2.4.3 The ltcalc Lexical Analyzer.

Until now, we relied on Bison’s defaults to enable location tracking. The next step is to rewrite the lexical analyzer, and make it able to feed the parser with the token locations, as it already does for semantic values.

To this end, we must take into account every single character of the input text, to avoid the computed locations of being fuzzy or wrong:

int
yylex (void)
{
  int c;

  /* Skip white space.  */
  while ((c = getchar ()) == ' ' || c == '\t')
    ++yylloc.last_column;

  /* Step.  */
  yylloc.first_line = yylloc.last_line;
  yylloc.first_column = yylloc.last_column;

  /* Process numbers.  */
  if (isdigit (c))
    {
      yylval = c - '0';
      ++yylloc.last_column;
      while (isdigit (c = getchar ()))
        {
          ++yylloc.last_column;
          yylval = yylval * 10 + c - '0';
        }
      ungetc (c, stdin);
      return NUM;
    }

  /* Return end-of-input.  */
  if (c == EOF)
    return 0;

  /* Return a single char, and update location.  */
  if (c == '\n')
    {
      ++yylloc.last_line;
      yylloc.last_column = 0;
    }
  else
    ++yylloc.last_column;
  return c;
}

Basically, the lexical analyzer performs the same processing as before: it skips blanks and tabs, and reads numbers or single-character tokens. In addition, it updates yylloc, the global variable (of type YYLTYPE) containing the token’s location.

Now, each time this function returns a token, the parser has its number as well as its semantic value, and its location in the text. The last needed change is to initialize yylloc, for example in the controlling function:

int
main (void)
{
  yylloc.first_line = yylloc.last_line = 1;
  yylloc.first_column = yylloc.last_column = 0;
  return yyparse ();
}

Remember that computing locations is not a matter of syntax. Every character must be associated to a location update, whether it is in valid input, in comments, in literal strings, and so on.