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* doc/bison.texinfo (How Can I Reset @code{yyparse}): New.

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Akim Demaille
2003-03-29 13:29:41 +00:00
parent fef4cb511e
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@@ -1,3 +1,7 @@
2003-03-29 Akim Demaille <akim@epita.fr>
* doc/bison.texinfo (How Can I Reset @code{yyparse}): New.
2003-03-13 Paul Eggert <eggert@twinsun.com>
* .cvsignore: Add configure.lineno.
@@ -35,7 +39,7 @@
Quote AC_FOREACH variable-expansions properly.
The 2003-01-03 obstack.h change also needs merging.
{end of changes requiring merging}
* lib/stdbool.h.in, m4/alloca.m4, m4/dirname.m4, m4/dos.m4,
m4/getopt.m4, m4/hash.m4, m4/malloc.m4, m4/memchr.m4,
m4/memrchr.m4, m4/obstack.m4, m4/onceonly.m4, m4/quote.m4,

85
doc/bison.texinfo
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@@ -284,6 +284,7 @@ Invoking Bison
Frequently Asked Questions
* Parser Stack Overflow:: Breaking the Stack Limits
* How Can I Reset @code{yyparse}:: @code{yyparse} Keeps some State
* Strings are Destroyed:: @code{yylval} Loses Track of Strings
Copying This Manual
@@ -6353,6 +6354,7 @@ are addressed.
@menu
* Parser Stack Overflow:: Breaking the Stack Limits
* How Can I Reset @code{yyparse}:: @code{yyparse} Keeps some State
* Strings are Destroyed:: @code{yylval} Loses Track of Strings
@end menu
@@ -6367,6 +6369,89 @@ message. What can I do?
This question is already addressed elsewhere, @xref{Recursion,
,Recursive Rules}.
@node How Can I Reset @code{yyparse}
@section How Can I Reset @code{yyparse}
The following phenomenon gives raise to several incarnations,
resulting in the following typical questions:
@display
I invoke @code{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 @code{yyparse}'s error flag?
@end display
@noindent
or
@display
My parser includes support for a @samp{#include} like feature, in
which case I run @code{yyparse} from @code{yyparse}. This fails
although I did specify I needed a @code{%pure-parser}.
@end display
These problems are not related to Bison itself, but with the 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,
@file{first-line.l}:
@verbatim
%{
#include <stdio.h>
#include <stdlib.h>
%}
%%
.*\n ECHO; return 1;
%%
int
yyparse (const char *file)
{
yyin = fopen (file, "r");
if (!yyin)
exit (2);
/* One token only. */
yylex ();
if (!fclose (yyin))
exit (3);
return 0;
}
int
main ()
{
yyparse ("input");
yyparse ("input");
return 0;
}
@end verbatim
@noindent
If the file @file{input} contains
@verbatim
input:1: Hello,
input:2: World!
@end verbatim
@noindent
then instead of getting twice the first line, you get:
@example
$ @kbd{flex -ofirst-line.c first-line.l}
$ @kbd{gcc -ofirst-line first-line.c -ll}
$ @kbd{./first-line}
input:1: Hello,
input:2: World!
@end example
Therefore, whenever you change @code{yyin}, you must tell the Lex
generated scanner to discard its current buffer, and to switch to the
new one. This depends upon your implementation of Lex, see its
documentation for more. For instance, in the case of Flex, a simple
call @samp{yyrestart (yyin)} suffices after each change to
@code{yyin}.
@node Strings are Destroyed
@section Strings are Destroyed