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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-type macro definitions to be available there. To do this, use the -d option when you run Bison, so that it will write these macro definitions into a separate header file name.tab.h which you can include in the other source files that need it. See Invoking Bison.

• Calling Convention: How yyparse calls yylex.
• Token Values: How yylex must return the semantic value of the token it has read.
• Token Positions: How yylex must return the text position (line number, etc.) of the token, if the actions want that.
• Pure Calling: How the calling convention differs in a pure parser (see A Pure (Reentrant) Parser).