1YACC(1P) POSIX Programmer's Manual YACC(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 yacc — yet another compiler compiler (DEVELOPMENT)
13
15 yacc [-dltv] [-b file_prefix] [-p sym_prefix] grammar
16
18 The yacc utility shall read a description of a context-free grammar in
19 grammar and write C source code, conforming to the ISO C standard, to a
20 code file, and optionally header information into a header file, in the
21 current directory. The generated source code shall not depend on any
22 undefined, unspecified, or implementation-defined behavior, except in
23 cases where it is copied directly from the supplied grammar, or in
24 cases that are documented by the implementation. The C code shall
25 define a function and related routines and macros for an automaton that
26 executes a parsing algorithm meeting the requirements in Algorithms.
27 The form and meaning of the grammar are described in the EXTENDED
29 DESCRIPTION section.
30 The C source code and header file shall be produced in a form suitable
32 as input for the C compiler (see c99).
33
35 The yacc utility shall conform to the Base Definitions volume of
36 POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines, except for
37 Guideline 9.
38 The following options shall be supported:
40 -b file_prefix
42 Use file_prefix instead of y as the prefix for all output
43 filenames. The code file y.tab.c, the header file y.tab.h
44 (created when -d is specified), and the description file
45 y.output (created when -v is specified), shall be changed to
46 file_prefix.tab.c, file_prefix.tab.h, and file_prefix.output,
47 respectively.
48 -d Write the header file; by default only the code file is writ‐
50 ten. See the OUTPUT FILES section.
51 -l Produce a code file that does not contain any #line con‐
53 structs. If this option is not present, it is unspecified
54 whether the code file or header file contains #line direc‐
55 tives. This should only be used after the grammar and the
56 associated actions are fully debugged.
57 -p sym_prefix
59 Use sym_prefix instead of yy as the prefix for all external
60 names produced by yacc. The names affected shall include the
61 functions yyparse(), yylex(), and yyerror(), and the vari‐
62 ables yylval, yychar, and yydebug. (In the remainder of this
63 section, the six symbols cited are referenced using their
64 default names only as a notational convenience.) Local names
65 may also be affected by the -p option; however, the -p option
66 shall not affect #define symbols generated by yacc.
67 -t Modify conditional compilation directives to permit compila‐
69 tion of debugging code in the code file. Runtime debugging
70 statements shall always be contained in the code file, but by
71 default conditional compilation directives prevent their com‐
72 pilation.
73 -v Write a file containing a description of the parser and a
75 report of conflicts generated by ambiguities in the grammar.
76
78 The following operand is required:
79 grammar A pathname of a file containing instructions, hereafter
81 called grammar, for which a parser is to be created. The for‐
82 mat for the grammar is described in the EXTENDED DESCRIPTION
83 section.
84
86 Not used.
87
89 The file grammar shall be a text file formatted as specified in the
90 EXTENDED DESCRIPTION section.
91
93 The following environment variables shall affect the execution of yacc:
94 LANG Provide a default value for the internationalization vari‐
96 ables that are unset or null. (See the Base Definitions vol‐
97 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
98 ables for the precedence of internationalization variables
99 used to determine the values of locale categories.)
100 LC_ALL If set to a non-empty string value, override the values of
102 all the other internationalization variables.
103 LC_CTYPE Determine the locale for the interpretation of sequences of
105 bytes of text data as characters (for example, single-byte as
106 opposed to multi-byte characters in arguments and input
107 files).
108 LC_MESSAGES
110 Determine the locale that should be used to affect the format
111 and contents of diagnostic messages written to standard
112 error.
113 NLSPATH Determine the location of message catalogs for the processing
115 of LC_MESSAGES.
116 The LANG and LC_* variables affect the execution of the yacc utility as
118 stated. The main() function defined in Yacc Library shall call:
119 setlocale(LC_ALL, "")
122 and thus the program generated by yacc shall also be affected by the
124 contents of these variables at runtime.
125
127 Default.
128
130 Not used.
131
133 If shift/reduce or reduce/reduce conflicts are detected in grammar,
134 yacc shall write a report of those conflicts to the standard error in
135 an unspecified format.
136 Standard error shall also be used for diagnostic messages.
138
140 The code file, the header file, and the description file shall be text
141 files. All are described in the following sections.
142 Code File
144 This file shall contain the C source code for the yyparse() function.
145 It shall contain code for the various semantic actions with macro sub‐
146 stitution performed on them as described in the EXTENDED DESCRIPTION
147 section. It also shall contain a copy of the #define statements in the
148 header file. If a %union declaration is used, the declaration for
149 YYSTYPE shall also be included in this file.
150 Header File
152 The header file shall contain #define statements that associate the
153 token numbers with the token names. This allows source files other than
154 the code file to access the token codes. If a %union declaration is
155 used, the declaration for YYSTYPE and an extern YYSTYPE yylval declara‐
156 tion shall also be included in this file.
157 Description File
159 The description file shall be a text file containing a description of
160 the state machine corresponding to the parser, using an unspecified
161 format. Limits for internal tables (see Limits) shall also be reported,
162 in an implementation-defined manner. (Some implementations may use
163 dynamic allocation techniques and have no specific limit values to
164 report.)
