blob: f37440228fad8a04d4093d9891c36bfb84a24ccc [file] [log] [blame]
** Bug fixes
Warnings about uninitialized yylloc in yyparse have been fixed.
Restored C90 compliance (yet no report was ever made).
** Diagnostics are improved
*** Changes in the format of error messages
This used to be the format of many error reports:
input.y:2.7-12: %type redeclaration for exp
input.y:1.7-12: previous declaration
It is now:
input.y:1.7-12: previous declaration
*** New format for error reports: carets
Caret errors have been added to Bison:
%type <sval> exp
input.y:1.7-12: previous declaration
%type <ival> exp
The default behaviour for now is still not to display these unless
** New value for %define variable: api.pure full
"%define api.pure full".
** New %define variable: api.location.type (,,
for locations. When defined, Bison no longer generates the position.hh
then responsible to define her type.
under the name "location_type" (which is maintained for backward
For consistency,'s %define variables location_type and
position_type are deprecated in favor of api.location.type and
** Exception safety (
The parse function now catches exceptions, uses the %destructors to
before re-throwing the exception.
This feature is somewhat experimental. User feedback would be
** Graph improvements in DOT and XSLT
numbered and left-justified.
The reductions are now explicitly represented as transitions to other
diamond shaped nodes.
processing, with minor (documented) differences.
** %language is no longer an experimental feature.
** Documentation
The sections about shift/reduce and reduce/reduce conflicts resolution
have been fixed and extended.
Although introduced more than four years ago, XML and Graphviz reports
were not properly documented.
The translation of mid-rule actions is now described.
We consider compiler warnings about Bison generated parsers to be bugs.
Rather than working around them in your own project, please consider
reporting them to us.
** Bug fixes
Other issues in the test suite have been addressed.
Nul characters are correctly displayed in error messages.
When possible, yylloc is correctly initialized before calling yylex. It
** Bug fixes
Bugs and portability issues in the test suite have been fixed.
users to the appropriate place to report them.
Stray Info files shipped by accident are removed.
generated, are removed.
All the generated headers are self-contained.
** Header guards (yacc.c, glr.c,
In order to avoid collisions, the header guards are now
For instance the header generated from
%define api.prefix "calc"
%defines "lib/parse.h"
will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
warnings such as:
input.c: In function 'yyparse':
function [-Wmaybe-uninitialized]
*++yyvsp = yylval;
Warnings from clang ("equality comparison with extraneous parentheses" and
** Bug fixes
suite have been fixed.
invalid C++. This is fixed.
** Spurious spaces and end-of-lines
** Future Changes
** Bug fixes
*** Type names are now properly escaped.
*** set_debug_level and debug_level work as expected.
*** Stray @ or $ in actions
now does.
** Type names in actions
will display two values for each typed and untyped symbol (provided
** Future Changes
The next major release of Bison will drop support for the following
deprecated features. Please report disagreements to
*** K&R C parsers
*** Features deprecated since Bison 1.875
%lex-param, will no longer be supported.
Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
*** The generated header will be included (yacc.c)
Instead of duplicating the content of the generated header (definition of
** Generated Parser Headers
*** Guards (yacc.c, glr.c,
parsers ( For instance, with --defines=foo.h:
#ifndef YY_FOO_H
# define YY_FOO_H
#endif /* !YY_FOO_H */
*** New declarations (yacc.c, glr.c)
The generated header now declares yydebug and yyparse. Both honor
--name-prefix=bar_, and yield
int bar_parse (void);
rather than
#define yyparse bar_parse
int yyparse (void);
in order to facilitate the inclusion of several parser headers inside a
single compilation unit.
*** Exported symbols in C++
The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
generated headers from a single compilation unit.
longer defined.
** New %define variable: api.prefix
Now that the generated headers are more complete and properly protected
against multiple inclusions, constant names, such as YYSTYPE are a
would introduce backward compatibility issues in projects not expecting
The following examples compares both:
%token <ival> FOO %token <ival> FOO
%% %%
exp: 'a'; exp: 'a';
bison generates:
#ifndef BAR_FOO_H #ifndef BAR_FOO_H
# define BAR_FOO_H # define BAR_FOO_H
# ifndef YYDEBUG | # ifndef BAR_DEBUG
> # if defined YYDEBUG
> # if YYDEBUG
> # define BAR_DEBUG 1
> # else
> # define BAR_DEBUG 0
> # endif
> # else
# define YYDEBUG 0 | # define BAR_DEBUG 0
> # endif
# endif | # endif
extern int bar_debug; extern int bar_debug;
# endif # endif
/* Tokens. */ /* Tokens. */
enum yytokentype { | enum bar_tokentype {
FOO = 258 FOO = 258
}; };
# endif # endif
typedef union YYSTYPE | typedef union BAR_STYPE
{ {
int ival; int ival;
#endif #endif
extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
int bar_parse (void); int bar_parse (void);
** Future changes:
** yacc.c: YYBACKUP works as expected.
** glr.c improvements:
*** Location support is eliminated when not requested:
not requested, and therefore not even usable.
*** __attribute__ is preserved:
when -std is passed to GCC).
** several fixes:
The Java parser no longer throws ArrayIndexOutOfBoundsException if the
** Changes for C++:
*** C++11 compatibility:
or higher.
*** Header guards
name for preprocessor guards, for instance:
case characters are converted to upper case, and series of
non-alphanumerical characters are converted to an underscore.
*** C++ locations:
The position and location constructors (and their initialize methods)
accept new arguments for line and column. Several issues in the
documentation were fixed.
