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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 |
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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.1‐2017, Section 12.2, Utility Syntax Guidelines, except for
Guideline 9.
The following options shall be supported:
-b file_prefix
Use file_prefix instead of 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. See the OUTPUT FILES section.
-l Produce a code file that does not contain any #line
constructs. If this option is not present, it is
unspecified whether the code file or header file
contains #line directives. This should only be used
after the grammar and the associated actions are fully
debugged.
-p sym_prefix
Use sym_prefix instead of yy as the prefix for all
external names produced by yacc. The names affected
shall include the functions yyparse(), yylex(), and
yyerror(), and the variables yylval, yychar, and
yydebug. (In the remainder of this section, the six
symbols cited are referenced using their default names
only as a notational convenience.) Local names may also
be affected by the -p option; however, the -p option
shall not affect #define symbols generated by yacc.
-t Modify conditional compilation directives to permit
compilation of debugging code in the code file. Runtime
debugging statements shall always be contained in the
code file, but by default conditional compilation
directives prevent their compilation.
-v Write a file containing a description of the parser and
a report of conflicts generated by ambiguities in the
grammar.
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.1‐2017, Section 8.2,
Internationalization Variables for the precedence of
internationalization variables used to determine the
values of locale categories.)
LC_ALL If set to a non-empty string value, override the values
of all the other internationalization variables.
LC_CTYPE Determine the locale for the interpretation of
sequences of bytes of text data as characters (for
example, single-byte as opposed to multi-byte
characters in arguments and input files).
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
The LANG and LC_* variables affect the execution of the yacc
utility as stated. The main() function defined in Yacc Library
shall call:
setlocale(LC_ALL, "")
and thus the program generated by yacc shall also be affected by
the contents of these variables at runtime.
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.1‐2017, in this case <tag> does not
represent a metavariable, but the literal angle bracket
characters surrounding a symbol.) The use of tag specifies that
the tokens named on this line shall be of the same C type as the
union member referenced by tag. This is discussed in more detail
below.
For lists used to define tokens, the first appearance of a given
token can be followed by a positive integer (as a string of
decimal digits). If this is done, the underlying value assigned
to it for lexical purposes shall be taken to be that number.
The following declares name to be a token:
%token [<tag>] name [number] [name [number]]...
If tag is present, the C type for all tokens on this line shall
be declared to be the type referenced by tag. If a positive
integer, number, follows a name, that value shall be assigned to
the token.
The following declares name to be a token, and assigns precedence
to it:
%left [<tag>] name [number] [name [number]]...
%right [<tag>] name [number] [name [number]]...
One or more lines, each beginning with one of these symbols, can
appear in this section. All tokens on the same line have the same
precedence level and associativity; the lines are in order of
increasing precedence or binding strength. %left denotes that
the operators on that line are left associative, and %right
similarly denotes right associative operators. If tag is present,
it shall declare a C type for names as described for %token.
The following declares name to be a token, and indicates that
this cannot be used associatively:
%nonassoc [<tag>] name [number] [name [number]]...
If the parser encounters associative use of this token it reports
an error. If tag is present, it shall declare a C type for names
as described for %token.
The following declares that union member names are non-terminals,
and thus it is required to have a tag field at its beginning:
%type <tag> name...
Because it deals with non-terminals only, assigning a token
number or using a literal is also prohibited. If this construct
is present, yacc shall perform type checking; if this construct
is not present, the parse stack shall hold only the int type.
Every name used in grammar not defined by a %token, %left,
%right, or %nonassoc declaration is assumed to represent a non-
terminal symbol. The yacc utility shall report an error for any
non-terminal symbol that does not appear on the left side of at
least one grammar rule.
Once the type, precedence, or token number of a name is
specified, it shall not be changed. If the first declaration of a
token does not assign a token number, yacc shall assign a token
number. Once this assignment is made, the token number shall not
be changed by explicit assignment.
The following declarators do not follow the previous pattern.
The following declares the non-terminal name to be the start
symbol, which represents the largest, most general structure
described by the grammar rules:
%start name
By default, it is the left-hand side of the first grammar rule;
this default can be overridden with this declaration.
The following declares the yacc value stack to be a union of the
various types of values desired.
%union { body of union (in C) }
The body of the union shall not contain unbalanced curly brace
preprocessing tokens.
By default, the values returned by actions (see below) and the
lexical analyzer shall be of type int. The yacc utility keeps
track of types, and it shall insert corresponding union member
names in order to perform strict type checking of the resulting
parser.
Alternatively, given that at least one <tag> construct is used,
the union can be declared in a header file (which shall be
included in the declarations section by using a #include
construct within %{ and %}), and a typedef used to define the
symbol YYSTYPE to represent this union. The effect of %union is
to provide the declaration of YYSTYPE directly from the yacc
input.
C-language declarations and definitions can appear in the
declarations section, enclosed by the following marks:
%{ ... %}
These statements shall be copied into the code file, and have
global scope within it so that they can be used in the rules and
program sections. The statements shall not contain "%}" outside a
comment, string literal, or multi-character constant.
The application shall ensure that the declarations section is
terminated by the token %%.
Grammar Rules in yacc
The rules section defines the context-free grammar to be accepted
by the function yacc generates, and associates with those rules
C-language actions and additional precedence information. The
grammar is described below, and a formal definition follows.
