1YACC(1) User Commands YACC(1)
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6 Yacc - an LALR(1) parser generator
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9 yacc [ -BdgilLPrtvVy ] [ -b file_prefix ] [ -H defines_file ] [ -o out‐
10 put_file ] [ -p symbol_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-time configuration for bty‐
27 acc).
28 -d causes the header file y.tab.h to be written. It contains #de‐
30 fine's for the token identifiers.
31 -H defines_file
33 causes #define's for the token identifiers to be written to the
34 given defines_file rather than the y.tab.h file used by the -d op‐
35 tion.
36 -g The -g option causes a graphical description of the generated
38 LALR(1) parser to be written to the file y.dot in graphviz format,
39 ready to be processed by dot(1).
40 -i The -i option causes a supplementary header file y.tab.i to be
42 written. It contains extern declarations and supplementary #de‐
43 fine's as needed to map the conventional yacc yy-prefixed names to
44 whatever the -p option may specify. The code file, e.g., y.tab.c
45 is modified to #include this file as well as the y.tab.h file, en‐
46 forcing consistent usage of the symbols defined in those files.
47 The supplementary header file makes it simpler to separate compi‐
49 lation of lex- and yacc-files.
50 -l If the -l option is not specified, yacc will insert #line direc‐
52 tives in the generated code. The #line directives let the C com‐
53 piler relate errors in the generated code to the user's original
54 code. If the -l option is specified, yacc will not insert the
55 #line directives. #line directives specified by the user will be
56 retained.
57 -L enable position processing, e.g., “%locations” (compile-time con‐
59 figuration for btyacc).
60 -o output_file
62 specify the filename for the parser file. If this option is not
63 given, the output filename is the file prefix concatenated with
64 the file suffix, e.g., y.tab.c. This overrides the -b option.
65 -p symbol_prefix
67 The -p option changes the prefix prepended to yacc-generated sym‐
68 bols to the string denoted by symbol_prefix. The default prefix
69 is the string yy.
70 -P create a reentrant parser, e.g., “%pure-parser”.
72 -r The -r option causes yacc to produce separate files for code and
74 tables. The code file is named y.code.c, and the tables file is
75 named y.tab.c. The prefix “y.” can be overridden using the -b op‐
76 tion.
77 -s suppress “#define” statements generated for string literals in a
79 “%token” statement, to more closely match original yacc behavior.
80 Normally when yacc sees a line such as
82 %token OP_ADD "ADD"
84 it notices that the quoted “ADD” is a valid C identifier, and gen‐
86 erates a #define not only for OP_ADD, but for ADD as well, e.g.,
87 #define OP_ADD 257
89 #define ADD 258
90 The original yacc does not generate the second “#define”. The -s
92 option suppresses this “#define”.
93 POSIX (IEEE 1003.1 2004) documents only names and numbers for
95 “%token”, though original yacc and bison also accept string liter‐
96 als.
97 -t The -t option changes the preprocessor directives generated by
99 yacc so that debugging statements will be incorporated in the com‐
100 piled code.
101 -v The -v option causes a human-readable description of the generated
103 parser to be written to the file y.output.
104 -V print the version number to the standard output.
106 -y yacc ignores this option, which bison supports for ostensible
108 POSIX compatibility.
109 The filename parameter is not optional. However, yacc accepts a single
111 “-” to read the grammar from the standard input. A double “--” marker
112 denotes the end of options. A single filename parameter is expected
113 after a “--” marker.
114
116 Yacc provides some extensions for compatibility with bison and other
117 implementations of yacc. The %destructor and %locations features are
118 available only if yacc has been configured and compiled to support the
119 back-tracking (btyacc) functionality. The remaining features are al‐
120 ways available:
121 %code keyword { code }
123 Adds the indicated source code at a given point in the output
124 file. The optional keyword tells yacc where to insert the code:
125 top just after the version-definition in the generated code-
127 file.
128 requires
130 just after the declaration of public parser variables. If
131 the -d option is given, the code is inserted at the begin‐
132 ning of the defines-file.
133 provides
135 just after the declaration of private parser variables. If
136 the -d option is given, the code is inserted at the end of
137 the defines-file.
138 If no keyword is given, the code is inserted at the beginning of
140 the section of code copied verbatim from the source file. Mul‐
141 tiple %code directives may be given; yacc inserts those into the
142 corresponding code- or defines-file in the order that they ap‐
143 pear in the source file.
144 %destructor { code } symbol+
146 defines code that is invoked when a symbol is automatically dis‐
147 carded during error recovery. This code can be used to reclaim
148 dynamically allocated memory associated with the corresponding
149 semantic value for cases where user actions cannot manage the
150 memory explicitly.
151 On encountering a parse error, the generated parser discards
153 symbols on the stack and input tokens until it reaches a state
154 that will allow parsing to continue. This error recovery ap‐
155 proach results in a memory leak if the YYSTYPE value is, or con‐
156 tains, pointers to dynamically allocated memory.
157 The bracketed code is invoked whenever the parser discards one
159 of the symbols. Within code, “$$” or “$<tag>$” designates the
160 semantic value associated with the discarded symbol, and “@$”
161 designates its location (see %locations directive).
