Notes for Flex and Bison

needed for course: practice of compiler design

a simple word count ==> done!

• gcc lex.yy.c -lfl
• actually has a default rule of . ECHO
  • turn off by %option nodefault

%{
int chars = 0;
int words = 0;
int lines = 0;
%}
%%
[a-zA-Z]+ { words++; chars += strlen(yytext); }
\n { chars++; lines++; }
. { chars++; }
%%
main(int argc, char **argv)
{
yylex();
printf("%8d%8d%8d\n", lines, words, chars);
}

BNF grammars

• no need to reduce left resursion?
• bison reports conflict when ambiguity detected
• bison only handle LL(1) or LR(1) grammar?

include bison generated .h file in .l file

well i found this

• http://westes.github.io/flex/manual/
• for reference when implmenting compiler for mini-c
• start states
  • kind of like that in automata
  • diff. state defines diff. scanners

yyterminate: return immediately

only with % yylineno will flex maintain yylineno

• still need to initialize to 1 manually

important(?) regex from pp.38/58

Bison

• for most of the cases, only LALR(1) supported
• no ambiguity, no more than one look ahead char.
• create a union for AST
  • declare which token holds which value
  • and need to assign corresponding value in .l file
• define precedence in bison
  • %left, %right

define user function

• user function also parsed to a AST

left recursive better in bison

• right recursive cause large stack

skip Chap. 4 for SQL parsing