165
167 The yacc command accepts a language that is used to define a grammar
168 for a target language to be parsed by the tables and code generated by
169 yacc. The language accepted by yacc as a grammar for the target lan‐
170 guage is described below using the yacc input language itself.
171 The input grammar includes rules describing the input structure of the
173 target language and code to be invoked when these rules are recognized
174 to provide the associated semantic action. The code to be executed
175 shall appear as bodies of text that are intended to be C-language code.
176 These bodies of text shall not contain C-language trigraphs. The C-lan‐
177 guage inclusions are presumed to form a correct function when processed
178 by yacc into its output files. The code included in this way shall be
179 executed during the recognition of the target language.
180 Given a grammar, the yacc utility generates the files described in the
182 OUTPUT FILES section. The code file can be compiled and linked using
183 c99. If the declaration and programs sections of the grammar file did
184 not include definitions of main(), yylex(), and yyerror(), the compiled
185 output requires linking with externally supplied versions of those
186 functions. Default versions of main() and yyerror() are supplied in the
187 yacc library and can be linked in by using the -l y operand to c99.
188 The yacc library interfaces need not support interfaces with other than
189 the default yy symbol prefix. The application provides the lexical ana‐
190 lyzer function, yylex(); the lex utility is specifically designed to
191 generate such a routine.
192 Input Language
194 The application shall ensure that every specification file consists of
195 three sections in order: declarations, grammar rules, and programs,
196 separated by double <percent-sign> characters ("%%"). The declarations
197 and programs sections can be empty. If the latter is empty, the preced‐
198 ing "%%" mark separating it from the rules section can be omitted.
199 The input is free form text following the structure of the grammar
201 defined below.
202 Lexical Structure of the Grammar
204 The <blank>, <newline>, and <form-feed> character shall be ignored,
205 except that the application shall ensure that they do not appear in
206 names or multi-character reserved symbols. Comments shall be enclosed
207 in "/* ... */", and can appear wherever a name is valid.
208 Names are of arbitrary length, made up of letters, periods ('.'),
210 underscores ('_'), and non-initial digits. Uppercase and lowercase let‐
211 ters are distinct. Conforming applications shall not use names begin‐
212 ning in yy or YY since the yacc parser uses such names. Many of the
213 names appear in the final output of yacc, and thus they should be cho‐
214 sen to conform with any additional rules created by the C compiler to
215 be used. In particular they appear in #define statements.
216 A literal shall consist of a single character enclosed in single-quote
218 characters. All of the escape sequences supported for character con‐
219 stants by the ISO C standard shall be supported by yacc.
220 The relationship with the lexical analyzer is discussed in detail
222 below.
223 The application shall ensure that the NUL character is not used in
225 grammar rules or literals.
226 Declarations Section
228 The declarations section is used to define the symbols used to define
229 the target language and their relationship with each other. In particu‐
230 lar, much of the additional information required to resolve ambiguities
231 in the context-free grammar for the target language is provided here.
232 Usually yacc assigns the relationship between the symbolic names it
234 generates and their underlying numeric value. The declarations section
235 makes it possible to control the assignment of these values.
236 It is also possible to keep semantic information associated with the
238 tokens currently on the parse stack in a user-defined C-language union,
239 if the members of the union are associated with the various names in
240 the grammar. The declarations section provides for this as well.
241 The first group of declarators below all take a list of names as argu‐
243 ments. That list can optionally be preceded by the name of a C union
244 member (called a tag below) appearing within '<' and '>'. (As an
245 exception to the typographical conventions of the rest of this volume
246 of POSIX.1‐2017, in this case <tag> does not represent a metavariable,
247 but the literal angle bracket characters surrounding a symbol.) The use
248 of tag specifies that the tokens named on this line shall be of the
249 same C type as the union member referenced by tag. This is discussed
250 in more detail below.
251 For lists used to define tokens, the first appearance of a given token
253 can be followed by a positive integer (as a string of decimal digits).
254 If this is done, the underlying value assigned to it for lexical pur‐
255 poses shall be taken to be that number.
256 The following declares name to be a token:
258 %token [<tag>] name [number] [name [number]]...
261 If tag is present, the C type for all tokens on this line shall be
263 declared to be the type referenced by tag. If a positive integer, num‐
264 ber, follows a name, that value shall be assigned to the token.
265 The following declares name to be a token, and assigns precedence to
267 it:
268 %left [<tag>] name [number] [name [number]]...
271 %right [<tag>] name [number] [name [number]]...
272 One or more lines, each beginning with one of these symbols, can appear
274 in this section. All tokens on the same line have the same precedence
275 level and associativity; the lines are in order of increasing prece‐
276 dence or binding strength. %left denotes that the operators on that
277 line are left associative, and %right similarly denotes right associa‐
278 tive operators. If tag is present, it shall declare a C type for names
279 as described for %token.
280 The following declares name to be a token, and indicates that this can‐
282 not be used associatively:
283 %nonassoc [<tag>] name [number] [name [number]]...
286 If the parser encounters associative use of this token it reports an
288 error. If tag is present, it shall declare a C type for names as
289 described for %token.
290 The following declares that union member names are non-terminals, and
292 thus it is required to have a tag field at its beginning:
293 %type <tag> name...
