mkohlhaas / Flex-Bison

Flex Bison

This is the code for the O'Reilly book flex & bison by John Levine.

Original code at ftp://ftp.iecc.com/pub/file/flexbison.zip.

flex and bison website.

• Three sections separated by '%%':
  • Definitions Section
  • Rules Section
  • User Code Section
• flex is not matching my patterns in the same order that I defined them.
  • flex picks the rule that matches the most text (i.e. the longest possible input string).
• yyin
• yyout
• yylex()
• yyparse()
• yytext
• yyleng → lenght of current token
• yywrap() [called when reached EOF, returns non-zero to indicate that no further files need process processing; obsolete] → use: %option noyywrap
• void yyrestart(FILE *new_file) → may be called to point yyin at a new input file
• YY_BUFFER_STATE yy_create_buffer (FILE *file, int size) → used for multiple input files
• yylval
• yyerror() → There is a default implementation provided by the library, but you can write your own.
  • default implementation: int yyerror (char const *);
• yylineno: flex provides this global variable for tracking line numbers. You are responsible for its management.
• %option nodefault → no default rule for unmatched input (recommended!)
• %s or %x (start conditions; inclusive/exclusive): inclusive → rules with no start conditions at all will also be active
  • In effect, the state defines a different scanner, with its own rules.
  • flex always defines INITIAL state
  • Exclusive states are usually more useful.
  • In action code, the macro BEGIN switches to a different start state.
  • Patterns are tagged with start state names in angle brackets to indicate in which state(s) the pattern is active.
• input() reads the next character from the input stream, e.g. to eat up C comments
• There are a number of special directives which can be included within an action:
  • ECHO: copies yytext to the scanner's output.
• yyterminate()
• %option yylineno : The %yylineno option tells flex to define an integer variable called yylineno and to maintain the current line number in it. What that means is that every time the scanner reads a newline character, it increments yylineno, and if the scanner backs up over a newline (using some features we’ll get to later), it decrements it. It’s still up to you to initialize yylineno to 1 at the beginning of each file and to save and restore it if you’re handling include files.
• %option case-insensitive
• An action consisting solely of a vertical bar ("|") tells flex that the action for this rule is the same as the action for the next rule.
• Named patterns; expand with {ame}
• /* ... */ is a comment, // is not!, comment must be indented by at least one space!
• flex: 16K buffer limit
• bison: default action → $$ = $1;
• bison: A %start declaration identifies the top-level rule, so we don’t have to put it at the beginning of the parser.
• bison: e.g. definition of a recursive data structure, e.g. a list:
  • right recursive → stmt ; list rather
  • left recursive → list stmt ;