YACC(1) User Commands YACC(1)
NAME
Yacc - an LALR(1) parser generator
SYNOPSIS
yacc [ -BdgilLPrtvVy ] [ -b file_prefix ] [ -o output_file ] [ -p sym-
bol_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 lan-
guage. 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 bty-
acc).
-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 #de-
fine'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, en-
forcing consistent usage of the symbols defined in those files.
The supplementary header file makes it simpler to separate compi-
lation of lex- and yacc-files.
-l If the -l option is not specified, yacc will insert #line direc-
tives in the generated code. The #line directives let the C com-
piler 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 con-
figuration 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 sym-
bols 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 op-
tion.
-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 gen-
erates 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 liter-
als.
-t The -t option changes the preprocessor directives generated by
yacc so that debugging statements will be incorporated in the com-
piled 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 al-
ways available:
%destructor { code } symbol+
defines code that is invoked when a symbol is automatically dis-
carded 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 ap-
proach results in a memory leak if the YYSTYPE value is, or con-
tains, 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 "@$"
designates its location (see %locations directive).
A per-symbol destructor is defined by listing a grammar symbol
in symbol+. A per-type destructor is defined by listing a se-
mantic type tag (e.g., "<some_tag>") in symbol+; in this case,
the parser will invoke code whenever it discards any grammar
symbol that has that semantic type tag, unless that symbol has
its own per-symbol destructor.
Two categories of default destructor are supported that are in-
voked when discarding any grammar symbol that has no per-symbol
and no per-type destructor:
o the code for "<*>" is used for grammar symbols that have an
explicitly declared semantic type tag (via "%type");
o the code for "<>" is used for grammar symbols that have no
declared semantic type tag.
%expect number
tells yacc the expected number of shift/reduce conflicts. That
makes it only report the number if it differs.
%expect-rr number
tell yacc the expected number of reduce/reduce conflicts. That
makes it only report the number if it differs. This is (unlike
bison) allowable in LALR parsers.
%locations
tells yacc to enable management of position information associ-
ated with each token, provided by the lexer in the global vari-
able yylloc, similar to management of semantic value information
provided in yylval.
As for semantic values, locations can be referenced within ac-
tions using @$ 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 loca-
tion 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 assign-
ment 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 cus-
tomized parser.
%pure-parser
Most variables (other than yydebug and yynerrs) are allocated on
the stack within yyparse, making the parser reasonably reen-
trant.
%token-table
Make the parser's names for tokens available in the yytname ar-
ray. However, yacc does not predefine "$end", "$error" or "$un-
defined" 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:
o Yacc accepts an equals mark preceding the left curly brace of an
action (as in the original grammar file ftp.y):
| STAT CRLF
= {
statcmd();
}
o Yacc and bison emit code in different order, and in particular bi-
son makes forward reference to common functions such as yylex, yy-
parse and yyerror without providing prototypes.
o Bison's support for "%expect" is broken in more than one release.
For best results using bison, delete that directive.
o Bison has no equivalent for some of yacc's commmand-line options,
relying on directives embedded in the grammar file.
o 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.
Berkeley Yacc January 1, 2014 YACC(1)