** Changes in the manual:
*** %printer is documented
*** Several improvements have been made:
Named references are motivated. The description of the automaton
index entries were fixed. Some other errors were fixed.
** Building bison:
*** Conflicting prototypes with recent/modified Flex.
some modified versions of 2.5.35, which have modified function prototypes.
*** Warnings during the build procedure have been eliminated.
This includes warnings with some compilers, unexpected behavior of tools
*** The install-pdf target works properly:
halts in the middle of its course.
* Changes in version 2.5 (2011-05-14):
** Grammar symbol names can now contain non-initial dashes:
Consistently with directives (such as %error-verbose) and with
%define variables (e.g. push-pull), grammar symbol names may contain
dashes in any position except the beginning. This is a GNU
extension over POSIX Yacc. Thus, use of this extension is reported
by -Wyacc and rejected in Yacc mode (--yacc).
** Named references:
Historically, Yacc and Bison have supported positional references
($n, $$) to allow access to symbol values from inside of semantic
actions code.
Starting from this version, Bison can also accept named references.
When no ambiguity is possible, original symbol names may be used
as named references:
if_stmt : "if" cond_expr "then" then_stmt ';'
{ $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
In the more common case, explicit names may be declared:
stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
{ $res = mk_if_stmt($cond, $then, $else); }
Location information is also accessible using @name syntax. When
accessing symbol names containing dots or dashes, explicit bracketing
($[sym.1]) must be used.
These features are experimental in this version. More user feedback
will help to stabilize them.
** IELR(1) and canonical LR(1):
IELR(1) is a minimal LR(1) parser table generation algorithm. That
because canonical LR(1)'s extra parser states may contain duplicate
significantly reduce the complexity of developing of a grammar.
Bison can now generate IELR(1) and canonical LR(1) parser tables in
place of its traditional LALR(1) parser tables, which remain the
default. You can specify the type of parser tables in the grammar
file with these directives:
%define lr.type lalr
%define lr.type ielr
%define lr.type canonical-lr
The default-reduction optimization in the parser tables can also be
adjusted using "%define lr.default-reductions". For details on both
of these features, see the new section "Tuning LR" in the Bison
These features are experimental. More user feedback will help to
stabilize them.
** LAC (Lookahead Correction) for syntax error handling:
Canonical LR, IELR, and LALR can suffer from a couple of problems
upon encountering a syntax error. First, the parser might perform
additional parser stack reductions before discovering the syntax
error. Such reductions can perform user semantic actions that are
unexpected because they are based on an invalid token, and they
cause error recovery to begin in a different syntactic context than
the one in which the invalid token was encountered. Second, when
verbose error messages are enabled (with %error-verbose or the
obsolete "#define YYERROR_VERBOSE"), the expected token list in the
syntax error message can both contain invalid tokens and omit valid
The culprits for the above problems are %nonassoc, default
reductions in inconsistent states, and parser state merging. Thus,
IELR and LALR suffer the most. Canonical LR can suffer only if
%nonassoc is used or if default reductions are enabled for
inconsistent states.
LAC is a new mechanism within the parsing algorithm that solves
these problems for canonical LR, IELR, and LALR without sacrificing
%nonassoc, default reductions, or state merging. When LAC is in
use, canonical LR and IELR behave almost exactly the same for both
syntactically acceptable and syntactically unacceptable input.
While LALR still does not support the full language-recognition
power of canonical LR and IELR, LAC at least enables LALR's syntax
error handling to correctly reflect LALR's language-recognition
Currently, LAC is only supported for deterministic parsers in C.
You can enable LAC with the following directive:
%define parse.lac full
See the new section "LAC" in the Bison manual for additional
details including a few caveats.
LAC is an experimental feature. More user feedback will help to
stabilize it.
** %define improvements:
*** Can now be invoked via the command line:
Each of these command-line options
is equivalent to this grammar file declaration
%define NAME ["VALUE"]
except that the manner in which Bison processes multiple definitions
for the same NAME differs. Most importantly, -F and --force-define
quietly override %define, but -D and --define do not. For further
details, see the section "Bison Options" in the Bison manual.
*** Variables renamed:
The following %define variables
have been renamed to
The old names are now deprecated but will be maintained indefinitely
for backward compatibility.
*** Values no longer need to be quoted in the grammar file:
If a %define value is an identifier, it no longer needs to be placed
within quotations marks. For example,
%define api.push-pull "push"
can be rewritten as
%define api.push-pull push
*** Unrecognized variables are now errors not warnings.
*** Multiple invocations for any variable is now an error not a warning.
** Unrecognized %code qualifiers are now errors not warnings.
** Character literals not of length one:
Previously, Bison quietly converted all character literals to length
one. For example, without warning, Bison interpreted the operators in
the following grammar to be the same token:
exp: exp '++'
| exp '+' exp
Bison now warns when a character literal is not of length one. In
some future release, Bison will start reporting an error instead.
** Destructor calls fixed for lookaheads altered in semantic actions:
Previously for deterministic parsers in C, if a user semantic action
altered yychar, the parser in some cases used the old yychar value to
determine which destructor to call for the lookahead upon a syntax
error or upon parser return. This bug has been fixed.
