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PROGRAM:

NAME


yacc — yet another compiler compiler (DEVELOPMENT)

SYNOPSIS


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

DESCRIPTION


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

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

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

OPTIONS


The yacc utility shall conform to the Base Definitions volume of POSIX.1‐2008, 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.

OPERANDS


The following operand is required:

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

STDIN


Not used.

INPUT FILES


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

ENVIRONMENT VARIABLES


The following environment variables shall affect the execution of yacc:

LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, 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.

ASYNCHRONOUS EVENTS


Default.

STDOUT


Not used.

STDERR


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

Standard error shall also be used for diagnostic messages.

OUTPUT FILES


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

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

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

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

EXTENDED DESCRIPTION


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

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

Given a grammar, the yacc utility generates the files described in the OUTPUT FILES
section. The code file can be compiled and linked using c99. If the declaration and
programs sections of the grammar file did not include definitions of main(), yylex(), and
yyerror(), the compiled output requires linking with externally supplied versions of those
functions. Default versions of main() and yyerror() are supplied in the yacc library and
can be linked in by using the −l 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 │ │
LimitMaximumDescription
├───────────┼─────────┼────────────────────────────────┤
│{NTERMS} │ 126 │ Number of tokens. │
│{NNONTERM} │ 200 │ Number of non-terminals. │
│{NPROD} │ 300 │ Number of rules. │
│{NSTATES} │ 600 │ Number of states. │
│{MEMSIZE} │ 5200 │ Length of rules. The total │
│ │ │ length, in names (tokens and │
│ │ │ non-terminals), of all the │
│ │ │ rules of the grammar. The │
│ │ │ left-hand side is counted for │
│ │ │ each rule, even if it is not │
│ │ │ explicitly repeated, as │
│ │ │ specified in Grammar Rules in
│ │ │ yacc. │
│{ACTSIZE} │ 4000 │ Number of actions. ``Actions'' │
│ │ │ here (and in the description │
│ │ │ file) refer to parser actions │
│ │ │ (shift, reduce, and so on) not │
│ │ │ to semantic actions defined in │
│ │ │ Grammar Rules in yacc. │
└───────────┴─────────┴────────────────────────────────┘

EXIT STATUS


The following exit values shall be returned:

0 Successful completion.

>0 An error occurred.

CONSEQUENCES OF ERRORS


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

The following sections are informative.

APPLICATION USAGE


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

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

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

EXAMPLES


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

c99 y.tab.c −l y

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

c99 y.tab.c lex.yy.c −l y −l l

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

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

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

setlocale(LC_ALL, "");

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

#include <stdio.h>

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

RATIONALE


The references in Referenced Documents may be helpful in constructing the parser
generator. The referenced DeRemer and Pennello article (along with the works it
references) describes a technique to generate parsers that conform to this volume of
POSIX.1‐2008. 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.

FUTURE DIRECTIONS


None.

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