296 Because it deals with non-terminals only, assigning a token number or
298 using a literal is also prohibited. If this construct is present, yacc
299 shall perform type checking; if this construct is not present, the
300 parse stack shall hold only the int type.
301 Every name used in grammar not defined by a %token, %left, %right, or
303 %nonassoc declaration is assumed to represent a non-terminal symbol.
304 The yacc utility shall report an error for any non-terminal symbol that
305 does not appear on the left side of at least one grammar rule.
306 Once the type, precedence, or token number of a name is specified, it
308 shall not be changed. If the first declaration of a token does not
309 assign a token number, yacc shall assign a token number. Once this
310 assignment is made, the token number shall not be changed by explicit
311 assignment.
312 The following declarators do not follow the previous pattern.
314 The following declares the non-terminal name to be the start symbol,
316 which represents the largest, most general structure described by the
317 grammar rules:
318 %start name
321 By default, it is the left-hand side of the first grammar rule; this
323 default can be overridden with this declaration.
324 The following declares the yacc value stack to be a union of the vari‐
326 ous types of values desired.
327 %union { body of union (in C) }
330 The body of the union shall not contain unbalanced curly brace prepro‐
332 cessing tokens.
333 By default, the values returned by actions (see below) and the lexical
335 analyzer shall be of type int. The yacc utility keeps track of types,
336 and it shall insert corresponding union member names in order to per‐
337 form strict type checking of the resulting parser.
338 Alternatively, given that at least one <tag> construct is used, the
340 union can be declared in a header file (which shall be included in the
341 declarations section by using a #include construct within %{ and %}),
342 and a typedef used to define the symbol YYSTYPE to represent this
343 union. The effect of %union is to provide the declaration of YYSTYPE
344 directly from the yacc input.
345 C-language declarations and definitions can appear in the declarations
347 section, enclosed by the following marks:
348 %{ ... %}
351 These statements shall be copied into the code file, and have global
353 scope within it so that they can be used in the rules and program sec‐
354 tions. The statements shall not contain "%}" outside a comment, string
355 literal, or multi-character constant.
356 The application shall ensure that the declarations section is termi‐
358 nated by the token %%.
359 Grammar Rules in yacc
361 The rules section defines the context-free grammar to be accepted by
362 the function yacc generates, and associates with those rules C-language
363 actions and additional precedence information. The grammar is described
364 below, and a formal definition follows.
365 The rules section is comprised of one or more grammar rules. A grammar
367 rule has the form:
368 A : BODY ;
371 The symbol A represents a non-terminal name, and BODY represents a
373 sequence of zero or more names, literals, and semantic actions that can
374 then be followed by optional precedence rules. Only the names and lit‐
375 erals participate in the formation of the grammar; the semantic actions
376 and precedence rules are used in other ways. The <colon> and the <semi‐
377 colon> are yacc punctuation. If there are several successive grammar
378 rules with the same left-hand side, the <vertical-line> ('|') can be
379 used to avoid rewriting the left-hand side; in this case the <semi‐
380 colon> appears only after the last rule. The BODY part can be empty (or
381 empty of names and literals) to indicate that the non-terminal symbol
382 matches the empty string.
383 The yacc utility assigns a unique number to each rule. Rules using the
385 vertical bar notation are distinct rules. The number assigned to the
386 rule appears in the description file.
387 The elements comprising a BODY are:
389 name, literal
391 These form the rules of the grammar: name is either a token
392 or a non-terminal; literal stands for itself (less the lexi‐
393 cally required quotation marks).
394 semantic action
396 With each grammar rule, the user can associate actions to be
397 performed each time the rule is recognized in the input
398 process. (Note that the word ``action'' can also refer to the
399 actions of the parser—shift, reduce, and so on.)
400 These actions can return values and can obtain the values
402 returned by previous actions. These values are kept in
403 objects of type YYSTYPE (see %union). The result value of
404 the action shall be kept on the parse stack with the left-
405 hand side of the rule, to be accessed by other reductions as
406 part of their right-hand side. By using the <tag> information
407 provided in the declarations section, the code generated by
408 yacc can be strictly type checked and contain arbitrary
409 information. In addition, the lexical analyzer can provide
410 the same kinds of values for tokens, if desired.
411 An action is an arbitrary C statement and as such can do
413 input or output, call subprograms, and alter external vari‐
414 ables. An action is one or more C statements enclosed in
415 curly braces '{' and '}'. The statements shall not contain
416 unbalanced curly brace preprocessing tokens.
417 Certain pseudo-variables can be used in the action. These are
419 macros for access to data structures known internally to
420 yacc.
421 $$ The value of the action can be set by assigning it
423 to $$. If type checking is enabled and the type of
424 the value to be assigned cannot be determined, a
425 diagnostic message may be generated.
426 $number This refers to the value returned by the component
428 specified by the token number in the right side of
429 a rule, reading from left to right; number can be
430 zero or negative. If number is zero or negative, it
431 refers to the data associated with the name on the
432 parser's stack preceding the leftmost symbol of the
433 current rule. (That is, "$0" refers to the name
434 immediately preceding the leftmost name in the cur‐
435 rent rule to be found on the parser's stack and
436 "$-1" refers to the symbol to its left.) If number
437 refers to an element past the current point in the
438 rule, or beyond the bottom of the stack, the result
439 is undefined. If type checking is enabled and the
440 type of the value to be assigned cannot be deter‐
441 mined, a diagnostic message may be generated.
