1YACC(1) User Commands YACC(1)
2
6 Yacc - an LALR(1) parser generator
7
9 yacc [ -BdgilLPrtvVy ] [ -b file_prefix ] [ -o output_file ] [ -p sym‐
10 bol_prefix ] filename
11
13 Yacc reads the grammar specification in the file filename and generates
14 an LALR(1) parser for it. The parsers consist of a set of LALR(1)
15 parsing tables and a driver routine written in the C programming lan‐
16 guage. Yacc normally writes the parse tables and the driver routine to
17 the file y.tab.c.
18 The following options are available:
20 -b file_prefix
22 The -b option changes the prefix prepended to the output file
23 names to the string denoted by file_prefix. The default prefix is
24 the character y.
25 -B create a backtracking parser (compile-type configuration for bty‐
27 acc).
28 -d The -d option causes the header file y.tab.h to be written. It
30 contains #define's for the token identifiers.
31 -g The -g option causes a graphical description of the generated
33 LALR(1) parser to be written to the file y.dot in graphviz format,
34 ready to be processed by dot(1).
35 -i The -i option causes a supplementary header file y.tab.i to be
37 written. It contains extern declarations and supplementary
38 #define's as needed to map the conventional yacc yy-prefixed names
39 to whatever the -p option may specify. The code file, e.g.,
40 y.tab.c is modified to #include this file as well as the y.tab.h
41 file, enforcing consistent usage of the symbols defined in those
42 files.
43 The supplementary header file makes it simpler to separate compi‐
45 lation of lex- and yacc-files.
46 -l If the -l option is not specified, yacc will insert #line direc‐
48 tives in the generated code. The #line directives let the C com‐
49 piler relate errors in the generated code to the user's original
50 code. If the -l option is specified, yacc will not insert the
51 #line directives. #line directives specified by the user will be
52 retained.
53 -L enable position processing, e.g., “%locations” (compile-type con‐
55 figuration for btyacc).
56 -o output_file
58 specify the filename for the parser file. If this option is not
59 given, the output filename is the file prefix concatenated with
60 the file suffix, e.g., y.tab.c. This overrides the -b option.
61 -p symbol_prefix
63 The -p option changes the prefix prepended to yacc-generated sym‐
64 bols to the string denoted by symbol_prefix. The default prefix
65 is the string yy.
66 -P create a reentrant parser, e.g., “%pure-parser”.
68 -r The -r option causes yacc to produce separate files for code and
70 tables. The code file is named y.code.c, and the tables file is
71 named y.tab.c. The prefix “y.” can be overridden using the -b
72 option.
73 -s suppress “#define” statements generated for string literals in a
75 “%token” statement, to more closely match original yacc behavior.
76 Normally when yacc sees a line such as
78 %token OP_ADD "ADD"
80 it notices that the quoted “ADD” is a valid C identifier, and gen‐
82 erates a #define not only for OP_ADD, but for ADD as well, e.g.,
83 #define OP_ADD 257
85 #define ADD 258
86 The original yacc does not generate the second “#define”. The -s
88 option suppresses this “#define”.
89 POSIX (IEEE 1003.1 2004) documents only names and numbers for
91 “%token”, though original yacc and bison also accept string liter‐
92 als.
93 -t The -t option changes the preprocessor directives generated by
95 yacc so that debugging statements will be incorporated in the com‐
96 piled code.
97 -v The -v option causes a human-readable description of the generated
99 parser to be written to the file y.output.
100 -V print the version number to the standard output.
102 -y yacc ignores this option, which bison supports for ostensible
104 POSIX compatibility.
105
107 yacc provides some extensions for compatibility with bison and other
108 implementations of yacc. The %destructor and %locations features are
109 available only if yacc has been configured and compiled to support the
110 back-tracking (btyacc) functionality. The remaining features are
111 always available:
112 %destructor { code } symbol+
114 defines code that is invoked when a symbol is automatically dis‐
115 carded during error recovery. This code can be used to reclaim
116 dynamically allocated memory associated with the corresponding
117 semantic value for cases where user actions cannot manage the
118 memory explicitly.
119 On encountering a parse error, the generated parser discards
121 symbols on the stack and input tokens until it reaches a state
122 that will allow parsing to continue. This error recovery
123 approach results in a memory leak if the YYSTYPE value is, or
124 contains, pointers to dynamically allocated memory.
125 The bracketed code is invoked whenever the parser discards one
127 of the symbols. Within code, “$$” or “$<tag>$” designates the
128 semantic value associated with the discarded symbol, and “@$”
129 designates its location (see %locations directive).
130 A per-symbol destructor is defined by listing a grammar symbol
132 in symbol+. A per-type destructor is defined by listing a
133 semantic type tag (e.g., “<some_tag>”) in symbol+; in this case,
134 the parser will invoke code whenever it discards any grammar
135 symbol that has that semantic type tag, unless that symbol has
136 its own per-symbol destructor.
