(bison.info)Error Reporting

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The Error Reporting Function `yyerror'

   The Bison parser detects a "parse error" or "syntax error" whenever
it reads a token which cannot satisfy any syntax rule.  A action in the
grammar can also explicitly proclaim an error, using the macro
`YYERROR' (Note: Special Features for Use in Actions.).

   The Bison parser expects to report the error by calling an error
reporting function named `yyerror', which you must supply.  It is
called by `yyparse' whenever a syntax error is found, and it receives
one argument.  For a parse error, the string is normally
`"parse error"'.

   If you define the macro `YYERROR_VERBOSE' in the Bison declarations
section (Note: The Bison Declarations Section.),
then Bison provides a more verbose and specific error message string
instead of just plain `"parse error"'.  It doesn't matter what
definition you use for `YYERROR_VERBOSE', just whether you define it.

   The parser can detect one other kind of error: stack overflow.  This
happens when the input contains constructions that are very deeply
nested.  It isn't likely you will encounter this, since the Bison
parser extends its stack automatically up to a very large limit.  But
if overflow happens, `yyparse' calls `yyerror' in the usual fashion,
except that the argument string is `"parser stack overflow"'.

   The following definition suffices in simple programs:

     yyerror (s)
          char *s;
       fprintf (stderr, "%s\n", s);

   After `yyerror' returns to `yyparse', the latter will attempt error
recovery if you have written suitable error recovery grammar rules
(Note: Error Recovery.).  If recovery is impossible, `yyparse' will
immediately return 1.

   The variable `yynerrs' contains the number of syntax errors
encountered so far.  Normally this variable is global; but if you
request a pure parser (Note: A Pure (Reentrant) Parser.)
then it is a local variable which only the actions can access.

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