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
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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
30 #define'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
35 option.
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
43 #define's as needed to map the conventional yacc yy-prefixed names
44 to whatever the -p option may specify. The code file, e.g.,
45 y.tab.c is modified to #include this file as well as the y.tab.h
46 file, enforcing consistent usage of the symbols defined in those
47 files.
48 The supplementary header file makes it simpler to separate compi‐
50 lation of lex- and yacc-files.
51 -l If the -l option is not specified, yacc will insert #line direc‐
53 tives in the generated code. The #line directives let the C com‐
54 piler relate errors in the generated code to the user's original
55 code. If the -l option is specified, yacc will not insert the
56 #line directives. #line directives specified by the user will be
57 retained.
58 -L enable position processing, e.g., “%locations” (compile-time con‐
60 figuration for btyacc).
61 -o output_file
63 specify the filename for the parser file. If this option is not
64 given, the output filename is the file prefix concatenated with
65 the file suffix, e.g., y.tab.c. This overrides the -b option.
66 -p symbol_prefix
68 The -p option changes the prefix prepended to yacc-generated sym‐
69 bols to the string denoted by symbol_prefix. The default prefix
70 is the string yy.
71 -P create a reentrant parser, e.g., “%pure-parser”.
73 -r The -r option causes yacc to produce separate files for code and
75 tables. The code file is named y.code.c, and the tables file is
76 named y.tab.c. The prefix “y.” can be overridden using the -b
77 option.
78 -s suppress “#define” statements generated for string literals in a
80 “%token” statement, to more closely match original yacc behavior.
81 Normally when yacc sees a line such as
83 %token OP_ADD "ADD"
85 it notices that the quoted “ADD” is a valid C identifier, and gen‐
87 erates a #define not only for OP_ADD, but for ADD as well, e.g.,
88 #define OP_ADD 257
90 #define ADD 258
91 The original yacc does not generate the second “#define”. The -s
93 option suppresses this “#define”.
94 POSIX (IEEE 1003.1 2004) documents only names and numbers for
96 “%token”, though original yacc and bison also accept string liter‐
97 als.
98 -t The -t option changes the preprocessor directives generated by
100 yacc so that debugging statements will be incorporated in the com‐
101 piled code.
102 -v The -v option causes a human-readable description of the generated
104 parser to be written to the file y.output.
105 -V print the version number to the standard output.
107 -y yacc ignores this option, which bison supports for ostensible
109 POSIX compatibility.
110 The filename parameter is not optional. However, yacc accepts a single
112 “-” to read the grammar from the standard input. A double “--” marker
113 denotes the end of options. A single filename parameter is expected
114 after a “--” marker.
115
117 Yacc provides some extensions for compatibility with bison and other
118 implementations of yacc. The %destructor and %locations features are
119 available only if yacc has been configured and compiled to support the
120 back-tracking (btyacc) functionality. The remaining features are
121 always available:
122 %code keyword { code }
124 Adds the indicated source code at a given point in the output
125 file. The optional keyword tells yacc where to insert the code:
126 top just after the version-definition in the generated code-
128 file.
129 requires
131 just after the declaration of public parser variables. If
132 the -d option is given, the code is inserted at the begin‐
133 ning of the defines-file.
134 provides
136 just after the declaration of private parser variables. If
137 the -d option is given, the code is inserted at the end of
138 the defines-file.
139 If no keyword is given, the code is inserted at the beginning of
141 the section of code copied verbatim from the source file. Mul‐
142 tiple %code directives may be given; yacc inserts those into the
143 corresponding code- or defines-file in the order that they
144 appear in the source file.
145 %destructor { code } symbol+
147 defines code that is invoked when a symbol is automatically dis‐
148 carded during error recovery. This code can be used to reclaim
149 dynamically allocated memory associated with the corresponding
150 semantic value for cases where user actions cannot manage the
151 memory explicitly.
152 On encountering a parse error, the generated parser discards
154 symbols on the stack and input tokens until it reaches a state
155 that will allow parsing to continue. This error recovery
156 approach results in a memory leak if the YYSTYPE value is, or
157 contains, pointers to dynamically allocated memory.
158 The bracketed code is invoked whenever the parser discards one
160 of the symbols. Within code, “$$” or “$<tag>$” designates the
161 semantic value associated with the discarded symbol, and “@$”
162 designates its location (see %locations directive).
163 A per-symbol destructor is defined by listing a grammar symbol
165 in symbol+. A per-type destructor is defined by listing a
166 semantic type tag (e.g., “<some_tag>”) in symbol+; in this case,
167 the parser will invoke code whenever it discards any grammar
168 symbol that has that semantic type tag, unless that symbol has
169 its own per-symbol destructor.
