Compiling Yacc code - yacc

Question

Below is my yacc code to parse C source code. I am a little new to this and this is an already existing code.
{
%{
#include <stdio.h>
#include <string.h>
#include "Expression.c"
%}
%token Identifier
%token Number
%token '=' '+' '-' '*' '/' ',' ';' '(' ')' '{' '}' '[' ']' '<' '>'
%token INT
%token CHAR
%token FLOAT
%token LONG
%token DOUBLE
%token RETURN
%token IF
%token ELSE
%token EQ /* == */
%token BADTOKEN
%%
program
: function
{ $$ = $1; }
| program function
{ $$ = binaryNode("",$1,$2);}
| error '}'
function:
typename Identifier '(' formal.arguments ')' function.body
{ $$ = attachAllChildren($2,$1,$4,$6); }
typename
: INT
{ $$ = leafNode("INT");}
| CHAR
{ $$ = leafNode("CHAR"); }
| DOUBLE
{ $$ = leafNode("DOUBLE"); }
| LONG
{ $$ = leafNode("LONG"); }
| FLOAT
{ $$ = leafNode("FLOAT"); }
formal.arguments
: /* empty */
{ $$ = NULL; }
| formal.argument.list
{ $$ = $1; }
formal.argument.list
: formal.argument
{ $$ = $1; }
| formal.argument.list ',' formal.argument
{ $$ = binaryNode(",", $1, $3); }
formal.argument
: typename Identifier
{ $$ = attachChild($2, $1); }
function.body
: '{' '}'
{ $$ = NULL; }
| '{' statements '}'
{ $$ = $2; }
statements
: statement
{ $$ = $1; }
| statements statement
{ $$ = attachChild($1,$2);}
statement
: declaration
{ $$ = $1; }
| RETURN expression ';' /* return statement */
{ $$ = unaryNode("RETURN", $2); }
| if.statement
{ $$ =$1; }
| term '=' expression ';' /* assignment */
{ $$ = binaryNode("=", $1, $3); }
| expression ';'
{ $$ = $1; }
| '{' statements '}'
{ $$ = $2; }
| ';' /* null statement */
{ $$ = NULL; }
declaration
: typename Identifier ';'
{ $$ = attachChild($2,$1); }
| typename Identifier '[' Number ']' ';' /* array */
{ $$ = attachSiblings($2, $1, $4); }
if.statement
: IF '(' expression ')' statement
{ $$ = ternaryNode("IF",$3,$5, NULL); }
| IF '(' expression ')' statement ELSE statement
{ $$ = ternaryNode("IF", $3, $5, $7); }
expression
: additive.expression
{ $$ = $1; }
| expression EQ additive.expression
{ $$ = binaryNode("=",$1, $3); }
| expression '>' additive.expression
{ $$ = binaryNode(">", $1, $3); }
| expression '<' additive.expression
{ $$ = binaryNode("<", $1, $3); }
additive.expression
: term
{ $$ = $1; }
| additive.expression '+' term
{ $$ = binaryNode("+", $1, $3);}
| additive.expression '-' term
{ $$ = binaryNode("-", $1, $3);}
term
: Identifier
{ $$ = leafNode($1);}
| Number
{ $$ = leafNode($1);}
| Identifier '(' opt.actual.arguments ')' /* function call */
{ $$ = attachChild($1,$3);}
| Identifier '[' expression ']' /* array access */
{ $$ = attachChild($1,$3); }
| '(' expression ')'
{ $$ = $2;}
opt.actual.arguments
: /* empty */
{ $$ = NULL;}
| actual.arguments
{ $$=$1; }
actual.arguments
: expression
{ $$ = $1; }
| actual.arguments ',' expression
{ $$ = binaryNode(",",$1, $3); }
%%
yyerror(msg)
char* msg;
{
#if !defined(YYBISON)
extern int yynerrs;
++yynerrs;
#endif
fprintf(stderr, "Error: %s\n",msg);
}
main()
{
extern int yynerrs;
yyparse();
fprintf(stderr, "%d errors.\n", yynerrs);
return 0;
}
}
On compiling the above code , I get a warning that there is 1 shift/reduce conflict in the code. How can I resolve this?

Answer 1

Use the -v option to yacc, which will produce a y.output file telling you where the conflict is and how its is triggered. Note that a conflict is NOT an error -- you still get a valid parser from yacc -- but that parser may not recognize exactly the language defined by your grammar.
In your case, you get something like:
State 81 conflicts: 1 shift/reduce
:
State 81
28 if.statement: IF '(' expression ')' statement .
29 | IF '(' expression ')' statement . ELSE statement
ELSE shift, and go to state 83
ELSE [reduce using rule 28 (if.statement)]
$default reduce using rule 28 (if.statement)
which tells you that you have the classic dangling else ambiguity, so you can probably just ignore the conflict, as the generated parser will resolve the ambiguity by binding the else to the closest if, which is probably what you want.