Manpages - yacc.1p

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.

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.

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.)

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.

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.

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 /name/s 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 /name/s as described for %token.

The following declares that union member name/s 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 %%.

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 A represents a non-terminal name, and BODY represents a sequence of zero or more /name/s, /literal/s, and /semantic action/s that can then be followed by optional /precedence rule/s. 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, “$0” refers to the name immediately preceding the leftmost name in the current rule to be found on the parser’s stack and “$-1” 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.

\(\)

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 %%.

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.

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 ';'
          ;

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.

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.

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.

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.

The following functions shall appear only in the yacc library accessible through the -l y 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 y and -l l operands given to c99 is significant; the application shall either provide its own /main/() function or ensure that -l y precedes -l l.

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.

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.

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/

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