YACC(1)                    POSIX Programmer's Manual                   YACC(1)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       yacc -- yet another compiler compiler (DEVELOPMENT)

SYNOPSIS
       yacc [[-dltv]] [[-b file__prefix]] [[-p sym__prefix]] grammar

DESCRIPTION
       The yacc utility shall read a description of a context-free grammar in
       grammar and write C source code, conforming to the ISO C standard, to a
       code file, and optionally header information into a header file, in the
       current directory. The generated source code shall not depend on any
       undefined, unspecified, or implementation-defined behavior, except in
       cases where it is copied directly from the supplied grammar, or in
       cases that are documented by the implementation. The C code shall
       define a function and related routines and macros for an automaton that
       executes a parsing algorithm meeting the requirements in Algorithms.

       The form and meaning of the grammar are described in the EXTENDED
       DESCRIPTION section.

       The C source code and header file shall be produced in a form suitable
       as input for the C compiler (see c99).

OPTIONS
       The yacc utility shall conform to the Base Definitions volume of
       POSIX.1-2017, Section 12.2, Utility Syntax Guidelines, except for
       Guideline 9.

       The following options shall be supported:

       -b file__prefix
                 Use file__prefix instead of yy as the prefix for all output
                 filenames. The code file y.tab.c, the header file y.tab.h
                 (created when -d is specified), and the description file
                 y.output (created when -v is specified), shall be changed to
                 file__prefix..ttaabb..cc, file__prefix..ttaabb..hh, and file__prefix..oouuttppuutt,
                 respectively.

       -d        Write the header file; by default only the code file is
                 written. See the OUTPUT FILES section.

       -l        Produce a code file that does not contain any #line
                 constructs. If this option is not present, it is unspecified
                 whether the code file or header file contains #line
                 directives. This should only be used after the grammar and
                 the associated actions are fully debugged.

       -p sym__prefix
                 Use sym__prefix instead of yy as the prefix for all external
                 names produced by yacc.  The names affected shall include the
                 functions yyparse(), yylex(), and yyerror(), and the
                 variables yylval, yychar, and yydebug.  (In the remainder of
                 this section, the six symbols cited are referenced using
                 their default names only as a notational convenience.) Local
                 names may also be affected by the -p option; however, the -p
                 option shall not affect #define symbols generated by yacc.

       -t        Modify conditional compilation directives to permit
                 compilation of debugging code in the code file. Runtime
                 debugging statements shall always be contained in the code
                 file, but by default conditional compilation directives
                 prevent their compilation.

       -v        Write a file containing a description of the parser and a
                 report of conflicts generated by ambiguities in the grammar.

OPERANDS
       The following operand is required:

       grammar   A pathname of a file containing instructions, hereafter
                 called grammar, for which a parser is to be created. The
                 format for the grammar is described in the EXTENDED
                 DESCRIPTION section.

STDIN
       Not used.

INPUT FILES
       The file grammar shall be a text file formatted as specified in the
       EXTENDED DESCRIPTION section.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of yacc:

       LANG      Provide a default value for the internationalization
                 variables that are unset or null. (See the Base Definitions
                 volume of POSIX.1-2017, Section 8.2, Internationalization
                 Variables for the precedence of internationalization
                 variables used to determine the values of locale categories.)

       LC__ALL    If set to a non-empty string value, override the values of
                 all the other internationalization variables.

       LC__CTYPE  Determine the locale for the interpretation of sequences of
                 bytes of text data as characters (for example, single-byte as
                 opposed to multi-byte characters in arguments and input
                 files).

       LC__MESSAGES
                 Determine the locale that should be used to affect the format
                 and contents of diagnostic messages written to standard
                 error.

       NLSPATH   Determine the location of message catalogs for the processing
                 of LC__MESSAGES.

       The LANG and LC___* variables affect the execution of the yacc utility as
       stated. The main() function defined in Yacc Library shall call:


           setlocale(LC_ALL, "")

       and thus the program generated by yacc shall also be affected by the
       contents of these variables at runtime.

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       Not used.

STDERR
       If shift/reduce or reduce/reduce conflicts are detected in grammar,
       yacc shall write a report of those conflicts to the standard error in
       an unspecified format.

       Standard error shall also be used for diagnostic messages.

OUTPUT FILES
       The code file, the header file, and the description file shall be text
       files. All are described in the following sections.

   Code File
       This file shall contain the C source code for the yyparse() function.
       It shall contain code for the various semantic actions with macro
       substitution performed on them as described in the EXTENDED DESCRIPTION
       section. It also shall contain a copy of the #define statements in the
       header file. If a %union declaration is used, the declaration for
       YYSTYPE shall also be included in this file.

   Header File
       The header file shall contain #define statements that associate the
       token numbers with the token names. This allows source files other than
       the code file to access the token codes. If a %union declaration is
       used, the declaration for YYSTYPE and an extern YYSTYPE yylval
       declaration shall also be included in this file.