442 $<tag>number
444 These correspond exactly to the corresponding sym‐
445 bols without the tag inclusion, but allow for
446 strict type checking (and preclude unwanted type
447 conversions). The effect is that the macro is
448 expanded to use tag to select an element from the
449 YYSTYPE union (using dataname.tag). This is par‐
450 ticularly useful if number is not positive.
451 $<tag>$ This imposes on the reference the type of the union
453 member referenced by tag. This construction is
454 applicable when a reference to a left context value
455 occurs in the grammar, and provides yacc with a
456 means for selecting a type.
457 Actions can occur anywhere in a rule (not just at the end);
459 an action can access values returned by actions to its left,
460 and in turn the value it returns can be accessed by actions
461 to its right. An action appearing in the middle of a rule
462 shall be equivalent to replacing the action with a new non-
463 terminal symbol and adding an empty rule with that non-termi‐
464 nal symbol on the left-hand side. The semantic action associ‐
465 ated with the new rule shall be equivalent to the original
466 action. The use of actions within rules might introduce con‐
467 flicts that would not otherwise exist.
468 By default, the value of a rule shall be the value of the
470 first element in it. If the first element does not have a
471 type (particularly in the case of a literal) and type check‐
472 ing is turned on by %type, an error message shall result.
473 precedence
475 The keyword %prec can be used to change the precedence level
476 associated with a particular grammar rule. Examples of this
477 are in cases where a unary and binary operator have the same
478 symbolic representation, but need to be given different
479 precedences, or where the handling of an ambiguous if-else
480 construction is necessary. The reserved symbol %prec can
481 appear immediately after the body of the grammar rule and can
482 be followed by a token name or a literal. It shall cause the
483 precedence of the grammar rule to become that of the follow‐
484 ing token name or literal. The action for the rule as a whole
485 can follow %prec.
486 If a program section follows, the application shall ensure that the
488 grammar rules are terminated by %%.
489 Programs Section
491 The programs section can include the definition of the lexical analyzer
492 yylex(), and any other functions; for example, those used in the
493 actions specified in the grammar rules. It is unspecified whether the
494 programs section precedes or follows the semantic actions in the output
495 file; therefore, if the application contains any macro definitions and
496 declarations intended to apply to the code in the semantic actions, it
497 shall place them within "%{ ... %}" in the declarations section.
498 Input Grammar
500 The following input to yacc yields a parser for the input to yacc.
501 This formal syntax takes precedence over the preceding text syntax
502 description.
503 The lexical structure is defined less precisely; Lexical Structure of
505 the Grammar defines most terms. The correspondence between the previous
506 terms and the tokens below is as follows.
507 IDENTIFIER This corresponds to the concept of name, given previously.
509 It also includes literals as defined previously.
510 C_IDENTIFIER
512 This is a name, and additionally it is known to be followed
513 by a <colon>. A literal cannot yield this token.
514 NUMBER A string of digits (a non-negative decimal integer).
516 TYPE, LEFT, MARK, LCURL, RCURL
518 These correspond directly to %type, %left, %%, %{, and %}.
519 { ... } This indicates C-language source code, with the possible
521 inclusion of '$' macros as discussed previously.
522 /* Grammar for the input to yacc. */
525 /* Basic entries. */
526 /* The following are recognized by the lexical analyzer. */
527 %token IDENTIFIER /* Includes identifiers and literals */
529 %token C_IDENTIFIER /* identifier (but not literal)
530 followed by a :. */
531 %token NUMBER /* [0-9][0-9]* */
532 /* Reserved words : %type=>TYPE %left=>LEFT, and so on */
534 %token LEFT RIGHT NONASSOC TOKEN PREC TYPE START UNION
536 %token MARK /* The %% mark. */
538 %token LCURL /* The %{ mark. */
539 %token RCURL /* The %} mark. */
540 /* 8-bit character literals stand for themselves; */
542 /* tokens have to be defined for multi-byte characters. */
543 %start spec
545 %%
547 spec : defs MARK rules tail
549 ;
550 tail : MARK
551 {
552 /* In this action, set up the rest of the file. */
553 }
554 | /* Empty; the second MARK is optional. */
555 ;
556 defs : /* Empty. */
557 | defs def
558 ;
559 def : START IDENTIFIER
560 | UNION
561 {
562 /* Copy union definition to output. */
563 }
564 | LCURL
565 {
566 /* Copy C code to output file. */
567 }
568 RCURL
569 | rword tag nlist
570 ;
571 rword : TOKEN
572 | LEFT
573 | RIGHT
574 | NONASSOC
575 | TYPE
576 ;
577 tag : /* Empty: union tag ID optional. */
578 | '<' IDENTIFIER '>'
579 ;
580 nlist : nmno
581 | nlist nmno
582 ;
583 nmno : IDENTIFIER /* Note: literal invalid with % type. */
584 | IDENTIFIER NUMBER /* Note: invalid with % type. */
585 ;
586 /* Rule section */
588 rules : C_IDENTIFIER rbody prec
590 | rules rule
591 ;
592 rule : C_IDENTIFIER rbody prec
593 | '|' rbody prec
594 ;
595 rbody : /* empty */
596 | rbody IDENTIFIER
597 | rbody act
598 ;
599 act : '{'
600 {
601 /* Copy action, translate $$, and so on. */
602 }
603 '}'
604 ;
605 prec : /* Empty */
606 | PREC IDENTIFIER
607 | PREC IDENTIFIER act
608 | prec ';'
609 ;
610 Conflicts
612 The parser produced for an input grammar may contain states in which
613 conflicts occur. The conflicts occur because the grammar is not
614 LALR(1). An ambiguous grammar always contains at least one LALR(1) con‐
615 flict. The yacc utility shall resolve all conflicts, using either
616 default rules or user-specified precedence rules.
