This manual is for @acronym{GNU} Bison (version @value{VERSION},
@value{UPDATED}), the @acronym{GNU} parser generator.
-Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 2003,
+Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
@quotation
Frequently Asked Questions
* Parser Stack Overflow:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* C++ Parsers:: Compiling Parsers with C++ Compilers
+* Implementing Loops:: Control Flow in the Calculator
Copying This Manual
Declare that the @var{code} must be invoked for each of the
@var{symbols} that will be discarded by the parser. The @var{code}
should use @code{$$} to designate the semantic value associated to the
-@var{symbols}. The additional parser parameters are also avaible
+@var{symbols}. The additional parser parameters are also available
(@pxref{Parser Function, , The Parser Function @code{yyparse}}).
@strong{Warning:} as of Bison 1.875, this feature is still considered as
@menu
* Parser Stack Overflow:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* C++ Parsers:: Compiling Parsers with C++ Compilers
+* Implementing Loops:: Control Flow in the Calculator
@end menu
@node Parser Stack Overflow
This question is already addressed elsewhere, @xref{Recursion,
,Recursive Rules}.
+@node How Can I Reset the Parser
+@section How Can I Reset the Parser
+
+The following phenomenon has several symptoms, 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 the error flag of @code{yyparse}?
+@end display
+
+@noindent
+or
+
+@display
+My parser includes support for an @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 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,
+@file{first-line.l}:
+
+@verbatim
+%{
+#include <stdio.h>
+#include <stdlib.h>
+%}
+%%
+.*\n ECHO; return 1;
+%%
+int
+yyparse (char const *file)
+{
+ yyin = fopen (file, "r");
+ if (!yyin)
+ exit (2);
+ /* One token only. */
+ yylex ();
+ if (fclose (yyin) != 0)
+ exit (3);
+ return 0;
+}
+
+int
+main (void)
+{
+ 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 the first line twice, 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 switch to the
+new one. This depends upon your implementation of Lex; see its
+documentation for more. For Flex, it suffices to call
+@samp{YY_FLUSH_BUFFER} after each change to @code{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 @samp{yy_switch_to_buffer} that manipulate multiple
+input buffers.
+
@node Strings are Destroyed
@section Strings are Destroyed
@display
-My parser seems to destroy old strings, or maybe it losses track of
+My parser seems to destroy old strings, or maybe it loses track of
them. Instead of reporting @samp{"foo", "bar"}, it reports
@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
@end display
@end example
+@node C++ Parsers
+@section C++ Parsers
+
+@display
+How can I generate parsers in C++?
+@end display
+
+We are working on a C++ output for Bison, but unfortunately, for lack
+of time, the skeleton is not finished. It is functional, but in
+numerous respects, it will require additional work which @emph{might}
+break backward compatibility. Since the skeleton for C++ is not
+documented, we do not consider ourselves bound to this interface,
+nevertheless, as much as possible we will try to keep compatibility.
+
+Another possibility is to use the regular C parsers, and to compile
+them with a C++ compiler. This works properly, provided that you bear
+some simple C++ rules in mind, such as not including ``real classes''
+(i.e., structure with constructors) in unions. Therefore, in the
+@code{%union}, use pointers to classes, or better yet, a single
+pointer type to the root of your lexical/syntactic hierarchy.
+
+
+@node Implementing Loops
+@section Implementing Loops
+
+@display
+My simple calculator supports variables, assignments, and functions,
+but how can I implement loops?
+@end display
+
+Although very pedagogical, the examples included in the document blur
+the distinction to make between the parser---whose job is to recover
+the structure of a text and to transmit it to subsequent modules of
+the program---and the processing (such as the execution) of this
+structure. This works well with so called straight line programs,
+i.e., precisely those that have a straightforward execution model:
+execute simple instructions one after the others.
+
+@cindex abstract syntax tree
+@cindex @acronym{AST}
+If you want a richer model, you will probably need to use the parser
+to construct a tree that does represent the structure it has
+recovered; this tree is usually called the @dfn{abstract syntax tree},
+or @dfn{@acronym{AST}} for short. Then, walking through this tree,
+traversing it in various ways, will enable treatments such as its
+execution or its translation, which will result in an interpreter or a
+compiler.
+
+This topic is way beyond the scope of this manual, and the reader is
+invited to consult the dedicated literature.
+
+
+
@c ================================================= Table of Symbols
@node Table of Symbols
@printindex cp
@bye
+
+@c LocalWords: texinfo setfilename settitle setchapternewpage finalout
+@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex
+@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry
+@c LocalWords: ifset vskip pt filll insertcopying sp ISBN Etienne Suvasa
+@c LocalWords: ifnottex yyparse detailmenu GLR RPN Calc var Decls Rpcalc
+@c LocalWords: rpcalc Lexer Gen Comp Expr ltcalc mfcalc Decl Symtab yylex
+@c LocalWords: yyerror pxref LR yylval cindex dfn LALR samp gpl BNF xref
+@c LocalWords: const int paren ifnotinfo AC noindent emph expr stmt findex
+@c LocalWords: glr YYSTYPE TYPENAME prog dprec printf decl init stmtMerge
+@c LocalWords: pre STDC GNUC endif yy YY alloca lf stddef stdlib YYDEBUG
+@c LocalWords: NUM exp subsubsection kbd Ctrl ctype EOF getchar isdigit
+@c LocalWords: ungetc stdin scanf sc calc ulator ls lm cc NEG prec yyerrok
+@c LocalWords: longjmp fprintf stderr preg yylloc YYLTYPE cos ln
+@c LocalWords: smallexample symrec val tptr FNCT fnctptr func struct sym
+@c LocalWords: fnct putsym getsym fname arith fncts atan ptr malloc sizeof
+@c LocalWords: strlen strcpy fctn strcmp isalpha symbuf realloc isalnum
+@c LocalWords: ptypes itype YYPRINT trigraphs yytname expseq vindex dtype
+@c LocalWords: Rhs YYRHSLOC LE nonassoc op deffn typeless typefull yynerrs
+@c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES
+@c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param
+@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP
+@c LocalWords: YYEMPTY YYRECOVERING yyclearin GE def UMINUS maybeword
+@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
+@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype
+@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args
+@c LocalWords: YYPRINTF infile ypp yxx outfile itemx vcg tex leaderfill
+@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll
+@c LocalWords: yyrestart nbar yytext fst snd osplit ntwo strdup AST
+@c LocalWords: YYSTACK DVI fdl printindex
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