   Description File
       The description file shall be a text file containing a description of
       the state machine corresponding to the parser, using an unspecified
       format. Limits for internal tables (see Limits) shall also be reported,
       in an implementation-defined manner. (Some implementations may use
       dynamic allocation techniques and have no specific limit values to
       report.)

EXTENDED DESCRIPTION
       The yacc command accepts a language that is used to define a grammar
       for a target language to be parsed by the tables and code generated by
       yacc.  The language accepted by yacc as a grammar for the target
       language is described below using the yacc input language itself.

       The input grammar includes rules describing the input structure of the
       target language and code to be invoked when these rules are recognized
       to provide the associated semantic action. The code to be executed
       shall appear as bodies of text that are intended to be C-language code.
       These bodies of text shall not contain C-language trigraphs. The C-
       language inclusions are presumed to form a correct function when
       processed by yacc into its output files. The code included in this way
       shall be executed during the recognition of the target language.

       Given a grammar, the yacc utility generates the files described in the
       OUTPUT FILES section. The code file can be compiled and linked using
       c99.  If the declaration and programs sections of the grammar file did
       not include definitions of main(), yylex(), and yyerror(), the compiled
       output requires linking with externally supplied versions of those
       functions. Default versions of main() and yyerror() are supplied in the
       yacc library and can be linked in by using the -l yy operand to c99.
       The yacc library interfaces need not support interfaces with other than
       the default yy symbol prefix. The application provides the lexical
       analyzer function, yylex(); the lex utility is specifically designed to
       generate such a routine.

   Input Language
       The application shall ensure that every specification file consists of
       three sections in order: declarations, grammar rules, and programs,
       separated by double <percent-sign> characters (""%%%%"").  The declarations
       and programs sections can be empty. If the latter is empty, the
       preceding ""%%%%"" mark separating it from the rules section can be
       omitted.

       The input is free form text following the structure of the grammar
       defined below.

   Lexical Structure of the Grammar
       The <blank>, <newline>, and <form-feed> character shall be ignored,
       except that the application shall ensure that they do not appear in
       names or multi-character reserved symbols. Comments shall be enclosed
       in ""//** ...... **//"", and can appear wherever a name is valid.

       Names are of arbitrary length, made up of letters, periods (''..''),
       underscores (''__''), and non-initial digits. Uppercase and lowercase
       letters are distinct.  Conforming applications shall not use names
       beginning in yy or YY since the yacc parser uses such names. Many of
       the names appear in the final output of yacc, and thus they should be
       chosen to conform with any additional rules created by the C compiler
       to be used. In particular they appear in #define statements.

       A literal shall consist of a single character enclosed in single-quote
       characters. All of the escape sequences supported for character
       constants by the ISO C standard shall be supported by yacc.

       The relationship with the lexical analyzer is discussed in detail
       below.

       The application shall ensure that the NUL character is not used in
       grammar rules or literals.

   Declarations Section
       The declarations section is used to define the symbols used to define
       the target language and their relationship with each other. In
       particular, much of the additional information required to resolve
       ambiguities in the context-free grammar for the target language is
       provided here.

       Usually yacc assigns the relationship between the symbolic names it
       generates and their underlying numeric value. The declarations section
       makes it possible to control the assignment of these values.

       It is also possible to keep semantic information associated with the
       tokens currently on the parse stack in a user-defined C-language union,
       if the members of the union are associated with the various names in
       the grammar. The declarations section provides for this as well.

       The first group of declarators below all take a list of names as
       arguments. That list can optionally be preceded by the name of a C
       union member (called a tag below) appearing within ''<<'' and ''>>''.  (As an
       exception to the typographical conventions of the rest of this volume
       of POSIX.1-2017, in this case <tag> does not represent a metavariable,
       but the literal angle bracket characters surrounding a symbol.) The use
       of tag specifies that the tokens named on this line shall be of the
       same C type as the union member referenced by tag.  This is discussed
       in more detail below.

       For lists used to define tokens, the first appearance of a given token
       can be followed by a positive integer (as a string of decimal digits).
       If this is done, the underlying value assigned to it for lexical
       purposes shall be taken to be that number.

       The following declares name to be a token:


           %token [[<tag>]] name [[number]] [[name [[number]]]]...

       If tag is present, the C type for all tokens on this line shall be
       declared to be the type referenced by tag.  If a positive integer,
       number, follows a name, that value shall be assigned to the token.

       The following declares name to be a token, and assigns precedence to
       it:


           %left [[<tag>]] name [[number]] [[name [[number]]]]...
           %right [[<tag>]] name [[number]] [[name [[number]]]]...

       One or more lines, each beginning with one of these symbols, can appear
       in this section. All tokens on the same line have the same precedence
       level and associativity; the lines are in order of increasing
       precedence or binding strength.  %left denotes that the operators on
       that line are left associative, and %right similarly denotes right
       associative operators. If tag is present, it shall declare a C type for
       names as described for %token.

       The following declares name to be a token, and indicates that this
       cannot be used associatively:


           %nonassoc [[<tag>]] name [[number]] [[name [[number]]]]...

