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NAME
Yacc - an LALR(1) parser generator
SYNOPSIS
yacc [ -BdgilLPrtvVy ] [ -b file_prefix ] [ -H defines_file ] [ -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-time configuration for
btyacc).
-d causes the header file y.tab.h to be written. It contains
#define's for the token identifiers.
-H defines_file
causes #define's for the token identifiers to be written to the
given defines_file rather than the y.tab.h file used by the -d
option.
-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
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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-time
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
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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.
The filename parameter is not optional. However, yacc accepts a
single ``-'' to read the grammar from the standard input. A double
``--'' marker denotes the end of options. A single filename parameter
is expected after a ``--'' marker.
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:
%code keyword { code }
Adds the indicated source code at a given point in the output
file. The optional keyword tells yacc where to insert the code:
top just after the version-definition in the generated code-
file.
requires
just after the declaration of public parser variables. If
the -d option is given, the code is inserted at the
beginning of the defines-file.
provides
just after the declaration of private parser variables.
If the -d option is given, the code is inserted at the end
of the defines-file.
If no keyword is given, the code is inserted at the beginning of
the section of code copied verbatim from the source file.
Multiple %code directives may be given; yacc inserts those into
the corresponding code- or defines-file in the order that they
appear in the source file.
%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
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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
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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,
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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 command-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.
+ Yacc accepts multiple parameters with %lex-param and %parse-param
in two forms
{type1 name1} {type2 name2} ...
{type1 name1, type2 name2 ...}
Bison accepts the latter (though undocumented), but depending on
the release may generate bad code.
+ Like bison, yacc will add parameters specified via %parse-param to
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yyparse, yyerror and (if configured for back-tracking) to the
destructor declared using %destructor. Bison puts the additional
parameters first for yyparse and yyerror but last for destructors.
Yacc matches this behavior.
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.
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