KOI: Lab Exercise 3

Resources:

• Thomas Niemann: A Compact Guide to Lex & Yacc is a good introduction to Yacc.
• PC versions of Bison and Flex can be downloaded from the same site. Click on Overview in the navigation bar to the left, and then choose "my version of Lex and Yacc". (Or use these local copies of the executables and the source: gnu.zip, gnusrc.zip.)
• Lecture 5 in this course was about Yacc.

About this version of Yacc

Assuming that you unzip gnu.zip in a directory called M:\Yacc, then the Bison executable will be called

M:\Yacc\bin\bison.exe

path=M:\Yacc\bin;%path%

bison -S "M:\Yacc\bin\bison.simple"

set BISON_SIMPLE=M:\Yacc\bin\bison.simple

bison -d language.y

Some more things to do:

• You must declare yyerror in the "definitions" part of the Yacc input file: extern void yyerror(char*);
• We must also define yyerror somewhere in the program, and a good place to do that is in the "subroutines" part of the Yacc input file.
• Yacc expects the scanner to be called yylex. Define such a function (again, in the "subroutines" part of the Yacc input file), and let it call the old scanner function, lexan.
• The parser function generated by Yacc will be called yyparse, but the program in which we will plug it in expects the parser function to be called parse. One way to handle this is to define a parse function, which just calls yyparse.
• Yacc generates its own token codes for ID, NUM etc. Obviously, the scanner must use the same token codes, and not the (different) ones that we used before. Therefore you should change the header file global.h, and replace the old definitions of token codes with an #include of the Yacc-generated header file language.tab.h.

Part A: The calculator

With a hand-coded parser, it was difficult to handle both assignments and expressions. Ley your Yacc grammar handle both, and see how easy it is!

Part B: More operators

• % (a synonym for mod)
• & (bitwise and)
• | (bitwise or)
• ?: (as in expr1 ? expr2 : expr3)
• <
• >

In C and in C++, the ?: operator only evaluates one of the expressions expr2 and expr3. Let your operator evaluate both expr2 and expr3

Part C: Yacc and C++

The original Yacc was designed to accept C code in the semantic actions, but later versions, such as Bison, also allows C++. The output from Bison, called for example language.tab.c, must be renamed to for example language.tab.cpp. (Some versions of Yacc, for example Bison version 1.28 for Linux, automatically genrates a file with a C++ extension if the input file ends with .yy instead of .y, but the PC Bison used in the labs doesn't.)

For everything to compile cleanly, you may have to add some declarations in the definitions part of the Yacc input file:

%{
  #include <stdlib.h>
  #include "global.h"
  extern int tokenval;
  extern void yyerror(char*);
  extern int yylex();
  static void *alloca(size_t size) { return malloc(size); }
%}

To avoid linking problems when mixing C and C++ in Borland C++, change all other C files that your program uses (main.c, lexer.c etc) to have C++ extensions (main.cpp, lexer.cpp etc).

(And yes, C and C++ are two different languages. You have to be a bit careful if you mix them in the same project.)

Report