       If the parser encounters associative use of this token it reports an
       error. If tag is present, it shall declare a C type for names as
       described for %token.

       The following declares that union member names are non-terminals, and
       thus it is required to have a tag field at its beginning:


           %type <tag> name...

       Because it deals with non-terminals only, assigning a token number or
       using a literal is also prohibited. If this construct is present, yacc
       shall perform type checking; if this construct is not present, the
       parse stack shall hold only the int type.

       Every name used in grammar not defined by a %token, %left, %right, or
       %nonassoc declaration is assumed to represent a non-terminal symbol.
       The yacc utility shall report an error for any non-terminal symbol that
       does not appear on the left side of at least one grammar rule.

       Once the type, precedence, or token number of a name is specified, it
       shall not be changed. If the first declaration of a token does not
       assign a token number, yacc shall assign a token number. Once this
       assignment is made, the token number shall not be changed by explicit
       assignment.

       The following declarators do not follow the previous pattern.

       The following declares the non-terminal name to be the start symbol,
       which represents the largest, most general structure described by the
       grammar rules:


           %start name

       By default, it is the left-hand side of the first grammar rule; this
       default can be overridden with this declaration.

       The following declares the yacc value stack to be a union of the
       various types of values desired.


           %union { body of union (in _C) }

       The body of the union shall not contain unbalanced curly brace
       preprocessing tokens.

       By default, the values returned by actions (see below) and the lexical
       analyzer shall be of type int.  The yacc utility keeps track of types,
       and it shall insert corresponding union member names in order to
       perform strict type checking of the resulting parser.

       Alternatively, given that at least one <tag> construct is used, the
       union can be declared in a header file (which shall be included in the
       declarations section by using a #include construct within %%{{ and %%}}),
       and a typedef used to define the symbol YYSTYPE to represent this
       union. The effect of %union is to provide the declaration of YYSTYPE
       directly from the yacc input.

       C-language declarations and definitions can appear in the declarations
       section, enclosed by the following marks:


           %{ ... %}

       These statements shall be copied into the code file, and have global
       scope within it so that they can be used in the rules and program
       sections. The statements shall not contain ""%%}}"" outside a comment,
       string literal, or multi-character constant.

       The application shall ensure that the declarations section is
       terminated by the token %%%%.

   Grammar Rules in yacc
       The rules section defines the context-free grammar to be accepted by
       the function yacc generates, and associates with those rules C-language
       actions and additional precedence information. The grammar is described
       below, and a formal definition follows.

       The rules section is comprised of one or more grammar rules. A grammar
       rule has the form:


           A : BODY ;

       The symbol AA represents a non-terminal name, and BODY represents a
       sequence of zero or more names, literals, and semantic actions that can
       then be followed by optional precedence rules.  Only the names and
       literals participate in the formation of the grammar; the semantic
       actions and precedence rules are used in other ways. The <colon> and
       the <semicolon> are yacc punctuation. If there are several successive
       grammar rules with the same left-hand side, the <vertical-line> (''||'')
       can be used to avoid rewriting the left-hand side; in this case the
       <semicolon> appears only after the last rule. The BODY part can be
       empty (or empty of names and literals) to indicate that the non-
       terminal symbol matches the empty string.

       The yacc utility assigns a unique number to each rule. Rules using the
       vertical bar notation are distinct rules. The number assigned to the
       rule appears in the description file.

       The elements comprising a BODY are:

       name, literal
                 These form the rules of the grammar: name is either a token
                 or a non-terminal; literal stands for itself (less the
                 lexically required quotation marks).

       semantic action
                 With each grammar rule, the user can associate actions to be
                 performed each time the rule is recognized in the input
                 process. (Note that the word ``action'' can also refer to the
                 actions of the parser--shift, reduce, and so on.)

                 These actions can return values and can obtain the values
                 returned by previous actions. These values are kept in
                 objects of type YYSTYPE (see %union).  The result value of
                 the action shall be kept on the parse stack with the left-
                 hand side of the rule, to be accessed by other reductions as
                 part of their right-hand side. By using the <tag> information
                 provided in the declarations section, the code generated by
                 yacc can be strictly type checked and contain arbitrary
                 information. In addition, the lexical analyzer can provide
                 the same kinds of values for tokens, if desired.

                 An action is an arbitrary C statement and as such can do
                 input or output, call subprograms, and alter external
                 variables. An action is one or more C statements enclosed in
                 curly braces ''{{'' and ''}}''.  The statements shall not contain
                 unbalanced curly brace preprocessing tokens.

                 Certain pseudo-variables can be used in the action. These are
                 macros for access to data structures known internally to
                 yacc.

                 $$        The value of the action can be set by assigning it
                           to $$. If type checking is enabled and the type of
                           the value to be assigned cannot be determined, a
                           diagnostic message may be generated.

