Modern compiler design [PDF]

%{
#include
#include "errors.h" /* Definition of global variable current_line_number, */
/* whose value is set by the lexer */
void yyerror(char *s) {
fprintf(stderr,"line %d, %s \n",current_line_number, s);
}
%}
%token PLUS MINUS TIMES DIVIDE IDENTIFIER
%%
exp : exp PLUS term
| exp MINUS term
| term
term : term TIMES IDENTIFIER
| term DIVIDE IDENTIFIER
| IDENTIFIER
Figure 5.2: A yacc .grm file that deals with errors in a more elegant fashion
5.1.3 Tokens With Values
So far, we have ignored the values of tokens. Many tokens – such as PLUS, MINUS, and IF, do not need
values, while other tokens – such as INTEGER LITERAL and IDENTIFIER, do need values. Our lexical
analyzer actually gave values to all tokens – each token was tagged with the line number on which that token
appeared. While we won’t be using the values of the tokens just yet, we do need to tell yacc which tokens
have values, and what those values are. First, we have to let yacc know what the possible token values are.
We do this with a %union declaration in the yacc definitions segment of the file. For instance, a
%union {
int integer_value;
char *string_value;
}
declaration in the yacc definitions segment of the file tells yacc that some of the tokens will have int
values, and some will have char * values. We now need to tell yacc which of the tokens can have what
values. A simple change to the token declarations does this for us. Consider the .grm file in Figure 5.3. The
token ID has the value char *, the token INTEGER LITERAL has the value int, and all other tokens have
no values.
Figure 5.4 shows a yacc file with that handles tokens with more complicated values, like those in the
programming project.
5.1.4 When Yacc Has Conflicts
Not all grammars are LR(1).Occasionally, when building the LR parse table, yacc will find duplicate entries.
There are two kinds of duplicate entries – shift-reduce conflicts and reduce-reduce conflicts. We will look at
each in turn
Shift-Reduce Conflicts
Shift-reduce conflicts arise when yacc can’t determine if a token should be shifted, or if a reduce should be
done instead. The canonical example of this problem is the dangling else. Consider the following two rules
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