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.
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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 %destructor { code } symbol+
124 defines code that is invoked when a symbol is automatically dis‐
125 carded during error recovery. This code can be used to reclaim
126 dynamically allocated memory associated with the corresponding
127 semantic value for cases where user actions cannot manage the
128 memory explicitly.
129 On encountering a parse error, the generated parser discards
131 symbols on the stack and input tokens until it reaches a state
132 that will allow parsing to continue. This error recovery
133 approach results in a memory leak if the YYSTYPE value is, or
134 contains, pointers to dynamically allocated memory.
135 The bracketed code is invoked whenever the parser discards one
137 of the symbols. Within code, “$$” or “$<tag>$” designates the
138 semantic value associated with the discarded symbol, and “@$”
139 designates its location (see %locations directive).
140 A per-symbol destructor is defined by listing a grammar symbol
142 in symbol+. A per-type destructor is defined by listing a
143 semantic type tag (e.g., “<some_tag>”) in symbol+; in this case,
144 the parser will invoke code whenever it discards any grammar
145 symbol that has that semantic type tag, unless that symbol has
146 its own per-symbol destructor.
147 Two categories of default destructor are supported that are
149 invoked when discarding any grammar symbol that has no per-sym‐
150 bol and no per-type destructor:
151 · the code for “<*>” is used for grammar symbols that have an
153 explicitly declared semantic type tag (via “%type”);
154 · the code for “<>” is used for grammar symbols that have no
156 declared semantic type tag.
157 %expect number
159 tells yacc the expected number of shift/reduce conflicts. That
160 makes it only report the number if it differs.
161 %expect-rr number
163 tell yacc the expected number of reduce/reduce conflicts. That
164 makes it only report the number if it differs. This is (unlike
165 bison) allowable in LALR parsers.
166 %locations
168 tells yacc to enable management of position information associ‐
169 ated with each token, provided by the lexer in the global vari‐
170 able yylloc, similar to management of semantic value information
171 provided in yylval.
172 As for semantic values, locations can be referenced within
174 actions using @$ to refer to the location of the left hand side
175 symbol, and @N (N an integer) to refer to the location of one of
176 the right hand side symbols. Also as for semantic values, when
177 a rule is matched, a default action is used the compute the
178 location represented by @$ as the beginning of the first symbol
179 and the end of the last symbol in the right hand side of the
180 rule. This default computation can be overridden by explicit
181 assignment to @$ in a rule action.
182 The type of yylloc is YYLTYPE, which is defined by default as:
184 typedef struct YYLTYPE {
186 int first_line;
187 int first_column;
188 int last_line;
189 int last_column;
190 } YYLTYPE;
191 YYLTYPE can be redefined by the user (YYLTYPE_IS_DEFINED must be
193 defined, to inhibit the default) in the declarations section of
194 the specification file. As in bison, the macro YYLLOC_DEFAULT
195 is invoked each time a rule is matched to calculate a position
196 for the left hand side of the rule, before the associated action
197 is executed; this macro can be redefined by the user.
198 This directive adds a YYLTYPE parameter to yyerror(). If the
200 %pure-parser directive is present, a YYLTYPE parameter is added
201 to yylex() calls.
202 %lex-param { argument-declaration }
204 By default, the lexer accepts no parameters, e.g., yylex(). Use
205 this directive to add parameter declarations for your customized
206 lexer.
207 %parse-param { argument-declaration }
209 By default, the parser accepts no parameters, e.g., yyparse().
210 Use this directive to add parameter declarations for your cus‐
211 tomized parser.
212 %pure-parser
214 Most variables (other than yydebug and yynerrs) are allocated on
215 the stack within yyparse, making the parser reasonably reen‐
216 trant.
217 %token-table
219 Make the parser's names for tokens available in the yytname
220 array. However, yacc does not predefine “$end”, “$error” or
221 “$undefined” in this array.
222
224 According to Robert Corbett,
225 Berkeley Yacc is an LALR(1) parser generator. Berkeley Yacc
227 has been made as compatible as possible with AT&T Yacc.
228 Berkeley Yacc can accept any input specification that
229 conforms to the AT&T Yacc documentation. Specifications
230 that take advantage of undocumented features of AT&T Yacc
231 will probably be rejected.
232 The rationale in
234 http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
236 documents some features of AT&T yacc which are no longer required for
238 POSIX compliance.
239 That said, you may be interested in reusing grammar files with some
241 other implementation which is not strictly compatible with AT&T yacc.
242 For instance, there is bison. Here are a few differences:
243 · Yacc accepts an equals mark preceding the left curly brace of an
245 action (as in the original grammar file ftp.y):
246 | STAT CRLF
248 = {
249 statcmd();
250 }
251 · Yacc and bison emit code in different order, and in particular
253 bison makes forward reference to common functions such as yylex,
254 yyparse and yyerror without providing prototypes.
255 · Bison's support for “%expect” is broken in more than one release.
257 For best results using bison, delete that directive.
258 · Bison has no equivalent for some of yacc's commmand-line options,
260 relying on directives embedded in the grammar file.
261 · Bison's “-y” option does not affect bison's lack of support for
263 features of AT&T yacc which were deemed obsolescent.
264 · Yacc accepts multiple parameters with %lex-param and %parse-param
266 in two forms
267 {type1 name1} {type2 name2} ...
269 {type1 name1, type2 name2 ...}
270 Bison accepts the latter (though undocumented), but depending on
272 the release may generate bad code.
273 · Like bison, yacc will add parameters specified via %parse-param to
275 yyparse, yyerror and (if configured for back-tracking) to the
276 destructor declared using %destructor. Bison puts the additional
277 parameters first for yyparse and yyerror but last for destructors.
278 Yacc matches this behavior.
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281 If there are rules that are never reduced, the number of such rules is
282 reported on standard error. If there are any LALR(1) conflicts, the
283 number of conflicts is reported on standard error.
284Berkeley Yacc June 16, 2019 YACC(1)