1YACC(1) User Commands YACC(1)
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6 Yacc - an LALR(1) parser generator
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9 yacc [ -BdghilLPrtvVy ] [ -b file_prefix ] [ -H defines_file ] [ -o
10 output_file ] [ -p symbol_prefix ] filename
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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 print a usage message.
33 -H defines_file
35 causes #define's for the token identifiers to be written to the
36 given defines_file rather than the y.tab.h file used by the -d op‐
37 tion.
38 -g The -g option causes a graphical description of the generated
40 LALR(1) parser to be written to the file y.dot in graphviz format,
41 ready to be processed by dot(1).
42 -i The -i option causes a supplementary header file y.tab.i to be
44 written. It contains extern declarations and supplementary #de‐
45 fine's as needed to map the conventional yacc yy-prefixed names to
46 whatever the -p option may specify. The code file, e.g., y.tab.c
47 is modified to #include this file as well as the y.tab.h file, en‐
48 forcing consistent usage of the symbols defined in those files.
49 The supplementary header file makes it simpler to separate compi‐
51 lation of lex- and yacc-files.
52 -l If the -l option is not specified, yacc will insert #line direc‐
54 tives in the generated code. The #line directives let the C com‐
55 piler relate errors in the generated code to the user's original
56 code. If the -l option is specified, yacc will not insert the
57 #line directives. #line directives specified by the user will be
58 retained.
59 -L enable position processing, e.g., “%locations” (compile-time con‐
61 figuration for btyacc).
62 -o output_file
64 specify the filename for the parser file. If this option is not
65 given, the output filename is the file prefix concatenated with
66 the file suffix, e.g., y.tab.c. This overrides the -b option.
67 -p symbol_prefix
69 The -p option changes the prefix prepended to yacc-generated sym‐
70 bols to the string denoted by symbol_prefix. The default prefix
71 is the string yy.
72 -P create a reentrant parser, e.g., “%pure-parser”.
74 -r The -r option causes yacc to produce separate files for code and
76 tables. The code file is named y.code.c, and the tables file is
77 named y.tab.c. The prefix “y.” can be overridden using the -b op‐
78 tion.
79 -s suppress “#define” statements generated for string literals in a
81 “%token” statement, to more closely match original yacc behavior.
82 Normally when yacc sees a line such as
84 %token OP_ADD "ADD"
86 it notices that the quoted “ADD” is a valid C identifier, and gen‐
88 erates a #define not only for OP_ADD, but for ADD as well, e.g.,
89 #define OP_ADD 257
91 #define ADD 258
92 The original yacc does not generate the second “#define”. The -s
94 option suppresses this “#define”.
95 POSIX (IEEE 1003.1 2004) documents only names and numbers for
97 “%token”, though original yacc and bison also accept string liter‐
98 als.
99 -t The -t option changes the preprocessor directives generated by
101 yacc so that debugging statements will be incorporated in the com‐
102 piled code.
103 -v The -v option causes a human-readable description of the generated
105 parser to be written to the file y.output.
106 -V print the version number to the standard output.
108 -y yacc ignores this option, which bison supports for ostensible
110 POSIX compatibility.
111 The filename parameter is not optional. However, yacc accepts a single
113 “-” to read the grammar from the standard input. A double “--” marker
114 denotes the end of options. A single filename parameter is expected
115 after a “--” marker.
116
118 Yacc provides some extensions for compatibility with bison and other
119 implementations of yacc. It accepts several long options which have
120 equivalents in yacc. The %destructor and %locations features are
121 available only if yacc has been configured and compiled to support the
122 back-tracking (btyacc) functionality. The remaining features are al‐
123 ways available:
124 %code keyword { code }
126 Adds the indicated source code at a given point in the output
127 file. The optional keyword tells yacc where to insert the code:
128 top just after the version-definition in the generated code-
130 file.
131 requires
133 just after the declaration of public parser variables. If
134 the -d option is given, the code is inserted at the begin‐
135 ning of the defines-file.
136 provides
138 just after the declaration of private parser variables. If
139 the -d option is given, the code is inserted at the end of
140 the defines-file.
141 If no keyword is given, the code is inserted at the beginning of
143 the section of code copied verbatim from the source file. Mul‐
144 tiple %code directives may be given; yacc inserts those into the
145 corresponding code- or defines-file in the order that they ap‐
146 pear in the source file.
147 %destructor { code } symbol+
149 defines code that is invoked when a symbol is automatically dis‐
150 carded during error recovery. This code can be used to reclaim
151 dynamically allocated memory associated with the corresponding
152 semantic value for cases where user actions cannot manage the
153 memory explicitly.
154 On encountering a parse error, the generated parser discards
156 symbols on the stack and input tokens until it reaches a state
157 that will allow parsing to continue. This error recovery ap‐
158 proach results in a memory leak if the YYSTYPE value is, or con‐
159 tains, pointers to dynamically allocated memory.
160 The bracketed code is invoked whenever the parser discards one
162 of the symbols. Within code, “$$” or “$<tag>$” designates the
163 semantic value associated with the discarded symbol, and “@$”
164 designates its location (see %locations directive).