** C++ parsers use YYRHSLOC:
Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
macro and use it in the default YYLLOC_DEFAULT. You are encouraged
to use it. If, for instance, your location structure has "first"
and "last" members, instead of
# define YYLLOC_DEFAULT(Current, Rhs, N) \
do \
if (N) \
{ \
(Current).first = (Rhs)[1].location.first; \
(Current).last = (Rhs)[N].location.last; \
} \
else \
{ \
(Current).first = (Current).last = (Rhs)[0].location.last; \
} \
while (false)
# define YYLLOC_DEFAULT(Current, Rhs, N) \
do \
if (N) \
{ \
(Current).first = YYRHSLOC (Rhs, 1).first; \
(Current).last = YYRHSLOC (Rhs, N).last; \
} \
else \
{ \
(Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
} \
while (false)
The default implementation of YYLLOC_DEFAULT used to be issued in
the header file. It is now output in the implementation file, after
the user %code sections so that its #ifndef guard does not try to
override the user's YYLLOC_DEFAULT if provided.
** YYFAIL now produces warnings and Java parsers no longer implement it:
YYFAIL has existed for many years as an undocumented feature of
deterministic parsers in C generated by Bison. More recently, it was
a documented feature of Bison's experimental Java parsers. As
promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
semantic action now produces a deprecation warning, and Java parsers
no longer implement YYFAIL at all. For further details, including a
discussion of how to suppress C preprocessor warnings about YYFAIL
being unused, see the Bison 2.4.2 NEWS entry.
** Temporary hack for adding a semicolon to the user action:
Previously, Bison appended a semicolon to every user action for
reductions when the output language defaulted to C (specifically, when
neither %yacc, %language, %skeleton, or equivalent command-line
options were specified). This allowed actions such as
instead of
As a first step in removing this misfeature, Bison now issues a
warning when it appends a semicolon. Moreover, in cases where Bison
cannot easily determine whether a semicolon is needed (for example, an
action ending with a cpp directive or a braced compound initializer),
it no longer appends one. Thus, the C compiler might now complain
about a missing semicolon where it did not before. Future releases of
Bison will cease to append semicolons entirely.
** Verbose syntax error message fixes:
When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
specified, syntax error messages produced by the generated parser
include the unexpected token as well as a list of expected tokens.
The effect of %nonassoc on these verbose messages has been corrected
in two ways, but a more complete fix requires LAC, described above:
*** When %nonassoc is used, there can exist parser states that accept no
tokens, and so the parser does not always require a lookahead token
in order to detect a syntax error. Because no unexpected token or
expected tokens can then be reported, the verbose syntax error
message described above is suppressed, and the parser instead
reports the simpler message, "syntax error". Previously, this
suppression was sometimes erroneously triggered by %nonassoc when a
lookahead was actually required. Now verbose messages are
suppressed only when all previous lookaheads have already been
shifted or discarded.
*** Previously, the list of expected tokens erroneously included tokens
that would actually induce a syntax error because conflicts for them
were resolved with %nonassoc in the current parser state. Such
tokens are now properly omitted from the list.
*** Expected token lists are still often wrong due to state merging
(from LALR or IELR) and default reductions, which can both add
invalid tokens and subtract valid tokens. Canonical LR almost
completely fixes this problem by eliminating state merging and
default reductions. However, there is one minor problem left even
when using canonical LR and even after the fixes above. That is,
if the resolution of a conflict with %nonassoc appears in a later
parser state than the one at which some syntax error is
discovered, the conflicted token is still erroneously included in
the expected token list. Bison's new LAC implementation,
described above, eliminates this problem and the need for
canonical LR. However, LAC is still experimental and is disabled
by default.
** Java skeleton fixes:
*** A location handling bug has been fixed.
*** The top element of each of the value stack and location stack is now
cleared when popped so that it can be garbage collected.
*** Parser traces now print the top element of the stack.
** -W/--warnings fixes:
*** Bison now properly recognizes the "no-" versions of categories:
For example, given the following command line, Bison now enables all
warnings except warnings for incompatibilities with POSIX Yacc:
bison -Wall,no-yacc gram.y
*** Bison now treats S/R and R/R conflicts like other warnings:
Previously, conflict reports were independent of Bison's normal
warning system. Now, Bison recognizes the warning categories
"conflicts-sr" and "conflicts-rr". This change has important
consequences for the -W and --warnings command-line options. For
bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
bison -Wnone gram.y # no conflicts are reported
bison -Werror gram.y # any conflict is an error
However, as before, if the %expect or %expect-rr directive is
specified, an unexpected number of conflicts is an error, and an
expected number of conflicts is not reported, so -W and --warning
then have no effect on the conflict report.
*** The "none" category no longer disables a preceding "error":
For example, for the following command line, Bison now reports
errors instead of warnings for incompatibilities with POSIX Yacc:
bison -Werror,none,yacc gram.y
*** The "none" category now disables all Bison warnings:
Previously, the "none" category disabled only Bison warnings for
which there existed a specific -W/--warning category. However,
given the following command line, Bison is now guaranteed to
suppress all warnings:
bison -Wnone gram.y
** Precedence directives can now assign token number 0:
Since Bison 2.3b, which restored the ability of precedence
directives to assign token numbers, doing so for token number 0 has
produced an assertion failure. For example:
%left END 0
This bug has been fixed.
* Changes in version 2.4.3 (2010-08-05):
** Bison now obeys -Werror and --warnings=error for warnings about
grammar rules that are useless in the parser due to conflicts.
** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
been fixed.
** Failures in the test suite for GCC 4.5 have been fixed.
** Failures in the test suite for some versions of Sun Studio C++ have
been fixed.
** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
warnings about undefined %prec identifiers will not be converted to
errors in Bison 2.5. They will remain warnings, which should be
sufficient for POSIX while avoiding backward compatibility issues.