137 Two categories of default destructor are supported that are
139 invoked when discarding any grammar symbol that has no per-sym‐
140 bol and no per-type destructor:
141 · the code for “<*>” is used for grammar symbols that have an
143 explicitly declared semantic type tag (via “%type”);
144 · the code for “<>” is used for grammar symbols that have no
146 declared semantic type tag.
147 %expect number
149 tells yacc the expected number of shift/reduce conflicts. That
150 makes it only report the number if it differs.
151 %expect-rr number
153 tell yacc the expected number of reduce/reduce conflicts. That
154 makes it only report the number if it differs. This is (unlike
155 bison) allowable in LALR parsers.
156 %locations
158 tells yacc to enable management of position information associ‐
159 ated with each token, provided by the lexer in the global vari‐
160 able yylloc, similar to management of semantic value information
161 provided in yylval.
162 As for semantic values, locations can be referenced within
164 actions using @$ to refer to the location of the left hand side
165 symbol, and @N (N an integer) to refer to the location of one of
166 the right hand side symbols. Also as for semantic values, when a
167 rule is matched, a default action is used the compute the loca‐
168 tion represented by @$ as the beginning of the first symbol and
169 the end of the last symbol in the right hand side of the rule.
170 This default computation can be overridden by explicit assign‐
171 ment to @$ in a rule action.
172 The type of yylloc is YYLTYPE, which is defined by default as:
174 typedef struct YYLTYPE {
176 int first_line;
177 int first_column;
178 int last_line;
179 int last_column;
180 } YYLTYPE;
181 YYLTYPE can be redefined by the user (YYLTYPE_IS_DEFINED must be
183 defined, to inhibit the default) in the declarations section of
184 the specification file. As in bison, the macro YYLLOC_DEFAULT
185 is invoked each time a rule is matched to calculate a position
186 for the left hand side of the rule, before the associated action
187 is executed; this macro can be redefined by the user.
188 This directive adds a YYLTYPE parameter to yyerror(). If the
190 %pure-parser directive is present, a YYLTYPE parameter is added
191 to yylex() calls.
192 %lex-param { argument-declaration }
194 By default, the lexer accepts no parameters, e.g., yylex(). Use
195 this directive to add parameter declarations for your customized
196 lexer.
197 %parse-param { argument-declaration }
199 By default, the parser accepts no parameters, e.g., yyparse().
200 Use this directive to add parameter declarations for your cus‐
201 tomized parser.
202 %pure-parser
204 Most variables (other than yydebug and yynerrs) are allocated on
205 the stack within yyparse, making the parser reasonably reen‐
206 trant.
207 %token-table
209 Make the parser's names for tokens available in the yytname
210 array. However, yacc does not predefine “$end”, “$error” or
211 “$undefined” in this array.
212
214 According to Robert Corbett,
215 Berkeley Yacc is an LALR(1) parser generator. Berkeley Yacc has been made
217 as compatible as possible with AT&T Yacc. Berkeley Yacc can accept any input
218 specification that conforms to the AT&T Yacc documentation. Specifications
219 that take advantage of undocumented features of AT&T Yacc will probably be
220 rejected.
221 The rationale in
223 http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
225 documents some features of AT&T yacc which are no longer required for
227 POSIX compliance.
228 That said, you may be interested in reusing grammar files with some
230 other implementation which is not strictly compatible with AT&T yacc.
231 For instance, there is bison. Here are a few differences:
232 · Yacc accepts an equals mark preceding the left curly brace of an
234 action (as in the original grammar file ftp.y):
235 | STAT CRLF
237 = {
238 statcmd();
239 }
240 · Yacc and bison emit code in different order, and in particular
242 bison makes forward reference to common functions such as yylex,
243 yyparse and yyerror without providing prototypes.
244 · Bison's support for “%expect” is broken in more than one release.
246 For best results using bison, delete that directive.
247 · Bison has no equivalent for some of yacc's commmand-line options,
249 relying on directives embedded in the grammar file.
250 · Bison's “-y” option does not affect bison's lack of support for
252 features of AT&T yacc which were deemed obsolescent.
253 · Yacc accepts multiple parameters with %lex-param and %parse-param
255 in two forms
256 {type1 name1} {type2 name2} ...
258 {type1 name1, type2 name2 ...}
259 Bison accepts the latter (though undocumented), but depending on
261 the release may generate bad code.
262 · Like bison, yacc will add parameters specified via %parse-param to
264 yyparse, yyerror and (if configured for back-tracking) to the
265 destructor declared using %destructor. Bison puts the additional
266 parameters first for yyparse and yyerror but last for destructors.
267 Yacc matches this behavior.
268
270 If there are rules that are never reduced, the number of such rules is
271 reported on standard error. If there are any LALR(1) conflicts, the
272 number of conflicts is reported on standard error.
273Berkeley Yacc October 5, 2014 YACC(1)