yylex

yylex is the lexical analyzer function produced by lexical analyzer generators such as Lex or Flex. It serves as the primary interface between the scanner and a parser (for example, one generated by Yacc or Bison). The function reads the input stream, matches characters against the patterns defined in the scanner, and returns token codes that identify the kind of token found.

In a typical setup with a parser, yylex returns an integer token value corresponding to the token type expected by the parser. If the matched token has a semantic value, the lexical analyzer places that value in a designated location, commonly yylval, before returning the token. The parser uses this value to perform its parsing actions. The scanner also often provides auxiliary information such as the current text of the token (yytext), its length (yyleng), and the current line number (yylineno).

There are differences in interface based on reentrancy. In non-reentrant configurations, yylex has the simple signature int yylex(void) and relies on global state. In reentrant configurations, the interface can involve a scanner object and a pointer to the semantic value, such as int yylex(YYSTYPE *yylval, void *scanner) or a similar form, allowing multiple scanners to operate without sharing global state.

End-of-input behavior is signaled by the scanner returning an end-of-file token, typically 0 or a dedicated YYEOF value. The yylex function is automatically generated from the scanner specification and works in concert with the parser to convert a stream of characters into a stream of tokens for syntactic analysis. See also Lex/Flex, Yacc/Bison, and semantic value handling with YYSTYPE.