** Minor documentation fixes.
* Changes in version 2.4.2 (2010-03-20):
** Some portability problems that resulted in failures and livelocks
in the test suite on some versions of at least Solaris, AIX, HP-UX,
RHEL4, and Tru64 have been addressed. As a result, fatal Bison
errors should no longer cause M4 to report a broken pipe on the
affected platforms.
** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
POSIX specifies that an error be reported for any identifier that does
not appear on the LHS of a grammar rule and that is not defined by
%token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
error report for the case when an identifier appears only after a
%prec directive. It is now restored. However, for backward
compatibility with recent Bison releases, it is only a warning for
now. In Bison 2.5 and later, it will return to being an error.
[Between the 2.4.2 and 2.4.3 releases, it was decided that this
warning will not be converted to an error in Bison 2.5.]
** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
** %code is now a permanent feature.
A traditional Yacc prologue directive is written in the form:
To provide a more flexible alternative, Bison 2.3b introduced the
%code directive with the following forms for C/C++:
%code {CODE}
%code requires {CODE}
%code provides {CODE}
%code top {CODE}
These forms are now considered permanent features of Bison. See the
%code entries in the section "Bison Declaration Summary" in the Bison
manual for a summary of their functionality. See the section
"Prologue Alternatives" for a detailed discussion including the
advantages of %code over the traditional Yacc prologue directive.
Bison's Java feature as a whole including its current usage of %code
is still considered experimental.
** YYFAIL is deprecated and will eventually be removed.
YYFAIL has existed for many years as an undocumented feature of
deterministic parsers in C generated by Bison. Previously, it was
documented for Bison's experimental Java parsers. YYFAIL is no longer
documented for Java parsers and is formally deprecated in both cases.
Users are strongly encouraged to migrate to YYERROR, which is
specified by POSIX.
Like YYERROR, you can invoke YYFAIL from a semantic action in order to
induce a syntax error. The most obvious difference from YYERROR is
that YYFAIL will automatically invoke yyerror to report the syntax
error so that you don't have to. However, there are several other
subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
used. For a more detailed discussion, see:
The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
deterministic parsers in C will continue to implement it. However,
because YYFAIL is already flawed, it seems futile to try to make new
Bison features compatible with it. Thus, during parser generation,
Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
rule action. In a later release, YYFAIL will be disabled for
%error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
be removed altogether.
There exists at least one case where Bison 2.5's YYFAIL warning will
be a false positive. Some projects add phony uses of YYFAIL and other
Bison-defined macros for the sole purpose of suppressing C
preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
To avoid Bison's future warning, such YYFAIL uses can be moved to the
epilogue (that is, after the second "%%") in the Bison input file. In
this release (2.4.2), Bison already generates its own code to suppress
C preprocessor warnings for YYFAIL, so projects can remove their own
phony uses of YYFAIL if compatibility with Bison releases prior to
2.4.2 is not necessary.
** Internationalization.
Fix a regression introduced in Bison 2.4: Under some circumstances,
message translations were not installed although supported by the
host system.
declarations have been fixed.
action for reductions. This allowed actions such as
instead of
Some grammars still depend on this "feature". Bison 2.4.1 restores
the previous behavior in the case of C output (specifically, when
neither %language or %skeleton or equivalent command-line options
are used) to leave more time for grammars depending on the old
behavior to be adjusted. Future releases of Bison will disable this
** A few minor improvements to the Bison manual.
* Changes in version 2.4 (2008-11-02):
** %language is an experimental feature.
We first introduced this feature in test release 2.3b as a cleaner
alternative to %skeleton. Since then, we have discussed the possibility of
modifying its effect on Bison's output file names. Thus, in this release,
we consider %language to be an experimental feature that will likely evolve
in future releases.
** Forward compatibility with GNU M4 has been improved.
** Several bugs in the C++ skeleton and the experimental Java skeleton have been
* Changes in version 2.3b (2008-05-27):
** The quotes around NAME that used to be required in the following directive
are now deprecated:
%define NAME "VALUE"
** The directive "%pure-parser" is now deprecated in favor of:
%define api.pure
which has the same effect except that Bison is more careful to warn about
unreasonable usage in the latter case.
** Push Parsing
Bison can now generate an LALR(1) parser in C with a push interface. That
is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
push one token at a time to the parser using "yypush_parse", which will
return to the caller after processing each token. By default, the push
interface is disabled. Either of the following directives will enable it:
%define api.push_pull "push" // Just push; does not require yylex.
%define api.push_pull "both" // Push and pull; requires yylex.
See the new section "A Push Parser" in the Bison manual for details.
feedback will help to stabilize it.
** The -g and --graph options now output graphs in Graphviz DOT format,
not VCG format. Like --graph, -g now also takes an optional FILE argument
and thus cannot be bundled with other short options.
** Java
Bison can now generate an LALR(1) parser in Java. The skeleton is
"data/". Consider using the new %language directive instead of
%skeleton to select it.
See the new section "Java Parsers" in the Bison manual for details.
The current Java interface is experimental and may evolve. More user
feedback will help to stabilize it.
** %language
This new directive specifies the programming language of the generated
parser, which can be C (the default), C++, or Java. Besides the skeleton
that Bison uses, the directive affects the names of the generated files if
the grammar file's name ends in ".y".
** XML Automaton Report
Bison can now generate an XML report of the LALR(1) automaton using the new
"--xml" option. The current XML schema is experimental and may evolve. More
user feedback will help to stabilize it.
** The grammar file may now specify the name of the parser header file using
%defines. For example:
%defines "parser.h"
** When reporting useless rules, useless nonterminals, and unused terminals,
Bison now employs the terms "useless in grammar" instead of "useless",
"useless in parser" instead of "never reduced", and "unused in grammar"
instead of "unused".