                 $number   This refers to the value returned by the component
                           specified by the token number in the right side of
                           a rule, reading from left to right; number can be
                           zero or negative. If number is zero or negative, it
                           refers to the data associated with the name on the
                           parser's stack preceding the leftmost symbol of the
                           current rule.  (That is, ""$$00"" refers to the name
                           immediately preceding the leftmost name in the
                           current rule to be found on the parser's stack and
                           ""$$--11"" refers to the symbol to its left.) If number
                           refers to an element past the current point in the
                           rule, or beyond the bottom of the stack, the result
                           is undefined. If type checking is enabled and the
                           type of the value to be assigned cannot be
                           determined, a diagnostic message may be generated.

                 $<tag>number
                           These correspond exactly to the corresponding
                           symbols without the tag inclusion, but allow for
                           strict type checking (and preclude unwanted type
                           conversions). The effect is that the macro is
                           expanded to use tag to select an element from the
                           YYSTYPE union (using dataname.tag).  This is
                           particularly useful if number is not positive.

                 $<tag>$   This imposes on the reference the type of the union
                           member referenced by tag.  This construction is
                           applicable when a reference to a left context value
                           occurs in the grammar, and provides yacc with a
                           means for selecting a type.

                 Actions can occur anywhere in a rule (not just at the end);
                 an action can access values returned by actions to its left,
                 and in turn the value it returns can be accessed by actions
                 to its right. An action appearing in the middle of a rule
                 shall be equivalent to replacing the action with a new non-
                 terminal symbol and adding an empty rule with that non-
                 terminal symbol on the left-hand side. The semantic action
                 associated with the new rule shall be equivalent to the
                 original action. The use of actions within rules might
                 introduce conflicts that would not otherwise exist.

                 By default, the value of a rule shall be the value of the
                 first element in it. If the first element does not have a
                 type (particularly in the case of a literal) and type
                 checking is turned on by %type, an error message shall
                 result.

       precedence
                 The keyword %prec can be used to change the precedence level
                 associated with a particular grammar rule. Examples of this
                 are in cases where a unary and binary operator have the same
                 symbolic representation, but need to be given different
                 precedences, or where the handling of an ambiguous if-else
                 construction is necessary. The reserved symbol %prec can
                 appear immediately after the body of the grammar rule and can
                 be followed by a token name or a literal. It shall cause the
                 precedence of the grammar rule to become that of the
                 following token name or literal. The action for the rule as a
                 whole can follow %prec.

       If a program section follows, the application shall ensure that the
       grammar rules are terminated by %%%%.

   Programs Section
       The programs section can include the definition of the lexical analyzer
       yylex(), and any other functions; for example, those used in the
       actions specified in the grammar rules. It is unspecified whether the
       programs section precedes or follows the semantic actions in the output
       file; therefore, if the application contains any macro definitions and
       declarations intended to apply to the code in the semantic actions, it
       shall place them within ""%%{{ ...... %%}}"" in the declarations section.

   Input Grammar
       The following input to yacc yields a parser for the input to yacc.
       This formal syntax takes precedence over the preceding text syntax
       description.

       The lexical structure is defined less precisely; Lexical Structure of
       the Grammar defines most terms. The correspondence between the previous
       terms and the tokens below is as follows.

       IDENTIFIER  This corresponds to the concept of name, given previously.
                   It also includes literals as defined previously.

       C_IDENTIFIER
                   This is a name, and additionally it is known to be followed
                   by a <colon>.  A literal cannot yield this token.

       NUMBER      A string of digits (a non-negative decimal integer).

       TYPE, LEFT, MARK, LCURL, RCURL
                   These correspond directly to %type, %left, %%%%, %%{{, and %%}}.

       {{ ...... }}     This indicates C-language source code, with the possible
                   inclusion of ''$$'' macros as discussed previously.


           /* Grammar for the input to yacc. */
           /* Basic entries. */
           /* The following are recognized by the lexical analyzer. */

           %token    IDENTIFIER      /* Includes identifiers and literals */
           %token    C_IDENTIFIER    /* identifier (but not literal)
                                        followed by a :. */
           %token    NUMBER          /* [0-9][0-9]* */

           /* Reserved words : %type=>TYPE %left=>LEFT, and so on */

           %token    LEFT RIGHT NONASSOC TOKEN PREC TYPE START UNION

           %token    MARK            /* The %% mark. */
           %token    LCURL           /* The %{ mark. */
           %token    RCURL           /* The %} mark. */

           /* 8-bit character literals stand for themselves; */
           /* tokens have to be defined for multi-byte characters. */