165 A per-symbol destructor is defined by listing a grammar symbol
167 in symbol+. A per-type destructor is defined by listing a se‐
168 mantic type tag (e.g., “<some_tag>”) in symbol+; in this case,
169 the parser will invoke code whenever it discards any grammar
170 symbol that has that semantic type tag, unless that symbol has
171 its own per-symbol destructor.
172 Two categories of default destructor are supported that are in‐
174 voked when discarding any grammar symbol that has no per-symbol
175 and no per-type destructor:
176 • the code for “<*>” is used for grammar symbols that have an
178 explicitly declared semantic type tag (via “%type”);
179 • the code for “<>” is used for grammar symbols that have no
181 declared semantic type tag.
182 %expect number
184 tells yacc the expected number of shift/reduce conflicts. That
185 makes it only report the number if it differs.
186 %expect-rr number
188 tell yacc the expected number of reduce/reduce conflicts. That
189 makes it only report the number if it differs. This is (unlike
190 bison) allowable in LALR parsers.
191 %locations
193 tells yacc to enable management of position information associ‐
194 ated with each token, provided by the lexer in the global vari‐
195 able yylloc, similar to management of semantic value information
196 provided in yylval.
197 As for semantic values, locations can be referenced within ac‐
199 tions using @$ to refer to the location of the left hand side
200 symbol, and @N (N an integer) to refer to the location of one of
201 the right hand side symbols. Also as for semantic values, when
202 a rule is matched, a default action is used the compute the lo‐
203 cation represented by @$ as the beginning of the first symbol
204 and the end of the last symbol in the right hand side of the
205 rule. This default computation can be overridden by explicit
206 assignment to @$ in a rule action.
207 The type of yylloc is YYLTYPE, which is defined by default as:
209 typedef struct YYLTYPE {
211 int first_line;
212 int first_column;
213 int last_line;
214 int last_column;
215 } YYLTYPE;
216 YYLTYPE can be redefined by the user (YYLTYPE_IS_DEFINED must be
218 defined, to inhibit the default) in the declarations section of
219 the specification file. As in bison, the macro YYLLOC_DEFAULT
220 is invoked each time a rule is matched to calculate a position
221 for the left hand side of the rule, before the associated action
222 is executed; this macro can be redefined by the user.
223 This directive adds a YYLTYPE parameter to yyerror(). If the
225 %pure-parser directive is present, a YYLTYPE parameter is added
226 to yylex() calls.
227 %lex-param { argument-declaration }
229 By default, the lexer accepts no parameters, e.g., yylex(). Use
230 this directive to add parameter declarations for your customized
231 lexer.
232 %parse-param { argument-declaration }
234 By default, the parser accepts no parameters, e.g., yyparse().
235 Use this directive to add parameter declarations for your cus‐
236 tomized parser.
237 %pure-parser
239 Most variables (other than yydebug and yynerrs) are allocated on
240 the stack within yyparse, making the parser reasonably reen‐
241 trant.
242 %token-table
244 Make the parser's names for tokens available in the yytname ar‐
245 ray. However, yacc does not predefine “$end”, “$error” or “$un‐
246 defined” in this array.
247
249 According to Robert Corbett,
250 Berkeley Yacc is an LALR(1) parser generator. Berkeley Yacc
252 has been made as compatible as possible with AT&T Yacc.
253 Berkeley Yacc can accept any input specification that
254 conforms to the AT&T Yacc documentation. Specifications
255 that take advantage of undocumented features of AT&T Yacc
256 will probably be rejected.
257 The rationale in
259 http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
261 documents some features of AT&T yacc which are no longer required for
263 POSIX compliance.
264 That said, you may be interested in reusing grammar files with some
266 other implementation which is not strictly compatible with AT&T yacc.
267 For instance, there is bison. Here are a few differences:
268 • Yacc accepts an equals mark preceding the left curly brace of an
270 action (as in the original grammar file ftp.y):
271 | STAT CRLF
273 = {
274 statcmd();
275 }
276 • Yacc and bison emit code in different order, and in particular bi‐
278 son makes forward reference to common functions such as yylex, yy‐
279 parse and yyerror without providing prototypes.
280 • Bison's support for “%expect” is broken in more than one release.
282 For best results using bison, delete that directive.
283 • Bison has no equivalent for some of yacc's command-line options,
285 relying on directives embedded in the grammar file.
286 • Bison's “-y” option does not affect bison's lack of support for
288 features of AT&T yacc which were deemed obsolescent.
289 • Yacc accepts multiple parameters with %lex-param and %parse-param
291 in two forms
292 {type1 name1} {type2 name2} ...
294 {type1 name1, type2 name2 ...}
295 Bison accepts the latter (though undocumented), but depending on
297 the release may generate bad code.
298 • Like bison, yacc will add parameters specified via %parse-param to
300 yyparse, yyerror and (if configured for back-tracking) to the de‐
301 structor declared using %destructor. Bison puts the additional pa‐
302 rameters first for yyparse and yyerror but last for destructors.
303 Yacc matches this behavior.
304
306 If there are rules that are never reduced, the number of such rules is
307 reported on standard error. If there are any LALR(1) conflicts, the
308 number of conflicts is reported on standard error.
309Berkeley Yacc March 28, 2021 YACC(1)