** Unreachable State Removal
Previously, Bison sometimes generated parser tables containing unreachable
states. A state can become unreachable during conflict resolution if Bison
disables a shift action leading to it from a predecessor state. Bison now:
1. Removes unreachable states.
2. Does not report any conflicts that appeared in unreachable states.
WARNING: As a result, you may need to update %expect and %expect-rr
directives in existing grammar files.
3. For any rule used only in such states, Bison now reports the rule as
"useless in parser due to conflicts".
This feature can be disabled with the following directive:
%define lr.keep_unreachable_states
See the %define entry in the "Bison Declaration Summary" in the Bison manual
for further discussion.
** Lookahead Set Correction in the ".output" Report
When instructed to generate a ".output" file including lookahead sets
(using "--report=lookahead", for example), Bison now prints each reduction's
lookahead set only next to the associated state's one item that (1) is
associated with the same rule as the reduction and (2) has its dot at the end
of its RHS. Previously, Bison also erroneously printed the lookahead set
next to all of the state's other items associated with the same rule. This
bug affected only the ".output" file and not the generated parser source
** --report-file=FILE is a new option to override the default ".output" file
** The "=" that used to be required in the following directives is now
%file-prefix "parser"
%name-prefix "c_"
%output "parser.c"
** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
Bison 2.3a provided a new set of directives as a more flexible alternative to
the traditional Yacc prologue blocks. Those have now been consolidated into
a single %code directive with an optional qualifier field, which identifies
the purpose of the code and thus the location(s) where Bison should generate
1. "%code {CODE}" replaces "%after-header {CODE}"
2. "%code requires {CODE}" replaces "%start-header {CODE}"
3. "%code provides {CODE}" replaces "%end-header {CODE}"
4. "%code top {CODE}" replaces "%before-header {CODE}"
See the %code entries in section "Bison Declaration Summary" in the Bison
manual for a summary of the new functionality. See the new section "Prologue
Alternatives" for a detailed discussion including the advantages of %code
over the traditional Yacc prologues.
The prologue alternatives are experimental. More user feedback will help to
determine whether they should become permanent features.
** Revised warning: unset or unused mid-rule values
Since Bison 2.2, Bison has warned about mid-rule values that are set but not
used within any of the actions of the parent rule. For example, Bison warns
about unused $2 in:
exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
Now, Bison also warns about mid-rule values that are used but not set. For
example, Bison warns about unset $$ in the mid-rule action in:
exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
However, Bison now disables both of these warnings by default since they
sometimes prove to be false alarms in existing grammars employing the Yacc
constructs $0 or $-N (where N is some positive integer).
To enable these warnings, specify the option "--warnings=midrule-values" or
"-W", which is a synonym for "--warnings=all".
** Default %destructor or %printer with "<*>" or "<>"
Bison now recognizes two separate kinds of default %destructor's and
1. Place "<*>" in a %destructor/%printer symbol list to define a default
%destructor/%printer for all grammar symbols for which you have formally
declared semantic type tags.
2. Place "<>" in a %destructor/%printer symbol list to define a default
%destructor/%printer for all grammar symbols without declared semantic
type tags.
Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
"<*>" and "<>" combined achieve the same effect with one exception: Bison no
longer applies any %destructor to a mid-rule value if that mid-rule value is
not actually ever referenced using either $$ or $n in a semantic action.
The default %destructor's and %printer's are experimental. More user
feedback will help to determine whether they should become permanent
See the section "Freeing Discarded Symbols" in the Bison manual for further
** %left, %right, and %nonassoc can now declare token numbers. This is required
by POSIX. However, see the end of section "Operator Precedence" in the Bison
manual for a caveat concerning the treatment of literal strings.
** The nonfunctional --no-parser, -n, and %no-parser options have been
completely removed from Bison.
* Changes in version 2.3a, 2006-09-13:
** Instead of %union, you can define and use your own union type
YYSTYPE if your grammar contains at least one <type> tag.
Your YYSTYPE need not be a macro; it can be a typedef.
This change is for compatibility with other Yacc implementations,
and is required by POSIX.
** Locations columns and lines start at 1.
In accordance with the GNU Coding Standards and Emacs.
** You may now declare per-type and default %destructor's and %printer's:
For example:
%union { char *string; }
%token <string> STRING1
%token <string> STRING2
%type <string> string1
%type <string> string2
%union { char character; }
%token <character> CHR
%type <character> chr
%destructor { free ($$); } %symbol-default
%destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
%destructor { } <character>
guarantees that, when the parser discards any user-defined symbol that has a
semantic type tag other than "<character>", it passes its semantic value to
"free". However, when the parser discards a "STRING1" or a "string1", it
also prints its line number to "stdout". It performs only the second
"%destructor" in this case, so it invokes "free" only once.
[Although we failed to mention this here in the 2.3a release, the default
%destructor's and %printer's were experimental, and they were rewritten in
future versions.]
** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
"--yacc", or "%yacc"), Bison no longer generates #define statements for
associating token numbers with token names. Removing the #define statements
helps to sanitize the global namespace during preprocessing, but POSIX Yacc
requires them. Bison still generates an enum for token names in all cases.
** Handling of traditional Yacc prologue blocks is now more consistent but
potentially incompatible with previous releases of Bison.
As before, you declare prologue blocks in your grammar file with the
"%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
prologue blocks that you've declared before the first %union. To generate
the post-prologue, Bison concatenates all prologue blocks that you've
declared after the first %union.
