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Welcome to BtYacc - BackTracking Yacc

Chris Dodd
Vadim Maslov


    This is original Berkeley Yacc modified by Chris Dodd and then further enhanced by Vadim Maslov of Siber Systems to better fit production environment (and to have less bugs, too).

    We temporarily dropped support for C in 3.0 and now only C++ is supported. So version 2.1 - the last version that supported C -- is also available. If you want to reincorporate C support into 3.0, you are welcome to send you changes to vadik\@siber.com.

    Siber Systems used BtYacc to develop commercial Cobol parser, which is a part of CobolTransformer toolkit.

BTYACC - backtracking YACC

    BTYACC was created by Chris Dodd using ideas from many places and lots of code from the Berkeley Yacc distribution, which is a public domain yacc clone put together by the good folks at Berkeley. This code is distributed with NO WARRANTEE and is public domain. It is certain to contain bugs, which you should report to: chrisd\@collins.com.

    Vadim Maslov of Siber Systems considerably modified BTYACC to make it suitable for production environment.

    Several people have suggested bug fixes that were incorporated into BtYacc.

    See the README.BYACC files for more about Berkeley Yacc and other sources of info.

     www.siber.com/btyacc/ is the current home of BtYacc. It is provided courtesy of Siber Systems.

Version 3.0 changes

by Vadim Maslov

    Changes mostly occurred in btyaccpa.ske file that contains the parsing shift/reduce/backtrack algorithm.

    Version 3.0 innovations focus on:

  • text position computation and propagation,
  • industrial-strength error processing and recovery.


Added mechanism for computing and propagating text position of tokens and non-terminals.

Compilers often need to build AST trees such that every node in a tree can relate to the parsed program source it came from. The following applications are very likely to need this:

  • debuggers that show actual source of the debugged program,
  • source-to-source translators that want unchanged parts of the tree to generate the unchanged code.

The new YYPOSN mechanism added in this version of BtYacc helps you in automating the text position computation and in assigning the computed text positions to the AST. This mechanism is successfully used in commercial parsers and source-to-source translators.

In standard Yaccs every token and every non-terminal has an YYSTYPE semantic value attached to it. In this new version every token and every non-terminal also has an YYPOSN text position attached to it. YYPOSN is a user-defined type that can be anything and that has a meaning of text position attached to token or non-terminal.

In addition to semantic value stack BtYacc now maintains text position stack. Behavior of the text position stack is similar to the behavior of the semantic value stack.

If using text position mechanism, you need to define the following:

YYPOSN Preprocessor variable that contains C/C++ type of the text position attached to every token and non-terminal.

yyposn Global variable of type YYPOSN. The lexer must assign text position of the returned token to yyposn, just like it assigns semantic value of the returned token to yylval.

YYREDUCEPOSNFUNC Preprocessor variable that points to unction that is called after the grammar rule reduction to reduce text positions located on the stack.

This function is called by BtYacc to reduce text positions. The function is called immediately after the regular rule reduction occurs.

The function has the following prototype:

void ReducePosn(YYPOSN &ret,
   YYPOSN *terms,
   YYSTYPE *term_vals,
   int term_no,
   int stk_pos,
   int yychar,
   YYPOSN &yyposn,
   UserType extra);

The function arguments are:


Reference to the text position returned by the rule. The function must write the computed text position returned by the rule to ret. This is analogue of the $$ semantic value.


Array of the right-hand side rule components YYPOSN text positions. These are analogues of $1, $2, ..., $N in the text position world.


Array of the right-hand side (RHS) rule components YYSTYPE values. These are the $1,...,$N themselves.


Number of the components in RHS of the reduced rule. Equal to size of arrays term_posns and term_vals. Also equal to N in $1,...,$N in the reduced rule.



stack position before the reduction.


Lookahead token that immediately follows the reduced RHS components.



of the token that immediately follows the reduced RHS components.


User-defined extra argument passed to ReducePosn.

Typically this function extracts text positions from the right-hand side rule components and either assigns them to the returned $$ structure/tree or if no $$ value is returned, puts them into the ret text position from where it will be picked up by the later reduced rules.


Extra user-defined argument passed to the ReducePosn function. This argument can use any variables defined in btyaccpa.ske.


Added code to btyaccpa.ske that automatically cleans up semantic semantic values and text positions of tokens and non-terminals that are discarded and deleted as a result of error processing.