617 Conflicts are either shift/reduce conflicts or reduce/reduce conflicts.
619 A shift/reduce conflict is where, for a given state and lookahead sym‐
620 bol, both a shift action and a reduce action are possible. A
621 reduce/reduce conflict is where, for a given state and lookahead sym‐
622 bol, reductions by two different rules are possible.
623 The rules below describe how to specify what actions to take when a
625 conflict occurs. Not all shift/reduce conflicts can be successfully
626 resolved this way because the conflict may be due to something other
627 than ambiguity, so incautious use of these facilities can cause the
628 language accepted by the parser to be much different from that which
629 was intended. The description file shall contain sufficient information
630 to understand the cause of the conflict. Where ambiguity is the reason
631 either the default or explicit rules should be adequate to produce a
632 working parser.
633 The declared precedences and associativities (see Declarations Section)
635 are used to resolve parsing conflicts as follows:
636 1. A precedence and associativity is associated with each grammar
638 rule; it is the precedence and associativity of the last token or
639 literal in the body of the rule. If the %prec keyword is used, it
640 overrides this default. Some grammar rules might not have both
641 precedence and associativity.
642 2. If there is a shift/reduce conflict, and both the grammar rule and
644 the input symbol have precedence and associativity associated with
645 them, then the conflict is resolved in favor of the action (shift
646 or reduce) associated with the higher precedence. If the prece‐
647 dences are the same, then the associativity is used; left associa‐
648 tive implies reduce, right associative implies shift, and non-asso‐
649 ciative implies an error in the string being parsed.
650 3. When there is a shift/reduce conflict that cannot be resolved by
652 rule 2, the shift is done. Conflicts resolved this way are counted
653 in the diagnostic output described in Error Handling.
654 4. When there is a reduce/reduce conflict, a reduction is done by the
656 grammar rule that occurs earlier in the input sequence. Conflicts
657 resolved this way are counted in the diagnostic output described in
658 Error Handling.
659 Conflicts resolved by precedence or associativity shall not be counted
661 in the shift/reduce and reduce/reduce conflicts reported by yacc on
662 either standard error or in the description file.
663 Error Handling
665 The token error shall be reserved for error handling. The name error
666 can be used in grammar rules. It indicates places where the parser can
667 recover from a syntax error. The default value of error shall be 256.
668 Its value can be changed using a %token declaration. The lexical ana‐
669 lyzer should not return the value of error.
670 The parser shall detect a syntax error when it is in a state where the
672 action associated with the lookahead symbol is error. A semantic
673 action can cause the parser to initiate error handling by executing the
674 macro YYERROR. When YYERROR is executed, the semantic action passes
675 control back to the parser. YYERROR cannot be used outside of semantic
676 actions.
677 When the parser detects a syntax error, it normally calls yyerror()
679 with the character string "syntax error" as its argument. The call
680 shall not be made if the parser is still recovering from a previous
681 error when the error is detected. The parser is considered to be recov‐
682 ering from a previous error until the parser has shifted over at least
683 three normal input symbols since the last error was detected or a
684 semantic action has executed the macro yyerrok. The parser shall not
685 call yyerror() when YYERROR is executed.
686 The macro function YYRECOVERING shall return 1 if a syntax error has
688 been detected and the parser has not yet fully recovered from it. Oth‐
689 erwise, zero shall be returned.
690 When a syntax error is detected by the parser, the parser shall check
692 if a previous syntax error has been detected. If a previous error was
693 detected, and if no normal input symbols have been shifted since the
694 preceding error was detected, the parser checks if the lookahead symbol
695 is an endmarker (see Interface to the Lexical Analyzer). If it is, the
696 parser shall return with a non-zero value. Otherwise, the lookahead
697 symbol shall be discarded and normal parsing shall resume.
698 When YYERROR is executed or when the parser detects a syntax error and
700 no previous error has been detected, or at least one normal input sym‐
701 bol has been shifted since the previous error was detected, the parser
702 shall pop back one state at a time until the parse stack is empty or
703 the current state allows a shift over error. If the parser empties the
704 parse stack, it shall return with a non-zero value. Otherwise, it shall
705 shift over error and then resume normal parsing. If the parser reads a
706 lookahead symbol before the error was detected, that symbol shall still
707 be the lookahead symbol when parsing is resumed.
708 The macro yyerrok in a semantic action shall cause the parser to act as
710 if it has fully recovered from any previous errors. The macro yyclearin
711 shall cause the parser to discard the current lookahead token. If the
712 current lookahead token has not yet been read, yyclearin shall have no
713 effect.