162 A per-symbol destructor is defined by listing a grammar symbol
164 in symbol+. A per-type destructor is defined by listing a se‐
165 mantic type tag (e.g., “<some_tag>”) in symbol+; in this case,
166 the parser will invoke code whenever it discards any grammar
167 symbol that has that semantic type tag, unless that symbol has
168 its own per-symbol destructor.
169 Two categories of default destructor are supported that are in‐
171 voked when discarding any grammar symbol that has no per-symbol
172 and no per-type destructor:
173 • the code for “<*>” is used for grammar symbols that have an
175 explicitly declared semantic type tag (via “%type”);
176 • the code for “<>” is used for grammar symbols that have no
178 declared semantic type tag.
179 %expect number
181 tells yacc the expected number of shift/reduce conflicts. That
182 makes it only report the number if it differs.
183 %expect-rr number
185 tell yacc the expected number of reduce/reduce conflicts. That
186 makes it only report the number if it differs. This is (unlike
187 bison) allowable in LALR parsers.
188 %locations
190 tells yacc to enable management of position information associ‐
191 ated with each token, provided by the lexer in the global vari‐
192 able yylloc, similar to management of semantic value information
193 provided in yylval.
194 As for semantic values, locations can be referenced within ac‐
196 tions using @$ to refer to the location of the left hand side
197 symbol, and @N (N an integer) to refer to the location of one of
198 the right hand side symbols. Also as for semantic values, when
199 a rule is matched, a default action is used the compute the lo‐
200 cation represented by @$ as the beginning of the first symbol
201 and the end of the last symbol in the right hand side of the
202 rule. This default computation can be overridden by explicit
203 assignment to @$ in a rule action.
204 The type of yylloc is YYLTYPE, which is defined by default as:
206 typedef struct YYLTYPE {
208 int first_line;
209 int first_column;
210 int last_line;
211 int last_column;
212 } YYLTYPE;
213 YYLTYPE can be redefined by the user (YYLTYPE_IS_DEFINED must be
215 defined, to inhibit the default) in the declarations section of
216 the specification file. As in bison, the macro YYLLOC_DEFAULT
217 is invoked each time a rule is matched to calculate a position
218 for the left hand side of the rule, before the associated action
219 is executed; this macro can be redefined by the user.
220 This directive adds a YYLTYPE parameter to yyerror(). If the
222 %pure-parser directive is present, a YYLTYPE parameter is added
223 to yylex() calls.
224 %lex-param { argument-declaration }
226 By default, the lexer accepts no parameters, e.g., yylex(). Use
227 this directive to add parameter declarations for your customized
228 lexer.
229 %parse-param { argument-declaration }
231 By default, the parser accepts no parameters, e.g., yyparse().
232 Use this directive to add parameter declarations for your cus‐
233 tomized parser.
234 %pure-parser
236 Most variables (other than yydebug and yynerrs) are allocated on
237 the stack within yyparse, making the parser reasonably reen‐
238 trant.
239 %token-table
241 Make the parser's names for tokens available in the yytname ar‐
242 ray. However, yacc does not predefine “$end”, “$error” or “$un‐
243 defined” in this array.
244
246 According to Robert Corbett,
247 Berkeley Yacc is an LALR(1) parser generator. Berkeley Yacc
249 has been made as compatible as possible with AT&T Yacc.
250 Berkeley Yacc can accept any input specification that
251 conforms to the AT&T Yacc documentation. Specifications
252 that take advantage of undocumented features of AT&T Yacc
253 will probably be rejected.
254 The rationale in
256 http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
258 documents some features of AT&T yacc which are no longer required for
260 POSIX compliance.
261 That said, you may be interested in reusing grammar files with some
263 other implementation which is not strictly compatible with AT&T yacc.
264 For instance, there is bison. Here are a few differences:
265 • Yacc accepts an equals mark preceding the left curly brace of an
267 action (as in the original grammar file ftp.y):
268 | STAT CRLF
270 = {
271 statcmd();
272 }
273 • Yacc and bison emit code in different order, and in particular bi‐
275 son makes forward reference to common functions such as yylex, yy‐
276 parse and yyerror without providing prototypes.
277 • Bison's support for “%expect” is broken in more than one release.
279 For best results using bison, delete that directive.
280 • Bison has no equivalent for some of yacc's command-line options,
282 relying on directives embedded in the grammar file.
283 • Bison's “-y” option does not affect bison's lack of support for
285 features of AT&T yacc which were deemed obsolescent.
286 • Yacc accepts multiple parameters with %lex-param and %parse-param
288 in two forms
289 {type1 name1} {type2 name2} ...
291 {type1 name1, type2 name2 ...}
292 Bison accepts the latter (though undocumented), but depending on
294 the release may generate bad code.
295 • Like bison, yacc will add parameters specified via %parse-param to
297 yyparse, yyerror and (if configured for back-tracking) to the de‐
298 structor declared using %destructor. Bison puts the additional pa‐
299 rameters first for yyparse and yyerror but last for destructors.
300 Yacc matches this behavior.
301
303 If there are rules that are never reduced, the number of such rules is
304 reported on standard error. If there are any LALR(1) conflicts, the
305 number of conflicts is reported on standard error.
306Berkeley Yacc September 10, 2020 YACC(1)