170 Two categories of default destructor are supported that are
172 invoked when discarding any grammar symbol that has no per-sym‐
173 bol and no per-type destructor:
174 · the code for “<*>” is used for grammar symbols that have an
176 explicitly declared semantic type tag (via “%type”);
177 · the code for “<>” is used for grammar symbols that have no
179 declared semantic type tag.
180 %expect number
182 tells yacc the expected number of shift/reduce conflicts. That
183 makes it only report the number if it differs.
184 %expect-rr number
186 tell yacc the expected number of reduce/reduce conflicts. That
187 makes it only report the number if it differs. This is (unlike
188 bison) allowable in LALR parsers.
189 %locations
191 tells yacc to enable management of position information associ‐
192 ated with each token, provided by the lexer in the global vari‐
193 able yylloc, similar to management of semantic value information
194 provided in yylval.
195 As for semantic values, locations can be referenced within
197 actions using @$ to refer to the location of the left hand side
198 symbol, and @N (N an integer) to refer to the location of one of
199 the right hand side symbols. Also as for semantic values, when
200 a rule is matched, a default action is used the compute the
201 location represented by @$ as the beginning of the first symbol
202 and the end of the last symbol in the right hand side of the
203 rule. This default computation can be overridden by explicit
204 assignment to @$ in a rule action.
205 The type of yylloc is YYLTYPE, which is defined by default as:
207 typedef struct YYLTYPE {
209 int first_line;
210 int first_column;
211 int last_line;
212 int last_column;
213 } YYLTYPE;
214 YYLTYPE can be redefined by the user (YYLTYPE_IS_DEFINED must be
216 defined, to inhibit the default) in the declarations section of
217 the specification file. As in bison, the macro YYLLOC_DEFAULT
218 is invoked each time a rule is matched to calculate a position
219 for the left hand side of the rule, before the associated action
220 is executed; this macro can be redefined by the user.
221 This directive adds a YYLTYPE parameter to yyerror(). If the
223 %pure-parser directive is present, a YYLTYPE parameter is added
224 to yylex() calls.
225 %lex-param { argument-declaration }
227 By default, the lexer accepts no parameters, e.g., yylex(). Use
228 this directive to add parameter declarations for your customized
229 lexer.
230 %parse-param { argument-declaration }
232 By default, the parser accepts no parameters, e.g., yyparse().
233 Use this directive to add parameter declarations for your cus‐
234 tomized parser.
235 %pure-parser
237 Most variables (other than yydebug and yynerrs) are allocated on
238 the stack within yyparse, making the parser reasonably reen‐
239 trant.
240 %token-table
242 Make the parser's names for tokens available in the yytname
243 array. However, yacc does not predefine “$end”, “$error” or
244 “$undefined” in this array.
245
247 According to Robert Corbett,
248 Berkeley Yacc is an LALR(1) parser generator. Berkeley Yacc
250 has been made as compatible as possible with AT&T Yacc.
251 Berkeley Yacc can accept any input specification that
252 conforms to the AT&T Yacc documentation. Specifications
253 that take advantage of undocumented features of AT&T Yacc
254 will probably be rejected.
255 The rationale in
257 http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
259 documents some features of AT&T yacc which are no longer required for
261 POSIX compliance.
262 That said, you may be interested in reusing grammar files with some
264 other implementation which is not strictly compatible with AT&T yacc.
265 For instance, there is bison. Here are a few differences:
266 · Yacc accepts an equals mark preceding the left curly brace of an
268 action (as in the original grammar file ftp.y):
269 | STAT CRLF
271 = {
272 statcmd();
273 }
274 · Yacc and bison emit code in different order, and in particular
276 bison makes forward reference to common functions such as yylex,
277 yyparse and yyerror without providing prototypes.
278 · Bison's support for “%expect” is broken in more than one release.
280 For best results using bison, delete that directive.
281 · Bison has no equivalent for some of yacc's command-line options,
283 relying on directives embedded in the grammar file.
284 · Bison's “-y” option does not affect bison's lack of support for
286 features of AT&T yacc which were deemed obsolescent.
287 · Yacc accepts multiple parameters with %lex-param and %parse-param
289 in two forms
290 {type1 name1} {type2 name2} ...
292 {type1 name1, type2 name2 ...}
293 Bison accepts the latter (though undocumented), but depending on
295 the release may generate bad code.
296 · Like bison, yacc will add parameters specified via %parse-param to
298 yyparse, yyerror and (if configured for back-tracking) to the
299 destructor declared using %destructor. Bison puts the additional
300 parameters first for yyparse and yyerror but last for destructors.
301 Yacc matches this behavior.
302
304 If there are rules that are never reduced, the number of such rules is
305 reported on standard error. If there are any LALR(1) conflicts, the
306 number of conflicts is reported on standard error.
307Berkeley Yacc November 19, 2019 YACC(1)