714 The macro YYACCEPT shall cause the parser to return with the value
716 zero. The macro YYABORT shall cause the parser to return with a non-
717 zero value.
718 Interface to the Lexical Analyzer
720 The yylex() function is an integer-valued function that returns a token
721 number representing the kind of token read. If there is a value associ‐
722 ated with the token returned by yylex() (see the discussion of tag
723 above), it shall be assigned to the external variable yylval.
724 If the parser and yylex() do not agree on these token numbers, reliable
726 communication between them cannot occur. For (single-byte character)
727 literals, the token is simply the numeric value of the character in the
728 current character set. The numbers for other tokens can either be cho‐
729 sen by yacc, or chosen by the user. In either case, the #define con‐
730 struct of C is used to allow yylex() to return these numbers symboli‐
731 cally. The #define statements are put into the code file, and the
732 header file if that file is requested. The set of characters permitted
733 by yacc in an identifier is larger than that permitted by C. Token
734 names found to contain such characters shall not be included in the
735 #define declarations.
736 If the token numbers are chosen by yacc, the tokens other than literals
738 shall be assigned numbers greater than 256, although no order is
739 implied. A token can be explicitly assigned a number by following its
740 first appearance in the declarations section with a number. Names and
741 literals not defined this way retain their default definition. All
742 token numbers assigned by yacc shall be unique and distinct from the
743 token numbers used for literals and user-assigned tokens. If duplicate
744 token numbers cause conflicts in parser generation, yacc shall report
745 an error; otherwise, it is unspecified whether the token assignment is
746 accepted or an error is reported.
747 The end of the input is marked by a special token called the endmarker,
749 which has a token number that is zero or negative. (These values are
750 invalid for any other token.) All lexical analyzers shall return zero
751 or negative as a token number upon reaching the end of their input. If
752 the tokens up to, but excluding, the endmarker form a structure that
753 matches the start symbol, the parser shall accept the input. If the
754 endmarker is seen in any other context, it shall be considered an
755 error.
756 Completing the Program
758 In addition to yyparse() and yylex(), the functions yyerror() and
759 main() are required to make a complete program. The application can
760 supply main() and yyerror(), or those routines can be obtained from the
761 yacc library.
762 Yacc Library
764 The following functions shall appear only in the yacc library accessi‐
765 ble through the -l y operand to c99; they can therefore be redefined by
766 a conforming application:
767 int main(void)
769 This function shall call yyparse() and exit with an unspecified
770 value. Other actions within this function are unspecified.
771 int yyerror(const char *s)
773 This function shall write the NUL-terminated argument to standard
774 error, followed by a <newline>.
775 The order of the -l y and -l l operands given to c99 is significant;
777 the application shall either provide its own main() function or ensure
778 that -l y precedes -l l.
779 Debugging the Parser
781 The parser generated by yacc shall have diagnostic facilities in it
782 that can be optionally enabled at either compile time or at runtime (if
783 enabled at compile time). The compilation of the runtime debugging
784 code is under the control of YYDEBUG, a preprocessor symbol. If YYDEBUG
785 has a non-zero value, the debugging code shall be included. If its
786 value is zero, the code shall not be included.
787 In parsers where the debugging code has been included, the external int
789 yydebug can be used to turn debugging on (with a non-zero value) and
790 off (zero value) at runtime. The initial value of yydebug shall be
791 zero.
792 When -t is specified, the code file shall be built such that, if YYDE‐
794 BUG is not already defined at compilation time (using the c99 -D YYDE‐
795 BUG option, for example), YYDEBUG shall be set explicitly to 1. When
796 -t is not specified, the code file shall be built such that, if YYDEBUG
797 is not already defined, it shall be set explicitly to zero.
798 The format of the debugging output is unspecified but includes at least
800 enough information to determine the shift and reduce actions, and the
801 input symbols. It also provides information about error recovery.
802 Algorithms
804 The parser constructed by yacc implements an LALR(1) parsing algorithm
805 as documented in the literature. It is unspecified whether the parser
806 is table-driven or direct-coded.
807 A parser generated by yacc shall never request an input symbol from
809 yylex() while in a state where the only actions other than the error
810 action are reductions by a single rule.
811 The literature of parsing theory defines these concepts.
813 Limits
815 The yacc utility may have several internal tables. The minimum maximums
816 for these tables are shown in the following table. The exact meaning of
817 these values is implementation-defined. The implementation shall define
818 the relationship between these values and between them and any error
819 messages that the implementation may generate should it run out of
820 space for any internal structure. An implementation may combine groups
821 of these resources into a single pool as long as the total available to
822 the user does not fall below the sum of the sizes specified by this
823 section.