In the previous versions the discarded token and non-terminal semantic values were not cleaned that caused quite severe leaks. The only way to fix it was to add garbage collection to YYSTYPE class.

Now BtYacc skeleton calls delete functions for semantic values and positions of the discarded tokens and non-terminals.

You need to define the following functions that BtYacc calls when it needs to delete semantic value or text position.


User-defined function that is called by BtYacc to delete semantic value of the token or non-terminal.

The user-defined function must have the prototype:

    void DeleteYYval(YYSTYPE v, int type);

v is semantic value to delete, type is one of the following:

0 discarding token

1 discarding state

2 cleaning up stack when aborting


User-defined function that is called by BtYacc to delete text position of the token or non-terminal.

The user-defined function must have the prototype:

    void DeleteYYposn(YYPOSN p, int type);

v is semantic value to delete, type is one of the following:

0 discarding token

1 discarding state

2 cleaning up stack when aborting


User can define "detailed" syntax error processing function that reports an *exact* position of the token that caused the error.

If you define preprocessor variable YYERROR_DETAILED in your grammar then you need define the following error processing function:

void yyerror_detailed(char *text,
  int errt,
  YYSTYPE &errt_value,
  YYPOSN &errt_posn);

It receives the following arguments:

text Error message.

errt Code of the token that caused the error.

errt_value Value of the token that caused the error.

errt_posn Text position of token that caused error.


Dropped compatibility with C.

Compatibility with C became increasingly difficult to maintain as new features were added to btyaccpa.ske. So we dropped it. If anybody wants to make the new version compatible with C, we would gladly accept the changes.

Meanwhile we expect that you use C++ to write grammar actions and everything else in grammar files. Since C is (in a sense) subset of C++, your C-based grammar may work if you use C++ compiler to compile it.

Version 3.0 bugs fixed

Matthias Meixner fixed a bug: BtYacc does not correctly handle typenames, if one typename is a prefix of another one and if this type is used after the longer one. In this case BTYacc produces invalid code.

Version 2.1 changes

by Vadim Maslov

** Added preprocessor statements to BtYacc that are similar in function and behavior to C/C++ preprocessor statements.

These statements are used to:

  1. Introduce modularity into a grammar by breaking it into several *.y files and assembling different grammars from the *.y modules using %include and %ifdef.
  2. Have several versions of the same grammar by using %ifdef and $endif.
  3. To include automatically generated grammar fragment. For instance, we use %include to include automatically generated list of tokens.

Preprocessor statements are:


Define preprocessor variable named .


If preprocessor variable named is defined by %define, then process the text from this %ifdef to the closing %endif.


Closing bracket for %ifdef preprocessor statement. Only one nesting level of %ifdef-%endif is allowed.


Process contents of the file named . If is a relative name, it is looked up in a directory in which btyacc was started. Only one nesting level of %include is allowed.

Version 2.0 changes

by Vadim Maslov

** Changed 16-bit short numbers to 32-bit int numbers in grammar tables, so that huge grammar tables (tables that are larger than 32768 elements) resulting from huge grammars (Cobol grammar, for instance) can work correctly. You need to have 32-bit integer to index table bigger than 32768 elements, 16-bit integer is not enough.

The original BtYacc just generated non-working tables larger than 32768 elements without even notifying about the table overflow.

** Make error recovery work correctly when error happens while processing nested conflicts. Original BtYacc could infinitely cycle in certain situations that involved error recovery while in nested conflict.

More detailed explanation: when we have nested conflicts (conflict that happens while trial-processing another conflict), it leads btyacc into NP-complete searching of conflict tree. The ultimate goal is YYVALID operator that selects a particular branch of that tree as a valid one.

If no YYVALID is found on the tree, then error recovery takes over. The problem with this is that error recovery is started in the same state context that exists on the last surveyed branch of the conflict tree. Sometimes this last branch may be of zero length and it results in recovering to exactly the same state as existed before entering the conflict. BtYacc cycles then.

We solved this problem by memorizing the longest path in the conflict tree while browsing it. If we ever get into error recovery, we restore state that existed on the longest path. Effectively we say: if we have an error, let us move forward as far as we possibly could while we were browsing the conflict tree.

** Introduce YYVALID_NESTED operation in addition to simply YYVALID. When we have a nested conflict (conflict while processing in trial mode for another conflict), we want to relate YYVALID to a particular level of conflict being in trial.