Previous releases of Bison inserted the pre-prologue into both the header
file and the code file in all cases except for LALR(1) parsers in C. In the
latter case, Bison inserted it only into the code file. For parsers in C++,
the point of insertion was before any token definitions (which associate
token numbers with names). For parsers in C, the point of insertion was
after the token definitions.
Now, Bison never inserts the pre-prologue into the header file. In the code
file, it always inserts it before the token definitions.
** Bison now provides a more flexible alternative to the traditional Yacc
prologue blocks: %before-header, %start-header, %end-header, and
For example, the following declaration order in the grammar file reflects the
order in which Bison will output these code blocks. However, you are free to
declare these code blocks in your grammar file in whatever order is most
convenient for you:
%before-header {
/* Bison treats this block like a pre-prologue block: it inserts it into
* the code file before the contents of the header file. It does *not*
* insert it into the header file. This is a good place to put
* #include's that you want at the top of your code file. A common
* example is '#include "system.h"'. */
%start-header {
/* Bison inserts this block into both the header file and the code file.
* In both files, the point of insertion is before any Bison-generated
* token, semantic type, location type, and class definitions. This is a
* good place to define %union dependencies, for example. */
%union {
/* Unlike the traditional Yacc prologue blocks, the output order for the
* new %*-header blocks is not affected by their declaration position
* relative to any %union in the grammar file. */
%end-header {
* In both files, the point of insertion is after the Bison-generated
* definitions. This is a good place to declare or define public
* functions or data structures that depend on the Bison-generated
* definitions. */
%after-header {
/* Bison treats this block like a post-prologue block: it inserts it into
* the code file after the contents of the header file. It does *not*
* insert it into the header file. This is a good place to declare or
* define internal functions or data structures that depend on the
* Bison-generated definitions. */
If you have multiple occurrences of any one of the above declarations, Bison
will concatenate the contents in declaration order.
[Although we failed to mention this here in the 2.3a release, the prologue
alternatives were experimental, and they were rewritten in future versions.]
** The option "--report=look-ahead" has been changed to "--report=lookahead".
The old spelling still works, but is not documented and may be removed
in a future release.
* Changes in version 2.3, 2006-06-05:
** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
for compatibility with LALR(1) grammars.
** It is now documented that any definition of YYSTYPE or YYLTYPE should
* Changes in version 2.2, 2006-05-19:
** The distribution terms for all Bison-generated parsers now permit
using the parsers in nonfree programs. Previously, this permission
** %name-prefix changes the namespace name in C++ outputs.
** The C++ parsers export their token_type.
** Bison now allows multiple %union declarations, and concatenates
their contents together.
** New warning: unused values
if the symbols have destructors. For instance:
| exp "+" exp
most likely contains three errors, and could be rewritten as:
exp: exp "?" exp ":" exp
| exp "+" exp
However, if the original actions were really intended, memory leaks
values are used, e.g.:
** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
Destructors are now called when user code invokes YYABORT, YYACCEPT,
corresponding to the right-hand side of the current rule.
** %expect, %expect-rr
Incorrect numbers of expected conflicts are now actual errors,
instead of warnings.
** GLR, YACC parsers.
experimental printers) as per the documentation.
** Bison now warns if it finds a stray "$" or "@" in an action.
** %require "VERSION"
This specifies that the grammar file depends on features implemented
in Bison version VERSION or higher.
** The token and value types are now class members.
tokens are enumerations of the "yy::parser::token" struct, and the
semantic values have the "yy::parser::semantic_type" type.
'%define "global_tokens_and_yystype" "1"' triggers the global
definition of tokens and YYSTYPE. This change is suitable both
for previous releases of Bison, and this one.
fail using '%require "2.2"'.
** DJGPP support added.
* Changes in version 2.1, 2005-09-16:
** The C++ skeleton supports %lex-param.
** Bison-generated parsers now support the translation of diagnostics like
"syntax error" into languages other than English. The default
language is still English. For details, please see the new
Internationalization section of the Bison manual. Software
distributors should also see the new PACKAGING file. Thanks to
Bruno Haible for this new feature.
** Wording in the Bison-generated parsers has been changed slightly to
simplify translation. In particular, the message "memory exhausted"
always accurate for modern Bison-generated parsers.
** Destructors are now called when the parser aborts, for all symbols left
** When generating verbose diagnostics, Bison-generated parsers no longer
quote the literal strings associated with tokens. For example, for
unexpected "number"'.
* Changes in version 2.0, 2004-12-25:
** Possibly-incompatible changes
the manual to determine safe values for YYMAXDEPTH in that case.
- Error token location.
During error recovery, the location of the syntax error is updated
the shifted symbols thrown away during the first part of the error
recovery, and the lookahead rejected during the second part.
- Semicolon changes:
. Semicolons are now required after in-grammar declarations.
- Unescaped newlines are no longer allowed in character constants or
string literals. They were never portable, and GCC 3.4.0 has
dropped support for them. Better diagnostics are now generated if
forget a closing quote.
** New features
- GLR grammars now support locations.
- New directive: %initial-action.
This directive allows the user to run arbitrary code (including
initializing @$) from yyparse before parsing starts.
reduce/reduce conflicts in GLR parsers.
- %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
This is a GNU extension.
- The option "--report=lookahead" was changed to "--report=look-ahead".
[However, this was changed back after 2.3.]
- Experimental %destructor support has been added to
** Bug fixes
these violations will become errors again.
arbitrarily limited to 16-bit counts.
* Changes in version 1.875, 2003-01-01:
** The documentation license has been upgraded to version 1.2
of the GNU Free Documentation License.