824 Table: Internal Limits in yacc
826 ┌───────────┬─────────┬────────────────────────────────┐
828 │ │ Minimum │ │
829 │ Limit │ Maximum │ Description │
830 ├───────────┼─────────┼────────────────────────────────┤
831 │{NTERMS} │ 126 │ Number of tokens. │
832 │{NNONTERM} │ 200 │ Number of non-terminals. │
833 │{NPROD} │ 300 │ Number of rules. │
834 │{NSTATES} │ 600 │ Number of states. │
835 │{MEMSIZE} │ 5200 │ Length of rules. The total │
836 │ │ │ length, in names (tokens and │
837 │ │ │ non-terminals), of all the │
838 │ │ │ rules of the grammar. The │
839 │ │ │ left-hand side is counted for │
840 │ │ │ each rule, even if it is not │
841 │ │ │ explicitly repeated, as speci‐ │
842 │ │ │ fied in Grammar Rules in yacc. │
843 │{ACTSIZE} │ 4000 │ Number of actions. ``Actions'' │
844 │ │ │ here (and in the description │
845 │ │ │ file) refer to parser actions │
846 │ │ │ (shift, reduce, and so on) not │
847 │ │ │ to semantic actions defined in │
848 │ │ │ Grammar Rules in yacc. │
849 └───────────┴─────────┴────────────────────────────────┘
851 The following exit values shall be returned:
852 0 Successful completion.
854 >0 An error occurred.
856
858 If any errors are encountered, the run is aborted and yacc exits with a
859 non-zero status. Partial code files and header files may be produced.
860 The summary information in the description file shall always be pro‐
861 duced if the -v flag is present.
862 The following sections are informative.
864
866 Historical implementations experience name conflicts on the names
867 yacc.tmp, yacc.acts, yacc.debug, y.tab.c, y.tab.h, and y.output if more
868 than one copy of yacc is running in a single directory at one time. The
869 -b option was added to overcome this problem. The related problem of
870 allowing multiple yacc parsers to be placed in the same file was
871 addressed by adding a -p option to override the previously hard-coded
872 yy variable prefix.
873 The description of the -p option specifies the minimal set of function
875 and variable names that cause conflict when multiple parsers are linked
876 together. YYSTYPE does not need to be changed. Instead, the programmer
877 can use -b to give the header files for different parsers different
878 names, and then the file with the yylex() for a given parser can
879 include the header for that parser. Names such as yyclearerr do not
880 need to be changed because they are used only in the actions; they do
881 not have linkage. It is possible that an implementation has other
882 names, either internal ones for implementing things such as yyclearerr,
883 or providing non-standard features that it wants to change with -p.
884 Unary operators that are the same token as a binary operator in general
886 need their precedence adjusted. This is handled by the %prec advisory
887 symbol associated with the particular grammar rule defining that unary
888 operator. (See Grammar Rules in yacc.) Applications are not required
889 to use this operator for unary operators, but the grammars that do not
890 require it are rare.
891
893 Access to the yacc library is obtained with library search operands to
894 c99. To use the yacc library main():
895 c99 y.tab.c -l y
898 Both the lex library and the yacc library contain main(). To access
900 the yacc main():
901 c99 y.tab.c lex.yy.c -l y -l l
904 This ensures that the yacc library is searched first, so that its
906 main() is used.
907 The historical yacc libraries have contained two simple functions that
909 are normally coded by the application programmer. These functions are
910 similar to the following code:
911 #include <locale.h>
914 int main(void)
915 {
916 extern int yyparse();
917 setlocale(LC_ALL, "");
919 /* If the following parser is one created by lex, the
921 application must be careful to ensure that LC_CTYPE
922 and LC_COLLATE are set to the POSIX locale. */
923 (void) yyparse();
924 return (0);
925 }
926 #include <stdio.h>
928 int yyerror(const char *msg)
930 {
931 (void) fprintf(stderr, "%s\n", msg);
932 return (0);
933 }
934
936 The references in Referenced Documents may be helpful in constructing
937 the parser generator. The referenced DeRemer and Pennello article
938 (along with the works it references) describes a technique to generate
939 parsers that conform to this volume of POSIX.1‐2017. Work in this area
940 continues to be done, so implementors should consult current literature
941 before doing any new implementations. The original Knuth article is the
942 theoretical basis for this kind of parser, but the tables it generates
943 are impractically large for reasonable grammars and should not be used.
944 The ``equivalent to'' wording is intentional to assure that the best
945 tables that are LALR(1) can be generated.
946 There has been confusion between the class of grammars, the algorithms
948 needed to generate parsers, and the algorithms needed to parse the lan‐
949 guages. They are all reasonably orthogonal. In particular, a parser
950 generator that accepts the full range of LR(1) grammars need not gener‐
951 ate a table any more complex than one that accepts SLR(1) (a relatively
952 weak class of LR grammars) for a grammar that happens to be SLR(1).
953 Such an implementation need not recognize the case, either; table com‐
954 pression can yield the SLR(1) table (or one even smaller than that)
955 without recognizing that the grammar is SLR(1). The speed of an LR(1)
956 parser for any class is dependent more upon the table representation
957 and compression (or the code generation if a direct parser is gener‐
958 ated) than upon the class of grammar that the table generator handles.
959 The speed of the parser generator is somewhat dependent upon the class
961 of grammar it handles. However, the original Knuth article algorithms
962 for constructing LR parsers were judged by its author to be impracti‐
963 cally slow at that time. Although full LR is more complex than LALR(1),
964 as computer speeds and algorithms improve, the difference (in terms of
965 acceptable wall-clock execution time) is becoming less significant.
966 Potential authors are cautioned that the referenced DeRemer and Pen‐
968 nello article previously cited identifies a bug (an over-simplification
969 of the computation of LALR(1) lookahead sets) in some of the LALR(1)
970 algorithm statements that preceded it to publication. They should take
971 the time to seek out that paper, as well as current relevant work, par‐
972 ticularly Aho's.
