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YACC(1) General Commands Manual YACC(1)
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
yacc).
-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 yacc).
-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 The -P options instructs yacc to create a reentrant parser,
like `%pure-parser' does.
-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'.
-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.
IEEE Std 1003.1 ("POSIX.1") documents only names and numbers
for `%token', though the original yacc and bison(1) 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 prints the version number to the standard
output.
-v The -v option causes a human-readable description of the
generated parser to be written to the file y.output.
-y yacc ignores this option, which bison(1) supports for
ostensible POSIX compatibility.
EXTENSIONS
yacc provides some extensions for compatibility with bison(1) 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 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 it `$$' or `$<tag>$' designates
the semantic value associated with the discarded symbol, and
`@$' designates its location (see `%locations' directive).
A per-symbol destructor is defined by listing a grammar
symbol in symbol+. A per-type destructor is defined by
listing a semantic type tag (e.g., `<some_tag>') in symbol+;
in this case, the parser will invoke code whenever it
discards any grammar symbol that has that semantic type tag,
unless that symbol has its own per-symbol destructor.
Two categories of default destructor are supported that are
invoked when discarding any grammar symbol that has no per-
symbol and no per-type destructor:
The code for `<*>' is used for grammar symbols that have an
explicitly declared semantic type tag (via `%type');
The code for `<>' is used for grammar symbols that have no
declared semantic type tag.
%expect number
Tell yacc the expected number of shift/reduce conflicts.
That makes it only report the number if it differs.
%expect-rr number
Tell yacc the expected number of reduce/reduce conflicts.
That makes it only report the number if it differs. This is,
unlike bison(1), allowable in LALR(1) parsers.
%locations Tell yacc to enable management of position information
associated with each token, provided by the lexer in the
global variable yylloc, similar to management of semantic
value information provided in yylval.
As for semantic values, locations can be referenced within
actions using `@$' 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(1), 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 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(1). 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(1) emit code in different order, and in particular
bison(1) makes forward reference to common functions such as yylex(),
yyparse() and yyerror() without providing prototypes.
bison(1) support for `%expect' is broken in more than one release. For
best results using bison(1), delete that directive.
bison(1) has no equivalent for some of yacc's command-line options,
relying on directives embedded in the grammar file.
bison(1) -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(1) accepts the latter (though undocumented), but depending on the
release may generate bad code.
Like bison(1), yacc will add parameters specified via `%parse-param' to
yyparse(), yyerror() and (if configured for back-tracking) to the
destructor declared using `%destructor'.
bison(1) puts the additional parameters first for yyparse() and yyerror()
but last for destructors. yacc matches this behavior.
ENVIRONMENT
The following environment variable is referenced by yacc:
TMPDIR If the environment variable TMPDIR is set, the string denoted by
TMPDIR will be used as the name of the directory where the
temporary files are created.
TABLES
The names of the tables generated by this version of yacc are yylhs,
yylen, yydefred, yydgoto, yysindex, yyrindex, yygindex, yytable, and
yycheck. Two additional tables, yyname and yyrule, are created if
YYDEBUG is defined and non-zero.
FILES
y.code.c
y.tab.c
y.tab.h
y.output
/tmp/yacc.aXXXXXX
/tmp/yacc.tXXXXXX
/tmp/yacc.uXXXXXX
DIAGNOSTICS
If there are rules that are never reduced, the number of such rules is
written to the standard error. If there are any LALR(1) conflicts, the
number of conflicts is also written to the standard error.
STANDARDS
The yacc utility conforms to IEEE Std 1003.2 ("POSIX.2").
NetBSD 10.99 October 5, 2014 NetBSD 10.99