11.2 How Can I Reset the Parser

The following phenomenon has several symptoms, resulting in the following typical questions:

I invoke yyparse several times, and on correct input it works properly; but when a parse error is found, all the other calls fail too. How can I reset the error flag of yyparse?

or

My parser includes support for an ‘#include’-like feature, in which case I run yyparse from yyparse. This fails although I did specify ‘%define api.pure full’.

These problems typically come not from Bison itself, but from Lex-generated scanners. Because these scanners use large buffers for speed, they might not notice a change of input file. As a demonstration, consider the following source file, ‘first-line.l’:

%{
#include <stdio.h>
#include <stdlib.h>
%}

%%
.*\n    ECHO; return 1;
%%

int
yyparse (char const *file)
{
  yyin = fopen (file, "r");
  if (!yyin)
    {
      perror ("fopen");
      exit (EXIT_FAILURE);
    }

  /* One token only.  */
  yylex ();
  if (fclose (yyin) != 0)
    {
      perror ("fclose");
      exit (EXIT_FAILURE);
    }
  return 0;
}

int
main (void)
{
  yyparse ("input");
  yyparse ("input");
  return 0;
}

If the file ‘input’ contains

input:1: Hello,
input:2: World!

then instead of getting the first line twice, you get:

$ flex -ofirst-line.c first-line.l
$ gcc  -ofirst-line   first-line.c -ll
$ ./first-line
input:1: Hello,
input:2: World!

Therefore, whenever you change yyin, you must tell the Lex-generated scanner to discard its current buffer and switch to the new one. This depends upon your implementation of Lex; see its documentation for more. For Flex, it suffices to call ‘YY_FLUSH_BUFFER’ after each change to yyin. If your Flex-generated scanner needs to read from several input streams to handle features like include files, you might consider using Flex functions like ‘yy_switch_to_buffer’ that manipulate multiple input buffers.

If your Flex-generated scanner uses start conditions (see (flex)Start conditions section ‘Start conditions’ in The Flex Manual), you might also want to reset the scanner’s state, i.e., go back to the initial start condition, through a call to ‘BEGIN (0)’.

This document was generated by Rick Perry on December 29, 2013 using texi2html 1.82.