Ubuntu Manpage: Yacc - an LALR(1) parser generator

Provided by: byacc_20140715-1build1_amd64

NAME

       Yacc - an LALR(1) parser generator

SYNOPSIS

       yacc [ -BdgilLPrtvVy ] [ -b file_prefix ] [ -o output_file ] [ -p symbol_prefix ] filename

DESCRIPTION

Yacc reads the grammar specification in the file filename and generates an LALR(1) parser for it. The
parsers consist of a set of LALR(1) parsing tables and a driver routine written in the C programming
language. Yacc normally writes the parse tables and the driver routine to the file y.tab.c.

The following options are available:

-b file_prefix
The -b option changes the prefix prepended to the output file names to the string denoted by
file_prefix. The default prefix is the character y.

-B create a backtracking parser (compile-type configuration for btyacc).

-d The -d option causes the header file y.tab.h to be written. It contains #define's for the token
identifiers.

-g The -g option causes a graphical description of the generated LALR(1) parser to be written to the
file y.dot in graphviz format, ready to be processed by dot(1).

-i The -i option causes a supplementary header file y.tab.i to be written. It contains extern
declarations and supplementary #define's as needed to map the conventional yacc yy-prefixed names to
whatever the -p option may specify. The code file, e.g., y.tab.c is modified to #include this file
as well as the y.tab.h file, enforcing consistent usage of the symbols defined in those files.

The supplementary header file makes it simpler to separate compilation of lex- and yacc-files.

-l If the -l option is not specified, yacc will insert #line directives in the generated code. The
#line directives let the C compiler relate errors in the generated code to the user's original code.
If the -l option is specified, yacc will not insert the #line directives. #line directives
specified by the user will be retained.

-L enable position processing, e.g., “%locations” (compile-type configuration for btyacc).

-o output_file
specify the filename for the parser file. If this option is not given, the output filename is the
file prefix concatenated with the file suffix, e.g., y.tab.c. This overrides the -b option.

-p symbol_prefix
The -p option changes the prefix prepended to yacc-generated symbols to the string denoted by
symbol_prefix. The default prefix is the string yy.

-P create a reentrant parser, e.g., “%pure-parser”.

-r The -r option causes yacc to produce separate files for code and tables. The code file is named
y.code.c, and the tables file is named y.tab.c. The prefix “y.” can be overridden using the -b
option.

-s suppress “#define” statements generated for string literals in a “%token” statement, to more closely
match original yacc behavior.

Normally when yacc sees a line such as

%token OP_ADD "ADD"

it notices that the quoted “ADD” is a valid C identifier, and generates a #define not only for
OP_ADD, but for ADD as well, e.g.,

#define OP_ADD 257
#define ADD 258

The original yacc does not generate the second “#define”. The -s option suppresses this “#define”.

POSIX (IEEE 1003.1 2004) documents only names and numbers for “%token”, though original yacc and
bison also accept string literals.

-t The -t option changes the preprocessor directives generated by yacc so that debugging statements
will be incorporated in the compiled code.

-v The -v option causes a human-readable description of the generated parser to be written to the file
y.output.

-V print the version number to the standard output.

-y yacc ignores this option, which bison supports for ostensible POSIX compatibility.

EXTENSIONS

yacc provides some extensions for compatibility with bison and other implementations of yacc. The
%destructor and %locations features are available only if yacc has been configured and compiled to
support the back-tracking (btyacc) functionality. The remaining features are always available:

%destructor { code } symbol+
defines code that is invoked when a symbol is automatically discarded during error recovery. This
code can be used to reclaim dynamically allocated memory associated with the corresponding
semantic value for cases where user actions cannot manage the memory explicitly.

On encountering a parse error, the generated parser discards symbols on the stack and input tokens
until it reaches a state that will allow parsing to continue. This error recovery approach
results in a memory leak if the YYSTYPE value is, or contains, pointers to dynamically allocated
memory.

The bracketed code is invoked whenever the parser discards one of the symbols. Within code, “$$”
or “$<tag>$” designates the semantic value associated with the discarded symbol, and “@$”
designates its location (see %locations directive).

