|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
YACC(1P) POSIX Programmer's Manual YACC(1P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
yacc — yet another compiler compiler (DEVELOPMENT)
yacc [−dltv] [−b file_prefix] [−p sym_prefix] grammar
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(1p)).
The yacc utility shall conform to the Base Definitions volume of
POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines, except for
Guideline 9.
The following options shall be supported:
−b file_prefix
Use file_prefix instead of y 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.tab.c, file_prefix.tab.h, and
file_prefix.output, respectively.
−d Write the header file; by default only the code file is
written. The #define statements associate the token codes
assigned by yacc with the user-declared token names. This
allows source files other than y.tab.c to access the token
codes.
−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.
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.
Not used.
The file grammar shall be a text file formatted as specified in the
EXTENDED DESCRIPTION section.
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.1‐2008, 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.
Default.
Not used.
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.
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.)
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 y 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.1‐2008, 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 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-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.
$<tag>$ 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 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.
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. │
└───────────┴─────────┴────────────────────────────────┘
The following exit values shall be returned:
0 Successful completion.
>0 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 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.
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.
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);
}
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.1‐2008. 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.1‐2008
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.
None.
c99(1p), lex(1p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 YACC(1P)
Pages that refer to this page: cflow(1p), lex(1p), make(1p)