sometimes referred to as a lexical scanner.

In simple programs, yylex is often defined at the end of the Bison grammar file. If yylex is defined in a separate source file, you need to arrange for the token-kind definitions to be available there. To do this, use the -d option when you run Bison, so that it will write these definitions into the separate parser header file,, which you can include in the other source files that need it. See Invoking Bison.