A per-symbol destructor is defined by listing a grammar symbol in symbol+. A per-type destructor
is defined by listing a semantic type tag (e.g., “<some_tag>”) in symbol+; in this case, the
parser will invoke code whenever it discards any grammar symbol that has that semantic type tag,
unless that symbol has its own per-symbol destructor.

Two categories of default destructor are supported that are invoked when discarding any grammar
symbol that has no per-symbol and no per-type destructor:

• the code for “<*>” is used for grammar symbols that have an explicitly declared semantic type
tag (via “%type”);

• the code for “<>” is used for grammar symbols that have no declared semantic type tag.

%expect number
tells yacc the expected number of shift/reduce conflicts. That makes it only report the number if
it differs.

%expect-rr number
tell yacc the expected number of reduce/reduce conflicts. That makes it only report the number if
it differs. This is (unlike bison) allowable in LALR parsers.

%locations
tells yacc to enable management of position information associated with each token, provided by
the lexer in the global variable yylloc, similar to management of semantic value information
provided in yylval.

As for semantic values, locations can be referenced within actions using @$ to refer to the
location of the left hand side symbol, and @N (N an integer) to refer to the location of one of
the right hand side symbols. Also as for semantic values, when a rule is matched, a default action
is used the compute the location represented by @$ as the beginning of the first symbol and the
end of the last symbol in the right hand side of the rule. This default computation can be
overridden by explicit assignment to @$ in a rule action.

The type of yylloc is YYLTYPE, which is defined by default as:

typedef struct YYLTYPE {
int first_line;
int first_column;
int last_line;
int last_column;
} YYLTYPE;

YYLTYPE can be redefined by the user (YYLTYPE_IS_DEFINED must be defined, to inhibit the default)
in the declarations section of the specification file. As in bison, the macro YYLLOC_DEFAULT is
invoked each time a rule is matched to calculate a position for the left hand side of the rule,
before the associated action is executed; this macro can be redefined by the user.

This directive adds a YYLTYPE parameter to yyerror(). If the %pure-parser directive is present, a
YYLTYPE parameter is added to yylex() calls.

%lex-param { argument-declaration }
By default, the lexer accepts no parameters, e.g., yylex(). Use this directive to add parameter
declarations for your customized lexer.

%parse-param { argument-declaration }
By default, the parser accepts no parameters, e.g., yyparse(). Use this directive to add
parameter declarations for your customized parser.

%pure-parser
Most variables (other than yydebug and yynerrs) are allocated on the stack within yyparse, making
the parser reasonably reentrant.

%token-table
Make the parser's names for tokens available in the yytname array. However, yacc does not
predefine “$end”, “$error” or “$undefined” in this array.

PORTABILITY

According to Robert Corbett,

Berkeley Yacc is an LALR(1) parser generator. Berkeley Yacc has been made
as compatible as possible with AT&T Yacc. Berkeley Yacc can accept any input
specification that conforms to the AT&T Yacc documentation. Specifications
that take advantage of undocumented features of AT&T Yacc will probably be
rejected.

The rationale in

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html

documents some features of AT&T yacc which are no longer required for POSIX compliance.

That said, you may be interested in reusing grammar files with some other implementation which is not
strictly compatible with AT&T yacc. For instance, there is bison. Here are a few differences:

• Yacc accepts an equals mark preceding the left curly brace of an action (as in the original grammar
file ftp.y):

| STAT CRLF
= {
statcmd();
}

• Yacc and bison emit code in different order, and in particular bison makes forward reference to
common functions such as yylex, yyparse and yyerror without providing prototypes.

• Bison's support for “%expect” is broken in more than one release. For best results using bison,
delete that directive.

• Bison has no equivalent for some of yacc's commmand-line options, relying on directives embedded in
the grammar file.

• Bison's “-y” option does not affect bison's lack of support for features of AT&T yacc which were
deemed obsolescent.

DIAGNOSTICS

       If there are rules that are never reduced, the number of such rules is reported on  standard  error.   If
       there are any LALR(1) conflicts, the number of conflicts is reported on standard error.