Since we mostly anticipate only 2-level nested conflicts YYVALID_NESTED tells the parser to satisfy only the internal conflict. Therefore, in 1-level conflict situation YYVALID_NESTED acts like a regular YYVALID, but in 2-level conflict it is a no-op and the other YYVALID for outer conflict will be searched for.

** Improved handling of situation where /tmp directory is missing. Original btyacc just died quietly when /tmp directory was missing. We added code that states the problem explicitly. While on UNIX /tmp directory is always present, it may be missing on WIN32 systems, therefore diagnosing this situation is important.

Version 1.0 changes: BackTracking

by Chris Dodd

BTYACC is a modified version of yacc that supports automatic backtracking and semantic disambiguation to parse ambiguous grammars, as well as syntactic sugar for inherited attributes (which tend to introduce conflicts).

Whenever a btyacc generated parser runs into a shift-reduce or reduce-reduce error in the parse table, it remembers the current parse point (yacc stack and input stream state), and goes into trial parse mode. It then continues parsing, ignoring most rule actions. If it runs into an error (either through the parse table or through an action calling YYERROR), it backtracks to the most recent conflict point and tries a different alternative.

If it finds a successful parse (reaches the end of the input or an action calls YYVALID), it backtracks to the point where it first entered trial parse mode, and continues with a full parse (executing all actions), following the path of the successful trial.

Actions in btyacc come in two flavors -- {}-actions, which are only executed when not in trial mode, and []-actions which are executed regardless of mode. There are also inherited attributes, which look like arguments (they are enclosed in "()") and act like []-actions.

What this buys you:

* No more lexer feedback hack. In yacc grammars for C, a standard hack, know as the "lexer feedback hack" is used to find typedef names. The lexer uses semantic information to decide if any given identifier is a typedef-name or not and returns a special token. With btyacc, you no longer need to do this; the lexer should just always return an identifier. The btyacc grammar then needs a rule of the form:

typename: ID [ if (!IsTypeName(LookupId($1))) YYERROR; ]

While the hack works adequately well for parsing C, it becomes a nightmare when you try to parse something like C++, where treating an ID as a typedef becomes heavily dependent on context.

* Easy disambiguation via simple ordering. Btyacc runs its trials via the rule "try shifting first, then try reducing by the order that the conflicting rules appear in the input file". This means you can deal with semantic a disambiguation rule like:

[1] If it looks like a declaration it is, otherwise

[2] If it looks like an expression it is, otherwise

[3] it is a syntax error

[Ellis&Stroustrup, Annotated C++ Reference Manual, p93]

To deal with this, you need only put all the rules for declarations before the rules for expressions in the grammar file.

* No extra cost if you do not use it. Backtracking is only triggered when the parse hits a shift/reduce or reduce/reduce conflict in the table. If you have no conflicts in your grammar, there is no extra cost, other than some extra code which will never be invoked.

* C++ and ANSI C compatible parsers. The parsers produced by btyacc can be compiled with C++ correctly. If you "#define" YYSTYPE to be some C++ type with constructor and destructor, everything will work fine. My favorite is "#define YYSTYPE SmartPointer", where SmartPointer is a smart pointer type that does garbage collection on the pointed to objects.

BTYACC was originally written to make it easy to write a C++ parser (my goal was to be able to use the grammar out of the back of the ARM with as few modifications as possible). Anyone who has ever looked at Jim Roskind public domain C++ yacc grammar, or the yacc-based grammar used in g++ knows how difficult this is. BTYACC is very useful for parsing any ambiguous grammar, particularly ones that come from trying to merge two (or more) complete grammars.

Limitations of the backtracking: Currently, the generated parser does NO pruning of alternate parsing paths. To avoid an exponential explosion of possible paths (and parsing time), you need to manually tell the parser when it can throw away saved paths using YYVALID. In practice, this turns out to be fairly easy to do. A C++ parser (for example) can just put a [YYVALID;] after every complete declaration and statement rule, corresponding to pruning the backtracking state after seeing a ';' or '}' -- there will never be a situation in which it is useful to backtrack past either of these.

