Parser - Segmentation fault when calling yytext

Question

My parser is recognizing the grammar and indicating the correct error line using yylineno. I want to print the symbol wich caused the error.

int yyerror(string s)
{
  extern int yylineno;  // defined and maintained in lex.yy.c
  extern char *yytext;  // defined and maintained in lex.yy.c

  cerr << "error: " << s << " -> " << yytext << " @ line " << yylineno << endl;
  //exit(1);
}

I get this error when I write something not acceptable by the grammar:

error: syntax error -> Segmentation fault

Am I not supposed to used yytext? If not what variable contains the symbol that caused the syntax error?

Thanks

Answer 1

Depending on the version of lex you are using, yytext may be an array or may be a pointer. Since it is defined in a different compilation unit, if it is an array and you declare it as a pointer, you won't see any error messages from the compiler or linker (linker generally don't do type checking). Instead it will treat the first several characters in the array as a pointer and try to dereference it and probably crash.

If you are using flex, you can add a %pointer declaration to the first section of your .l file to ensure that it is a pointer and not an array

Answer 2

Are you using lex or flex? If you're using lex,yytext is a char[], not a char*.

EDIT If you aren't using flex you should be, it is superior in every way and has been from the moment of its appearance nearly 30 years ago. lex was obsoleted on that day.