How to prevent default "syntax error" in Bison

Question

As described in the header, I am using Bison and Flex to get a parser, yet I need to handle the error and continue after I find one. Thus I use:

Stmt:   Reference '=' Expr ';'                                { printf(" Reference = Expr ;\n");}
|       '{' Stmts '}'                                         { printf("{ Stmts }");}
|       WHILE '(' Bool ')' '{' Stmts '}'                      { printf(" WHILE ( Bool ) { Stmts } ");}
|       FOR NAME '=' Expr TO Expr BY Expr '{' Stmts '}'       { printf(" FOR NAME = Expr TO Expr BY Expr { Stmts } ");}
|       IF '(' Bool ')' THEN Stmt                             { printf(" IF ( Bool ) THEN Stmt ");}
|       IF '(' Bool ')' THEN Stmt ELSE Stmt                   { printf(" IF ( Bool ) THEN Stmt ELSE Stmt ");}
|       READ Reference ';'                                    { printf(" READ Reference ;");}
|       WRITE Expr ';'                                        { printf(" WRITE Expr ;");}
|       error ';'                                             { yyerror("Statement is not valid"); yyclearin; yyerrok;}
;

however, I always get a msg "syntax error" and I do not know where does it come from and how to prevent it so that my own "error code" will be executed. I am trying to do an error recovery here so that my parser will continue to parse the input till the EOF.

Chris Dodd Chris Dodd · Accepted Answer · 2020-03-11T07:24:49

People often confuse the purpose of error rules in yacc/bison -- they are for error RECOVERY, not for error HANDLING. So an error rule is not called in response to an error -- the error happens and then the error rule is used to recover.

If you want to handle the error yourself (so avoid printing a "syntax error" message), you need to define your own yyerror function (that is the error handler) that does something with "syntax error" string other than printing it. One option is to do nothing, and then print a message in your error recovery rule (eg, where you call yyerror, change it to printf instead). The problem being that if error recovery fails, you won't get any message (you will get a failure return from yyparse, so could print a message there).

How to prevent default "syntax error" in Bison

1 Answers