           %start    spec

           %%

           spec  : defs MARK rules tail
                 ;
           tail  : MARK
                 {
                   /* In this action, set up the rest of the file. */
                 }
                 | /* Empty; the second MARK is optional. */
                 ;
           defs  : /* Empty. */
                 |    defs def
                 ;
           def   : START IDENTIFIER
                 |    UNION
                 {
                   /* Copy union definition to output. */
                 }
                 |    LCURL
                 {
                   /* Copy C code to output file. */
                 }
                   RCURL
                 |    rword tag nlist
                 ;
           rword : TOKEN
                 | LEFT
                 | RIGHT
                 | NONASSOC
                 | TYPE
                 ;
           tag   : /* Empty: union tag ID optional. */
                 | '<' IDENTIFIER '>'
                 ;
           nlist : nmno
                 | nlist nmno
                 ;
           nmno  : IDENTIFIER         /* Note: literal invalid with % type. */
                 | IDENTIFIER NUMBER  /* Note: invalid with % type. */
                 ;

           /* Rule section */

           rules : C_IDENTIFIER rbody prec
                 | rules  rule
                 ;
           rule  : C_IDENTIFIER rbody prec
                 | '|' rbody prec
                 ;
           rbody : /* empty */
                 | rbody IDENTIFIER
                 | rbody act
                 ;
           act   : '{'
                   {
                     /* Copy action, translate $$, and so on. */
                   }
                   '}'
                 ;
           prec  : /* Empty */
                 | PREC IDENTIFIER
                 | PREC IDENTIFIER act
                 | prec ';'
                 ;

   Conflicts
       The parser produced for an input grammar may contain states in which
       conflicts occur. The conflicts occur because the grammar is not
       LALR(1). An ambiguous grammar always contains at least one LALR(1)
       conflict. The yacc utility shall resolve all conflicts, using either
       default rules or user-specified precedence rules.

       Conflicts are either shift/reduce conflicts or reduce/reduce conflicts.
       A shift/reduce conflict is where, for a given state and lookahead
       symbol, both a shift action and a reduce action are possible. A
       reduce/reduce conflict is where, for a given state and lookahead
       symbol, reductions by two different rules are possible.

       The rules below describe how to specify what actions to take when a
       conflict occurs. Not all shift/reduce conflicts can be successfully
       resolved this way because the conflict may be due to something other
       than ambiguity, so incautious use of these facilities can cause the
       language accepted by the parser to be much different from that which
       was intended. The description file shall contain sufficient information
       to understand the cause of the conflict. Where ambiguity is the reason
       either the default or explicit rules should be adequate to produce a
       working parser.

       The declared precedences and associativities (see Declarations Section)
       are used to resolve parsing conflicts as follows:

        1. A precedence and associativity is associated with each grammar
           rule; it is the precedence and associativity of the last token or
           literal in the body of the rule. If the %prec keyword is used, it
           overrides this default. Some grammar rules might not have both
           precedence and associativity.

        2. If there is a shift/reduce conflict, and both the grammar rule and
           the input symbol have precedence and associativity associated with
           them, then the conflict is resolved in favor of the action (shift
           or reduce) associated with the higher precedence. If the
           precedences are the same, then the associativity is used; left
           associative implies reduce, right associative implies shift, and
           non-associative implies an error in the string being parsed.

        3. When there is a shift/reduce conflict that cannot be resolved by
           rule 2, the shift is done. Conflicts resolved this way are counted
           in the diagnostic output described in Error Handling.

        4. When there is a reduce/reduce conflict, a reduction is done by the
           grammar rule that occurs earlier in the input sequence. Conflicts
           resolved this way are counted in the diagnostic output described in
           Error Handling.

       Conflicts resolved by precedence or associativity shall not be counted
       in the shift/reduce and reduce/reduce conflicts reported by yacc on
       either standard error or in the description file.

   Error Handling
       The token error shall be reserved for error handling. The name error
       can be used in grammar rules. It indicates places where the parser can
       recover from a syntax error. The default value of error shall be 256.
       Its value can be changed using a %token declaration. The lexical
       analyzer should not return the value of error.

       The parser shall detect a syntax error when it is in a state where the
       action associated with the lookahead symbol is error.  A semantic
       action can cause the parser to initiate error handling by executing the
       macro YYERROR. When YYERROR is executed, the semantic action passes
       control back to the parser. YYERROR cannot be used outside of semantic
       actions.

       When the parser detects a syntax error, it normally calls yyerror()
       with the character string "syntax error" as its argument. The call
       shall not be made if the parser is still recovering from a previous
       error when the error is detected. The parser is considered to be
       recovering from a previous error until the parser has shifted over at
       least three normal input symbols since the last error was detected or a
       semantic action has executed the macro yyerrok.  The parser shall not
       call yyerror() when YYERROR is executed.

       The macro function YYRECOVERING shall return 1 if a syntax error has
       been detected and the parser has not yet fully recovered from it.
       Otherwise, zero shall be returned.

       When a syntax error is detected by the parser, the parser shall check
       if a previous syntax error has been detected. If a previous error was
       detected, and if no normal input symbols have been shifted since the
       preceding error was detected, the parser checks if the lookahead symbol
       is an endmarker (see Interface to the Lexical Analyzer).  If it is, the
       parser shall return with a non-zero value. Otherwise, the lookahead
       symbol shall be discarded and normal parsing shall resume.