** syntax error processing
locations too. This fixes bugs in error-location computation.
- %destructor
It is now possible to reclaim the memory associated to symbols
discarded during error recovery. This feature is still experimental.
- %error-verbose
This new directive is preferred over YYERROR_VERBOSE.
- #defining yyerror to steal internal variables is discouraged.
It is not guaranteed to work forever.
** POSIX conformance
compatibility with Yacc.
- "parse error" -> "syntax error"
Bison now uniformly uses the term "syntax error"; formerly, the code
and manual sometimes used the term "parse error" instead. POSIX
requires "syntax error" in diagnostics, and it was thought better to
be consistent.
- The documentation now emphasizes that yylex and yyerror must be
declared before use. C99 requires this.
- Bison now parses C99 lexical constructs like UCNs and
output as "foo\\bar.y".
- Yacc command and library now available
The Bison distribution now installs a "yacc" command, as POSIX requires.
Also, Bison now installs a small library liby.a containing
implementations of Yacc-compatible yyerror and main functions.
This library is normally not useful, but POSIX requires it.
- Type clashes now generate warnings, not errors.
using typedef instead of defining it as a macro.
For consistency, YYLTYPE is also declared instead of defined.
** Other compatibility issues
- %union directives can now have a tag before the "{", e.g., the
directive "%union foo {...}" now generates the C code
"typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
This is for compatibility with both Yacc and Bison 1.35.
- ";" is output before the terminating "}" of an action, for
compatibility with Bison 1.35.
"conflicts: 2 shift/reduce, 1 reduce/reduce".
- "yystype" and "yyltype" are now obsolescent macros instead of being
withdrawn in a future release.
** GLR parser notes
- GLR and inline
C keyword "inline".
- "parsing stack overflow..." -> "parser stack overflow"
GLR parsers now report "parser stack overflow" as per the Bison manual.
** %parse-param and %lex-param
The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
additional context to yyparse and yylex. They suffer from several
- a single argument only can be added,
- their types are weak (void *),
- this context is not passed to ancillary functions such as yyerror,
- only yacc.c parsers support them.
The new %parse-param/%lex-param directives provide a more precise control.
For instance:
%parse-param {int *nastiness}
%lex-param {int *nastiness}
%parse-param {int *randomness}
results in the following signatures:
int yylex (int *nastiness);
int yyparse (int *nastiness, int *randomness);
or, if both %pure-parser and %locations are used:
int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
int yyparse (int *nastiness, int *randomness);
** Bison now warns if it detects conflicting outputs to the same file,
e.g., it generates a warning for "bison -d -o foo.h foo.y" since
that command outputs both code and header to foo.h.
** #line in output files
- --no-line works properly.
ago, but nobody noticed until we recently asked someone to try
building Bison with a K&R C compiler.
* Changes in version 1.75, 2002-10-14:
** Bison should now work on 64-bit hosts.
** Indonesian translation thanks to Tedi Heriyanto.
** GLR parsers
Fix spurious parse errors.
** Pure parsers
Some people redefine yyerror to steal yyparse' private variables.
Reenable this trick until an official feature replaces it.
** Type Clashes
In agreement with POSIX and with other Yaccs, leaving a default
untyped: ... typed;
but the converse remains an error:
typed: ... untyped;
** Values of mid-rule actions
The following code:
* Changes in version 1.50, 2002-10-04:
** GLR parsing
The declaration
ambiguities. Contributed by Paul Hilfinger.
Unfortunately Bison 1.50 does not work properly on 64-bit hosts
** Output Directory
specified, running "bison foo/bar.y" created "foo/bar.c". It
now creates "bar.c".
** Undefined token
The undefined token was systematically mapped to 2 which prevented
** Unknown token numbers
no longer the case.
** Error token
According to POSIX, the error token must be 256.
will be mapped onto another number.
** Verbose error messages
They no longer report "..., expecting error or..." for states where
error recovery is possible.
** End token
Defaults to "$end" instead of "$".
** Error recovery now conforms to documentation and to POSIX
the stack until it finds a state that allows shifting the error
** Traces
Popped tokens and nonterminals are now reported.
** Larger grammars
Larger grammars are now supported (larger token numbers, larger grammar
now these limits are 32 bits on most hosts.
** Explicit initial rule
graphs as rule 0.
** Useless rules
included them in the parsers. They are now actually removed.
** Useless rules, useless nonterminals
** Rules never reduced
Rules that can never be reduced because of conflicts are now
** Incorrect "Token not used"
On a grammar such as
%token useless useful
exp: '0' %prec useful;
bison reported both "useful" and "useless" as useless tokens.
** Revert the C++ namespace changes introduced in 1.31
as they caused too many portability hassles.
** Default locations
The latter is now removed: YYLLOC_DEFAULT is fully responsible of
the computation of @$.
** Token end-of-file
error messages instead of "$end", which remains being the default.
For instance
%token MYEOF 0
%token MYEOF 0 "end of file"
** Semantic parser
** New translations
Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
Croatian, thanks to Denis Lackovic.
** Incorrect token definitions
When given
%token 'a' "A"
bison used to output
#define 'a' 65
** Token definitions as enums
This lets debuggers display names instead of integers.
** Reports
produces additional information:
- itemset
complete the core item sets with their closure
- lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
explicitly associate lookahead tokens to items
- solved
describe shift/reduce conflicts solving.
Bison used to systematically output this information on top of
the report. Solved conflicts are now attached to their states.
