The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


yacc - yet another compiler compiler (DEVELOPMENT)


yacc [-dltv][-b file_prefix][-p sym_prefix] grammar


The yacc utility reads a description of a context-free grammar in file and writes 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 C code defines 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 are produced in a form suitable as input for the C compiler (see c89).


The yacc utility supports the XBD specification, Utility Syntax Guidelines  .

The following options are supported:

-b file_prefix
Use file_prefix instead of y as the prefix for all output filenames. The code file, the header file (created when -d is specified), and the description file y.output (created when -v is specified), will be changed to,, and file_prefix.output, respectively.
Write the header file; by default only the code file is written. The #define statements that associate the token codes assigned by yacc with the user-declared token names. This allows source files other than to access the token codes.
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 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 does not affect #define symbols generated by yacc.
Modify conditional compilation directives to permit compilation of debugging code in the code file. Run-time debugging statements will be always contained in the code file, but by default conditional compilation directives prevent their compilation.
Write a file containing a description of the parser and a report of conflicts generated by ambiguities in the grammar.


The following operand is required:
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.


Not used.


The file grammar must be a text file formatted as specified in the EXTENDED DESCRIPTION section.


The following environment variables affect the execution of yacc:
Provide a default value for the internationalisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.
If set to a non-empty string value, override the values of all the other internationalisation variables.
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single- as opposed to multi-byte characters in arguments and input files).
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
Determine the location of message catalogues 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 calls:

setlocale(LC_ALL, "")

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




Not used.


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

Standard error is also used for diagnostic messages.


The code file, the header file and the description file are text files. All are described in the following sections.
 Code file
This file will contain the C source code for the yyparse() routine. It will contain code for the various semantic actions with macro substitution performed on them as described in the EXTENDED DESCRIPTION section. It will also contain a copy of the #define statements in the header file. If a %union declaration is used, the declaration for YYSTYPE also will be included in this file.

The contents of the Program Section (see Programs Section ) of the input file will then be included.

 Header file
The header file will 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 also will be included in this file.
 Description file
The description file will 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 in the EXTENDED DESCRIPTION section) will also be reported, in an implementation-dependent manner. (Some implementations may use dynamic allocation techniques and have no specific limit values to report.)


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 recognised to provide the associated semantic action. The code to be executed will appear as bodies of text that are intended to be C-language code. 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 will 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 cc or c89. 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 version of those functions. Default versions of main() and yyerror() are supplied in the yacc library and can be linked in by using the -l y operand to cc or c89. The yacc library interfaces need not support interfaces with other than the default yy symbol prefix. The application provides the lexical analyser function, yylex(); the lex utility is specifically designed to generate such a routine.

 Input Language
Every specification file must consist of three sections in order: declarations , grammar rules and programs , separated by double percent signs (%%). 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 characters blank, newline and form-feed are ignored, except that they must not appear in names or multi-character reserved symbols. Comments must 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. Upper- and lower-case letters are distinct. Portable applications must 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 will appear in #define statements.

A literal consists of a single character enclosed in single-quotes ('). All of the escape sequences supported for character constants by the ISO C standard are supported by yacc.

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

The NUL character must not be 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 specification, 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 are to 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 will be taken to be that number.

%token [<tag>] name [number] [name [number]]...
Declares names to be a token. If tag is present, the C type for all tokens on this line will be declared to be the type referenced by tag. If a positive integer, number, follows a name, that value will be assigned to the token.
%left [<tag>] name [number] [name [number]]...
%right [<tag>] name [number] [name [number]]...
Declares name to be a token, and assigns precedence to it. 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 declares a C type for names as described for %token.
%nonassoc [<tag>] name [number] [name [number]]...
Declares name to be a token, and indicates that this cannot be used associatively. If the parser encounters associative use of this token it will report an error. If tag is present, it declares a C type for names as described for %token.
%type [<tag>] name...
Declares that union member names are non-terminals, and thus it is required to have a tag field at its beginning. Because it deals with non-terminals only, assigning a token number or using a literal is also prohibited. If this construct is present, yacc will perform type checking; if this construct is not present, the parse stack will hold only the int type.

Every name used in grammar undefined by a %token, %left, %right or %nonassoc declaration is assumed to represent a non-terminal symbol. The yacc utility will 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 will not be changed. If the first declaration of a token does not assign a token number, yacc will assign a token number. Once this assignment is made, the token number will not be changed by explicit assignment.

The following declarators do not follow the previous pattern.