       When YYERROR is executed or when the parser detects a syntax error and
       no previous error has been detected, or at least one normal input
       symbol has been shifted since the previous error was detected, the
       parser shall pop back one state at a time until the parse stack is
       empty or the current state allows a shift over error.  If the parser
       empties the parse stack, it shall return with a non-zero value.
       Otherwise, it shall shift over error and then resume normal parsing. If
       the parser reads a lookahead symbol before the error was detected, that
       symbol shall still be the lookahead symbol when parsing is resumed.

       The macro yyerrok in a semantic action shall cause the parser to act as
       if it has fully recovered from any previous errors. The macro yyclearin
       shall cause the parser to discard the current lookahead token. If the
       current lookahead token has not yet been read, yyclearin shall have no
       effect.

       The macro YYACCEPT shall cause the parser to return with the value
       zero. The macro YYABORT shall cause the parser to return with a non-
       zero value.

   Interface to the Lexical Analyzer
       The yylex() function is an integer-valued function that returns a token
       number representing the kind of token read. If there is a value
       associated with the token returned by yylex() (see the discussion of
       tag above), it shall be assigned to the external variable yylval.

       If the parser and yylex() do not agree on these token numbers, reliable
       communication between them cannot occur. For (single-byte character)
       literals, the token is simply the numeric value of the character in the
       current character set.  The numbers for other tokens can either be
       chosen by yacc, or chosen by the user. In either case, the #define
       construct of C is used to allow yylex() to return these numbers
       symbolically. The #define statements are put into the code file, and
       the header file if that file is requested. The set of characters
       permitted by yacc in an identifier is larger than that permitted by C.
       Token names found to contain such characters shall not be included in
       the #define declarations.

       If the token numbers are chosen by yacc, the tokens other than literals
       shall be assigned numbers greater than 256, although no order is
       implied. A token can be explicitly assigned a number by following its
       first appearance in the declarations section with a number. Names and
       literals not defined this way retain their default definition. All
       token numbers assigned by yacc shall be unique and distinct from the
       token numbers used for literals and user-assigned tokens. If duplicate
       token numbers cause conflicts in parser generation, yacc shall report
       an error; otherwise, it is unspecified whether the token assignment is
       accepted or an error is reported.

       The end of the input is marked by a special token called the endmarker,
       which has a token number that is zero or negative. (These values are
       invalid for any other token.) All lexical analyzers shall return zero
       or negative as a token number upon reaching the end of their input. If
       the tokens up to, but excluding, the endmarker form a structure that
       matches the start symbol, the parser shall accept the input. If the
       endmarker is seen in any other context, it shall be considered an
       error.

   Completing the Program
       In addition to yyparse() and yylex(), the functions yyerror() and
       main() are required to make a complete program. The application can
       supply main() and yyerror(), or those routines can be obtained from the
       yacc library.

   Yacc Library
       The following functions shall appear only in the yacc library
       accessible through the -l yy operand to c99; they can therefore be
       redefined by a conforming application:

       int main(void)
             This function shall call yyparse() and exit with an unspecified
             value. Other actions within this function are unspecified.

       int yyerror(const char *_s)
             This function shall write the NUL-terminated argument to standard
             error, followed by a <newline>.

       The order of the -l yy and -l ll operands given to c99 is significant;
       the application shall either provide its own main() function or ensure
       that -l yy precedes -l ll.

   Debugging the Parser
       The parser generated by yacc shall have diagnostic facilities in it
       that can be optionally enabled at either compile time or at runtime (if
       enabled at compile time).  The compilation of the runtime debugging
       code is under the control of YYDEBUG, a preprocessor symbol. If YYDEBUG
       has a non-zero value, the debugging code shall be included. If its
       value is zero, the code shall not be included.

       In parsers where the debugging code has been included, the external int
       yydebug can be used to turn debugging on (with a non-zero value) and
       off (zero value) at runtime. The initial value of yydebug shall be
       zero.

       When -t is specified, the code file shall be built such that, if
       YYDEBUG is not already defined at compilation time (using the c99 -D
       YYDEBUG option, for example), YYDEBUG shall be set explicitly to 1.
       When -t is not specified, the code file shall be built such that, if
       YYDEBUG is not already defined, it shall be set explicitly to zero.

       The format of the debugging output is unspecified but includes at least
       enough information to determine the shift and reduce actions, and the
       input symbols. It also provides information about error recovery.

   Algorithms
       The parser constructed by yacc implements an LALR(1) parsing algorithm
       as documented in the literature. It is unspecified whether the parser
       is table-driven or direct-coded.

       A parser generated by yacc shall never request an input symbol from
       yylex() while in a state where the only actions other than the error
       action are reductions by a single rule.

       The literature of parsing theory defines these concepts.

   Limits
       The yacc utility may have several internal tables. The minimum maximums
       for these tables are shown in the following table. The exact meaning of
       these values is implementation-defined. The implementation shall define
       the relationship between these values and between them and any error
       messages that the implementation may generate should it run out of
       space for any internal structure. An implementation may combine groups
       of these resources into a single pool as long as the total available to
       the user does not fall below the sum of the sizes specified by this
       section.