973 The -b option was added to provide a portable method for permitting
975 yacc to work on multiple separate parsers in the same directory. If a
976 directory contains more than one yacc grammar, and both grammars are
977 constructed at the same time (by, for example, a parallel make pro‐
978 gram), conflict results. While the solution is not historical practice,
979 it corrects a known deficiency in historical implementations. Corre‐
980 sponding changes were made to all sections that referenced the file‐
981 names y.tab.c (now ``the code file''), y.tab.h (now ``the header
982 file''), and y.output (now ``the description file'').
983 The grammar for yacc input is based on System V documentation. The tex‐
985 tual description shows there that the ';' is required at the end of the
986 rule. The grammar and the implementation do not require this. (The use
987 of C_IDENTIFIER causes a reduce to occur in the right place.)
988 Also, in that implementation, the constructs such as %token can be ter‐
990 minated by a <semicolon>, but this is not permitted by the grammar. The
991 keywords such as %token can also appear in uppercase, which is again
992 not discussed. In most places where '%' is used, <backslash> can be
993 substituted, and there are alternate spellings for some of the symbols
994 (for example, %LEFT can be "%<" or even "\<").
995 Historically, <tag> can contain any characters except '>', including
997 white space, in the implementation. However, since the tag must refer‐
998 ence an ISO C standard union member, in practice conforming implementa‐
999 tions need to support only the set of characters for ISO C standard
1000 identifiers in this context.
1001 Some historical implementations are known to accept actions that are
1003 terminated by a period. Historical implementations often allow '$' in
1004 names. A conforming implementation does not need to support either of
1005 these behaviors.
1006 Deciding when to use %prec illustrates the difficulty in specifying the
1008 behavior of yacc. There may be situations in which the grammar is not,
1009 strictly speaking, in error, and yet yacc cannot interpret it unambigu‐
1010 ously. The resolution of ambiguities in the grammar can in many
1011 instances be resolved by providing additional information, such as
1012 using %type or %union declarations. It is often easier and it usually
1013 yields a smaller parser to take this alternative when it is appropri‐
1014 ate.
1015 The size and execution time of a program produced without the runtime
1017 debugging code is usually smaller and slightly faster in historical
1018 implementations.
1019 Statistics messages from several historical implementations include the
1021 following types of information:
1022 n/512 terminals, n/300 non-terminals
1025 n/600 grammar rules, n/1500 states
1026 n shift/reduce, n reduce/reduce conflicts reported
1027 n/350 working sets used
1028 Memory: states, etc. n/15000, parser n/15000
1029 n/600 distinct lookahead sets
1030 n extra closures
1031 n shift entries, n exceptions
1032 n goto entries
1033 n entries saved by goto default
1034 Optimizer space used: input n/15000, output n/15000
1035 n table entries, n zero
1036 Maximum spread: n, Maximum offset: n
1037 The report of internal tables in the description file is left implemen‐
1039 tation-defined because all aspects of these limits are also implementa‐
1040 tion-defined. Some implementations may use dynamic allocation tech‐
1041 niques and have no specific limit values to report.
1042 The format of the y.output file is not given because specification of
1044 the format was not seen to enhance applications portability. The list‐
1045 ing is primarily intended to help human users understand and debug the
1046 parser; use of y.output by a conforming application script would be
1047 unusual. Furthermore, implementations have not produced consistent out‐
1048 put and no popular format was apparent. The format selected by the
1049 implementation should be human-readable, in addition to the requirement
1050 that it be a text file.
1051 Standard error reports are not specifically described because they are
1053 seldom of use to conforming applications and there was no reason to
1054 restrict implementations.
1055 Some implementations recognize "={" as equivalent to '{' because it
1057 appears in historical documentation. This construction was recognized
1058 and documented as obsolete as long ago as 1978, in the referenced Yacc:
1059 Yet Another Compiler-Compiler. This volume of POSIX.1‐2017 chose to
1060 leave it as obsolete and omit it.
1061 Multi-byte characters should be recognized by the lexical analyzer and
1063 returned as tokens. They should not be returned as multi-byte character
1064 literals. The token error that is used for error recovery is normally
1065 assigned the value 256 in the historical implementation. Thus, the
1066 token value 256, which is used in many multi-byte character sets, is
1067 not available for use as the value of a user-defined token.
1068
1070 None.
1071
1073 c99, lex
1074 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
1076 Variables, Section 12.2, Utility Syntax Guidelines
1077
1079 Portions of this text are reprinted and reproduced in electronic form
1080 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
1081 table Operating System Interface (POSIX), The Open Group Base Specifi‐
1082 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
1083 Electrical and Electronics Engineers, Inc and The Open Group. In the
1084 event of any discrepancy between this version and the original IEEE and
1085 The Open Group Standard, the original IEEE and The Open Group Standard
1086 is the referee document. The original Standard can be obtained online
1087 at http://www.opengroup.org/unix/online.html .
1088 Any typographical or formatting errors that appear in this page are
1090 most likely to have been introduced during the conversion of the source
1091 files to man page format. To report such errors, see https://www.ker‐
1092 nel.org/doc/man-pages/reporting_bugs.html .
1093IEEE/The Open Group 2017 YACC(1P)