yylval

yylval is a global variable used by the lexical analyzer and parser generated by tools such as Lex/Flex and Yacc/Bison to communicate the semantic value of the currently scanned token. It is of type YYSTYPE, a type defined by the parser (often via a %union declaration in the grammar). Before the parser processes a token, the lexer assigns the appropriate value to yylval and returns the token’s symbolic type.

During parsing, the token’s semantic value is available to the grammar actions through the standard semantic value mechanism, such as the $n and $$ annotations. The lexer typically writes to yylval when it recognizes a token that carries data, for example a numeric literal or an identifier, and then returns the corresponding token type to the parser.

In practice, implementations usually declare extern YYSTYPE yylval (or define YYSTYPE yylval) for the generated parser. The YYSTYPE type is defined by the grammar, and can be a primitive type, a struct, or a union (via %union) to accommodate multiple kinds of semantic data. This design enables the lexer to pass rich information to the parser without requiring additional interfaces.

Relative to modern toolchains, yylval remains the conventional channel for token values, even as parsing modes vary (such as reentrant or pure parsers). The exact interfacing details may differ, but the core idea is that yylval carries the semantic value of the lookahead token from the lexer to the parser.