                           Table: Internal Limits in yacc

               +-----------+---------+--------------------------------+
               |           | Minimum |                                |
               |  Limit    | Maximum |          Description           |
               +-----------+---------+--------------------------------+
               |{NTERMS}   |   126   | Number of tokens.              |
               |{NNONTERM} |   200   | Number of non-terminals.       |
               |{NPROD}    |   300   | Number of rules.               |
               |{NSTATES}  |   600   | Number of states.              |
               |{MEMSIZE}  |  5200   | Length of rules. The total     |
               |           |         | length, in names (tokens and   |
               |           |         | non-terminals), of all the     |
               |           |         | rules of the grammar. The      |
               |           |         | left-hand side is counted for  |
               |           |         | each rule, even if it is not   |
               |           |         | explicitly repeated, as        |
               |           |         | specified in Grammar Rules in  |
               |           |         | yacc.                          |
               |{ACTSIZE}  |  4000   | Number of actions. ``Actions'' |
               |           |         | here (and in the description   |
               |           |         | file) refer to parser actions  |
               |           |         | (shift, reduce, and so on) not |
               |           |         | to semantic actions defined in |
               |           |         | Grammar Rules in yacc.         |
               +-----------+---------+--------------------------------+
EXIT STATUS
       The following exit values shall be returned:

        0    Successful completion.

       >0    An error occurred.

CONSEQUENCES OF ERRORS
       If any errors are encountered, the run is aborted and yacc exits with a
       non-zero status. Partial code files and header files may be produced.
       The summary information in the description file shall always be
       produced if the -v flag is present.

       The following sections are informative.

APPLICATION USAGE
       Historical implementations experience name conflicts on the names
       yacc.tmp, yacc.acts, yacc.debug, y.tab.c, y.tab.h, and y.output if more
       than one copy of yacc is running in a single directory at one time. The
       -b option was added to overcome this problem. The related problem of
       allowing multiple yacc parsers to be placed in the same file was
       addressed by adding a -p option to override the previously hard-coded
       yy variable prefix.

       The description of the -p option specifies the minimal set of function
       and variable names that cause conflict when multiple parsers are linked
       together. YYSTYPE does not need to be changed. Instead, the programmer
       can use -b to give the header files for different parsers different
       names, and then the file with the yylex() for a given parser can
       include the header for that parser. Names such as yyclearerr do not
       need to be changed because they are used only in the actions; they do
       not have linkage. It is possible that an implementation has other
       names, either internal ones for implementing things such as yyclearerr,
       or providing non-standard features that it wants to change with -p.

       Unary operators that are the same token as a binary operator in general
       need their precedence adjusted. This is handled by the %prec advisory
       symbol associated with the particular grammar rule defining that unary
       operator. (See Grammar Rules in yacc.)  Applications are not required
       to use this operator for unary operators, but the grammars that do not
       require it are rare.

EXAMPLES
       Access to the yacc library is obtained with library search operands to
       c99.  To use the yacc library main():


           c99 y.tab.c -l y

       Both the lex library and the yacc library contain main().  To access
       the yacc main():


           c99 y.tab.c lex.yy.c -l y -l l

       This ensures that the yacc library is searched first, so that its
       main() is used.

       The historical yacc libraries have contained two simple functions that
       are normally coded by the application programmer. These functions are
       similar to the following code:


           #include <locale.h>
           int main(void)
           {
               extern int yyparse();

               setlocale(LC_ALL, "");

               /* If the following parser is one created by lex, the
                  application must be careful to ensure that LC_CTYPE
                  and LC_COLLATE are set to the POSIX locale. */
               (void) yyparse();
               return (0);
           }

           #include <stdio.h>

           int yyerror(const char *msg)
           {
               (void) fprintf(stderr, "%s\n", msg);
               return (0);
           }

RATIONALE
       The references in Referenced Documents may be helpful in constructing
       the parser generator. The referenced DeRemer and Pennello article
       (along with the works it references) describes a technique to generate
       parsers that conform to this volume of POSIX.1-2017. Work in this area
       continues to be done, so implementors should consult current literature
       before doing any new implementations. The original Knuth article is the
       theoretical basis for this kind of parser, but the tables it generates
       are impractically large for reasonable grammars and should not be used.
       The ``equivalent to'' wording is intentional to assure that the best
       tables that are LALR(1) can be generated.

       There has been confusion between the class of grammars, the algorithms
       needed to generate parsers, and the algorithms needed to parse the
       languages. They are all reasonably orthogonal. In particular, a parser
       generator that accepts the full range of LR(1) grammars need not
       generate a table any more complex than one that accepts SLR(1) (a
       relatively weak class of LR grammars) for a grammar that happens to be
       SLR(1). Such an implementation need not recognize the case, either;
       table compression can yield the SLR(1) table (or one even smaller than
       that) without recognizing that the grammar is SLR(1).  The speed of an
       LR(1) parser for any class is dependent more upon the table
       representation and compression (or the code generation if a direct
       parser is generated) than upon the class of grammar that the table
       generator handles.