Declares the non-terminal name to be the start symbol , which represents the largest, most general structure described by the grammar rules. By default, it is the left-hand side of the first grammar rule; this default can be overridden with this declaration.
%union { body of union (in C) }
Declares the yacc value stack to be a union of the various types of values desired. By default, the values returned by actions (see below) and the lexical analyser will be integers. The yacc utility keeps track of types, and will 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 will be included in the declarations section by using an #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 input.
%{ ... %}
C-language declarations and definitions can appear in the declarations section, enclosed by these marks. These statements will 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 declarations section must be 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 A 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 bar "|" 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:

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 recognised in the input process. (Note that the word "action" can also refer to the actions of the parsershift, reduce, and so on.) These actions can return values and can obtain the values returned by previous actions. These values will be kept in objects of type YYSTYPE (see %union). The result value of the action will 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 analyser 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 "}". 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.
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 it is, 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.
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 in the middle of a rule as well as 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 will 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 will 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 is 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 will result.

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 will 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 grammar rules must be terminated by %%.

 Programs Section
The programs section can include the definition of the lexical analyser yylex(), and any other functions, for example those used in the actions specified in the grammar rules. This is C-language code, and will be included in the code file after the tables and code generated by yacc. 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 must 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.

This corresponds to the concept of name, given previously. It also includes literals as defined previously.
This is a name, and additionally it is known to be followed by a colon. A literal cannot yield this token.
A string of digits (a non-negative decimal integer).
and so on
These correspond directly to %type, %left, %% and so on.
{ ... }
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 recognised by the lexical analyser */

%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    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
      |    UNION
        /* Copy union definition to output */
      |    LCURL
        /* Copy C code to output file */
      |    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 ';'

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 will 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 will 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 favour 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 will 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 is 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 is 256. Its value can be changed using a %token declaration. The lexical analyser should not return the value of error. (Multi-byte characters should be recognised by the lexical analyser 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 used in many multi-byte character sets, is not available for use as the value of a user-defined token.)

The parser will 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 will pass 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 will 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 will not call yyerror when YYERROR is executed.

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

When a syntax error is detected by the parser, the parser will 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 Analyser ). If it is, the parser will return with a non-zero value. Otherwise, the lookahead symbol will be discarded and normal parsing will 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 will 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 will return with a non-zero value. Otherwise, it will shift over error and then resume normal parsing. If the parser reads a lookahead symbol before the error was detected, that symbol will still be the lookahead symbol when parsing is resumed.

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

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

 Interface to the Lexical Analyser
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 will 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 (one 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 will not be included in the #define declarations.

If the token numbers are chosen by yacc, the tokens other than literals will 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 assigned token numbers will be unique and distinct from the token numbers used for literals. If duplicate token numbers cause conflicts in parser generation, yacc will 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 analysers will 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 will accept the input. If the endmarker is seen in any other context, it will 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 appear only in the yacc library accessible through the -l y operand to cc or c89; they can therefore be redefined by a portable application:
int main(void)
This function will call yyparse() and exit with an unspecified value. Other actions within this function are unspecified.
int yyerror(const char *s
This function will write the NUL-terminated argument to standard error, followed by a newline character.

The order of the -l y and -l l operands given to cc or c89 is significant; the application must either provide its own main() function or ensure that -l y precedes -l l.

 Debugging the Parser
The parser generated by yacc will have diagnostic facilities in it that can be optionally enabled at either compile time or at run time (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 will be included. If its value is zero, the code will 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 run time. The initial value of yydebug will be zero.

When -t is specified, the code file will be built such that, if YYDEBUG is not already defined at compilation time (using the c89 -D YYDEBUG option, for example), YYDEBUG will be set explicitly to 1. When -t is not specified, the code file will be built such that, if YYDEBUG is not already defined, it will 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 will 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-dependent. The implementation will 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.
Limit Minimum
{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
{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
Table: Internal Limits in yacc


The following exit values are returned:
Successful completion.
An error occurred.


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


Historical implementations experience name conflicts on the names yacc.tmp, yacc.acts, yacc.debug,, 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 will have 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.


Access to the yacc library is obtained with library search operands to cc or c89. To use the yacc library main():

c89 -l y

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

c89 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 library 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);


The IEEE PASC 1003.2 Interpretations Committee has forwarded concerns about parts of this interface definition to the IEEE PASC Shell and Utilities Working Group which is identifying the corrections. A future revision of this specification will align with IEEE Std. 1003.2b when finalised.


cc, c89, lex.

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