** Type clashes
Previous versions don't complain when there is a type clash on
%type <foo> bar
bar: '0' {} '0';
This is fixed.
** GNU M4 is now required when using Bison.
* Changes in version 1.35, 2002-03-25:
** C Skeleton
alignment and 64 bit architectures made this impossible.
generation exists, kludges were implemented in the parser to
kludge will be disabled.
* Changes in version 1.34, 2002-03-12:
** File name clashes are detected
$ bison foo.y -d -o foo.x
fatal error: header and parser would both be named "foo.x"
** A missing ";" at the end of a rule triggers a warning
In accordance with POSIX, and in agreement with other
Yacc implementations, Bison will mandate this semicolon in the near
future. This eases the implementation of a Bison parser of Bison
facilitate the transition, this release introduces a warning.
** Revert the C++ namespace changes introduced in 1.31, as they caused too
many portability hassles.
** DJGPP support added.
** Fix test suite portability problems.
* Changes in version 1.33, 2002-02-07:
** Fix C++ issues
Groff could not be compiled for the definition of size_t was lacking
under some conditions.
** Catch invalid @n
As is done with $n.
* Changes in version 1.32, 2002-01-23:
** Fix Yacc output file names
** Portability fixes
** Italian, Dutch translations
* Changes in version 1.31, 2002-01-14:
** Many Bug Fixes
** GNU Gettext and %expect
Bison dies on incorrect %expectations, we fear there will be
does not trigger an error when the input file is named "plural.y".
** Use of alloca in parsers
If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
malloc exclusively. Since 1.29, but was not NEWS'ed.
alloca is used only when compiled with GCC, to avoid portability
problems as on AIX.
** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
** When the generated parser lacks debugging code, YYDEBUG is now 0
(as POSIX requires) instead of being undefined.
** User Actions
ending semicolon. Now if in Yacc compatibility mode, the semicolon
** Better C++ compliance
The output parsers try to respect C++ namespaces.
** Reduced Grammars
Fixed bugs when reporting useless nonterminals.
** 64 bit hosts
The parsers work properly on 64 bit hosts.
** Error messages
Some calls to strerror resulted in scrambled or missing error messages.
** %expect
any warning.
** The verbose report includes the rule line numbers.
** Rule line numbers are fixed in traces.
** Swedish translation
** Parse errors
Verbose parse error messages from the parsers are better looking.
** Fixed parser memory leaks.
previous allocations were not freed.
** Fixed verbose output file.
Some newlines were missing.
Some conflicts in state descriptions were missing.
** Fixed conflict report.
Option -v was needed to get the result.
** %expect
Was not used.
Mismatches are errors, not warnings.
** Fixed incorrect processing of some invalid input.
** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
** Fixed some typos in the documentation.
** %token MY_EOF 0 is supported.
Before, MY_EOF was silently renumbered as 257.
** doc/refcard.tex is updated.
** %output, %file-prefix, %name-prefix.
** --output
New, aliasing "--output-file".
* Changes in version 1.30, 2001-10-26:
** "--defines" and "--graph" have now an optional argument which is the
output file name. "-d" and "-g" do not change; they do not take any
** "%source_extension" and "%header_extension" are removed, failed
** Portability fixes.
* Changes in version 1.29, 2001-09-07:
** The output file does not define const, as this caused problems when used
with common autoconfiguration schemes. If you still use ancient compilers
"-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
** Added "-g" and "--graph".
** The Bison manual is now distributed under the terms of the GNU FDL.
** The input and the output files has automatically a similar extension.
** Russian translation added.
** NLS support updated; should hopefully be less troublesome.
** Added the old Bison reference card.
** Added "--locations" and "%locations".
** Added "-S" and "--skeleton".
** "%raw", "-r", "--raw" is disabled.
** Special characters are escaped when output. This solves the problems
of the #line lines with path names including backslashes.
** New directives.
"%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
"%debug", "%source_extension" and "%header_extension".
** @$
Automatic location tracking.
* Changes in version 1.28, 1999-07-06:
** Should compile better now with K&R compilers.
** Added NLS.
** Fixed a problem with escaping the double quote character.
** There is now a FAQ.
* Changes in version 1.27:
** The make rule which prevented bison.simple from being created on
some systems has been fixed.
* Changes in version 1.26:
** Bison now uses Automake.
** New mailing lists: <> and <>.
** Token numbers now start at 257 as previously documented, not 258.
** Bison honors the TMPDIR environment variable.
** A couple of buffer overruns have been fixed.
** Problems when closing files should now be reported.
** Generated parsers should now work even on operating systems which do
not provide alloca().
* Changes in version 1.25, 1995-10-16:
** Errors in the input grammar are not fatal; Bison keeps reading
** Tokens can now be specified as multiple-character strings: for
of choosing a name like LESSEQ.
** The %token_table declaration says to write a table of tokens (names
table to recognize multiple-character string tokens, or for other
** The %no_lines declaration says not to generate any #line preprocessor
directives in the parser file.
** The %raw declaration says to use internal Bison token numbers, not
** The --no-parser option produces the parser tables without including
a switch statement body.
* Changes in version 1.23:
actually point to an object. Grammar actions can access the variable
by casting it to the proper pointer type.
Line numbers in output file corrected.
* Changes in version 1.22:
--help option added.
* Changes in version 1.20:
Output file does not redefine const for C++.
Copyright (C) 1995-2012 Free Software Foundation, Inc.
This file is part of Bison, the GNU Parser Generator.
(at your option) any later version.
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <>.
LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp calc yyo fval Wmaybe
LocalWords: yyvsp pragmas noreturn java's
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