The rules section is comprised of one or more grammar rules. A
grammar rule has the form:
A : BODY ;
The symbol 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.1‐2017. Work in this area continues to be done, so
implementors should consult current literature before doing any
new implementations. The original Knuth article is the
theoretical basis for this kind of parser, but the tables it
generates are impractically large for reasonable grammars and
should not be used. The ``equivalent to'' wording is intentional
to assure that the best tables that are LALR(1) can be generated.
There has been confusion between the class of grammars, the
algorithms needed to generate parsers, and the algorithms needed
to parse the languages. They are all reasonably orthogonal. In
particular, a parser generator that accepts the full range of
LR(1) grammars need not generate a table any more complex than
one that accepts SLR(1) (a relatively weak class of LR grammars)
for a grammar that happens to be SLR(1). Such an implementation
need not recognize the case, either; table compression can yield
the SLR(1) table (or one even smaller than that) without
recognizing that the grammar is SLR(1). The speed of an LR(1)
parser for any class is dependent more upon the table
representation and compression (or the code generation if a
direct parser is generated) than upon the class of grammar that
the table generator handles.
The speed of the parser generator is somewhat dependent upon the
class of grammar it handles. However, the original Knuth article
algorithms for constructing LR parsers were judged by its author
to be impractically slow at that time. Although full LR is more
complex than LALR(1), as computer speeds and algorithms improve,
the difference (in terms of acceptable wall-clock execution time)
is becoming less significant.
Potential authors are cautioned that the referenced DeRemer and
Pennello article previously cited identifies a bug (an over-
simplification of the computation of LALR(1) lookahead sets) in
some of the LALR(1) algorithm statements that preceded it to
publication. They should take the time to seek out that paper, as
well as current relevant work, particularly Aho's.
The -b option was added to provide a portable method for
permitting yacc to work on multiple separate parsers in the same
directory. If a directory contains more than one yacc grammar,
and both grammars are constructed at the same time (by, for
example, a parallel make program), conflict results. While the
solution is not historical practice, it corrects a known
deficiency in historical implementations. Corresponding changes
were made to all sections that referenced the filenames y.tab.c
(now ``the code file''), y.tab.h (now ``the header file''), and
y.output (now ``the description file'').
The grammar for yacc input is based on System V documentation.
The textual description shows there that the ';' is required at
the end of the rule. The grammar and the implementation do not
require this. (The use of C_IDENTIFIER causes a reduce to occur
in the right place.)
Also, in that implementation, the constructs such as %token can
be terminated by a <semicolon>, but this is not permitted by the
grammar. The keywords such as %token can also appear in
uppercase, which is again not discussed. In most places where '%'
is used, <backslash> can be substituted, and there are alternate
spellings for some of the symbols (for example, %LEFT can be "%<"
or even "\<").
Historically, <tag> can contain any characters except '>',
including white space, in the implementation. However, since the
tag must reference an ISO C standard union member, in practice
conforming implementations need to support only the set of
characters for ISO C standard identifiers in this context.
Some historical implementations are known to accept actions that
are terminated by a period. Historical implementations often
allow '$' in names. A conforming implementation does not need to
support either of these behaviors.
Deciding when to use %prec illustrates the difficulty in
specifying the behavior of yacc. There may be situations in
which the grammar is not, strictly speaking, in error, and yet
yacc cannot interpret it unambiguously. The resolution of
ambiguities in the grammar can in many instances be resolved by
providing additional information, such as using %type or %union
declarations. It is often easier and it usually yields a smaller
parser to take this alternative when it is appropriate.
The size and execution time of a program produced without the
runtime debugging code is usually smaller and slightly faster in
historical implementations.
Statistics messages from several historical implementations
include the following types of information:
n/512 terminals, n/300 non-terminals
n/600 grammar rules, n/1500 states
n shift/reduce, n reduce/reduce conflicts reported
n/350 working sets used
Memory: states, etc. n/15000, parser n/15000
n/600 distinct lookahead sets
n extra closures
n shift entries, n exceptions
n goto entries
n entries saved by goto default
Optimizer space used: input n/15000, output n/15000
n table entries, n zero
Maximum spread: n, Maximum offset: n
The report of internal tables in the description file is left
implementation-defined because all aspects of these limits are
also implementation-defined. Some implementations may use dynamic
allocation techniques and have no specific limit values to
report.
The format of the y.output file is not given because
specification of the format was not seen to enhance applications
portability. The listing is primarily intended to help human
users understand and debug the parser; use of y.output by a
conforming application script would be unusual. Furthermore,
implementations have not produced consistent output and no
popular format was apparent. The format selected by the
implementation should be human-readable, in addition to the
requirement that it be a text file.
Standard error reports are not specifically described because
they are seldom of use to conforming applications and there was
no reason to restrict implementations.
Some implementations recognize "={" as equivalent to '{' because
it appears in historical documentation. This construction was
recognized and documented as obsolete as long ago as 1978, in the
referenced Yacc: Yet Another Compiler-Compiler. This volume of
POSIX.1‐2017 chose to leave it as obsolete and omit it.
Multi-byte characters should be recognized by the lexical
analyzer and returned as tokens. They should not be returned as
multi-byte character literals. The token error that is used for
error recovery is normally assigned the value 256 in the
historical implementation. Thus, the token value 256, which is
used in many multi-byte character sets, is not available for use
as the value of a user-defined token.
None.
c99(1p), lex(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any
discrepancy between this version and the original IEEE and The
Open Group Standard, the original IEEE and The Open Group
Standard is the referee document. The original Standard can be
obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 YACC(1P)
Pages that refer to this page: cflow(1p), lex(1p), make(1p)
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