Inherited attributes in btyacc:

Inherited attributes look a lot like function arguments to non-terminals, which is what they end up being in a recursive descent parser, but NOT how they are implemented in btyacc. Basically they are just syntactic sugar for embedded semantic actions and $0, $-1, ... in normal yacc. btyacc gives you two big advantages besides just the syntax:

  1. it does type checking on the inherited attributes, so you do not have to specify $0 and makes sure you give the correct number of arguments (inherited attributes) to every use of a non-terminal.
  2. It "collapses" identical actions from that are produced from inherited attributes. This eliminates many potential reduce-reduce conflicts arising from the inherited attributes.

You use inherited attributes by declaring the types of the attributes in the preamble with a type declaration and declaring names of the attributes on the lhs of the yacc rule. You can of course have more than one rule with the same lhs, and you can even give them different names in each, but the type and number must be the same.

Here is a small example:

/* lhs takes 2 inherited attributes */
%type  lhs(, )
stuff(, )
lhs($i1, $i2) : { $$ = $i1 }
| lhs($i1, $i2) stuff($1,$i2) { $$ = $2; }

This is roughly equivalent to the following yacc code:

lhs :
{ $$ = $-1; }
| lhs [ $$ = $-1; ] [ $$ = $0; ] stuff
{ $$ = $4; }

See the file "test/t2.y" for a longer and more complete example. At the current time, the start symbol cannot have any arguments.

Variant parsers:

Btyacc supports the -S flag to use a different parser skeleton, changing the way that the parser is called and used. The skeleton "push.skel" is included to produce a "passive" parser that you feed tokens to (rather than having the parser call a separate yylex routine). With push.skel, yyparse is defined as follows:

int yyparse(int token, YYSTYPE yylval)

You should call yyparse repeatedly with successive tokens of input. It returns 0 if more input is needed, 1 for a successful parse, and -1 for an unrecoverable parse error.

Miscellaneous Features in ver. 1.0

by Chris Dodd

The -r option has been implemented. The -r option tells Yacc to put the read-only tables in y.tab.c and the code and variables in y.code.c. Keith Bostic asked for this option so that :yyfix could be eliminated.

The -l and -t options have been implemented. The -l option tells Yacc not to include #line directives in the code it produces. The -t option causes debugging code to be included in the compiled parser.

The code for error recovery has been changed to implement the same algorithm as AT&T Yacc. There will still be differences in the way error recovery works because AT&T Yacc uses more default reductions than Berkeley Yacc.

The environment variable TMPDIR determines the directory where temporary files will be created. If TMPDIR is defined, temporary files will be created in the directory whose pathname is the value of TMPDIR. By default, temporary files are created in /tmp.

The keywords are now case-insensitive. For example, %nonassoc, %NONASSOC, %NonAssoc, and %nOnAsSoC are all equivalent.

Commas and semicolons that are not part of C code are treated as commentary.

Line-end comments, as in BCPL, are permitted. Line-end comments begin with // and end at the next end-of-line. Line-end comments are permitted in C code; they are converted to C comments on output.

The form of y.output files has been changed to look more like those produced by AT&T Yacc.

A new kind of declaration has been added. The form of the declaration is

%ident string

where string is a sequence of characters beginning with a double quote and ending with either a double quote or the next end-of-line, whichever comes first. The declaration will cause a #ident directive to be written near the start of the output file.

If a parser has been compiled with debugging code, that code can be enabled by setting an environment variable. If the environment variable YYDEBUG is set to 0, debugging output is suppressed. If it is set to 1, debugging output is written to standard output.

Building BtYacc

by Chris Dodd and Vadim Maslov

We used GCC and GNU make to compile BtYacc both on UNIX and WIN32 paltforms. You are welcome to try different combinations of makes and compilers. Most likely it will work, but it may require Makefile changes.

There is no config script. Just type "make" and it should compile.

AWK. If you want to change file btyaccpa.ske (backtracking parser skeleton), you will need awk to compile it into skeleton.c file. We used GNU AWK (gawk) version 3.0.

It is known that using older versions of gawk may create problems in compilation, because older awks have problems with backslashes at the end of a line.

For MSDOS, there a "makefile.dos" that should do the trick. Note: makefile.dos was not tested for a long time.

The result of compilation should be a single executable called "btyacc" which you can install anywhere you like; it does not require any other files in the distribution to run.

Legal Stuff

by Chris Dodd and Vadim Maslov

In English: BtYacc is freeware. BtYacc is distributed with no warranty whatsoever. The author and any other contributors take no responsibility for any and all consequences of its use.


© 1998-2005 SoloTony (Antonio Solo) www.solotony.com