       The speed of the parser generator is somewhat dependent upon the class
       of grammar it handles. However, the original Knuth article algorithms
       for constructing LR parsers were judged by its author to be
       impractically slow at that time. Although full LR is more complex than
       LALR(1), as computer speeds and algorithms improve, the difference (in
       terms of acceptable wall-clock execution time) is becoming less
       significant.

       Potential authors are cautioned that the referenced DeRemer and
       Pennello article previously cited identifies a bug (an over-
       simplification of the computation of LALR(1) lookahead sets) in some of
       the LALR(1) algorithm statements that preceded it to publication. They
       should take the time to seek out that paper, as well as current
       relevant work, particularly Aho's.

       The -b option was added to provide a portable method for permitting
       yacc to work on multiple separate parsers in the same directory. If a
       directory contains more than one yacc grammar, and both grammars are
       constructed at the same time (by, for example, a parallel make
       program), conflict results. While the solution is not historical
       practice, it corrects a known deficiency in historical implementations.
       Corresponding changes were made to all sections that referenced the
       filenames y.tab.c (now ``the code file''), y.tab.h (now ``the header
       file''), and y.output (now ``the description file'').

       The grammar for yacc input is based on System V documentation. The
       textual description shows there that the '';;'' is required at the end of
       the rule. The grammar and the implementation do not require this. (The
       use of C_IDENTIFIER causes a reduce to occur in the right place.)

       Also, in that implementation, the constructs such as %token can be
       terminated by a <semicolon>, but this is not permitted by the grammar.
       The keywords such as %token can also appear in uppercase, which is
       again not discussed. In most places where ''%%'' is used, <backslash> can
       be substituted, and there are alternate spellings for some of the
       symbols (for example, %LEFT can be ""%%<<"" or even ""\\<<"").

       Historically, <tag> can contain any characters except ''>>'', including
       white space, in the implementation. However, since the tag must
       reference an ISO C standard union member, in practice conforming
       implementations need to support only the set of characters for ISO C
       standard identifiers in this context.

       Some historical implementations are known to accept actions that are
       terminated by a period. Historical implementations often allow ''$$'' in
       names. A conforming implementation does not need to support either of
       these behaviors.

       Deciding when to use %prec illustrates the difficulty in specifying the
       behavior of yacc.  There may be situations in which the grammar is not,
       strictly speaking, in error, and yet yacc cannot interpret it
       unambiguously. The resolution of ambiguities in the grammar can in many
       instances be resolved by providing additional information, such as
       using %type or %union declarations. It is often easier and it usually
       yields a smaller parser to take this alternative when it is
       appropriate.

       The size and execution time of a program produced without the runtime
       debugging code is usually smaller and slightly faster in historical
       implementations.

       Statistics messages from several historical implementations include the
       following types of information:


           _n/512 terminals, _n/300 non-terminals
           _n/600 grammar rules, _n/1500 states
           _n shift/reduce, _n reduce/reduce conflicts reported
           _n/350 working sets used
           Memory: states, etc. _n/15000, parser _n/15000
           _n/600 distinct lookahead sets
           _n extra closures
           _n shift entries, _n exceptions
           _n goto entries
           _n entries saved by goto default
           Optimizer space used: input _n/15000, output _n/15000
           _n table entries, _n zero
           Maximum spread: _n, Maximum offset: _n

       The report of internal tables in the description file is left
       implementation-defined because all aspects of these limits are also
       implementation-defined. Some implementations may use dynamic allocation
       techniques and have no specific limit values to report.

       The format of the y.output file is not given because specification of
       the format was not seen to enhance applications portability. The
       listing is primarily intended to help human users understand and debug
       the parser; use of y.output by a conforming application script would be
       unusual. Furthermore, implementations have not produced consistent
       output and no popular format was apparent. The format selected by the
       implementation should be human-readable, in addition to the requirement
       that it be a text file.

       Standard error reports are not specifically described because they are
       seldom of use to conforming applications and there was no reason to
       restrict implementations.

       Some implementations recognize ""=={{"" as equivalent to ''{{'' because it
       appears in historical documentation. This construction was recognized
       and documented as obsolete as long ago as 1978, in the referenced Yacc:
       Yet Another Compiler-Compiler. This volume of POSIX.1-2017 chose to
       leave it as obsolete and omit it.

       Multi-byte characters should be recognized by the lexical analyzer and
       returned as tokens. They should not be returned as multi-byte character
       literals. The token error that is used for error recovery is normally
       assigned the value 256 in the historical implementation. Thus, the
       token value 256, which is used in many multi-byte character sets, is
       not available for use as the value of a user-defined token.

FUTURE DIRECTIONS
       None.

SEE ALSO
       c99, lex

       The Base Definitions volume of POSIX.1-2017, Chapter _8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                              YACC(1)
