yacc(1) General Commands Manual yacc(1)
NAME
yacc - Generates an LR(1) parsing program from input consisting of a context-free grammar specification
SYNOPSIS
yacc [-vltds] [-b prefix] [-N number] [-p symbol_prefix] [-P pathname] grammar
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
yacc: XPG4, XPG4-UNIX
Refer to the standards(5) reference page for more information about industry standards and associated tags.
OPTIONS
Uses prefix instead of y as the prefix for all output filenames (prefix.tab.c, prefix.tab.h, and prefix.output). Produces the <y.tab.h>
file, which contains the #define statements that associate the yacc-assigned token codes with your token names. This allows source files
other than y.tab.c to access the token codes by including this header file. Includes no #line constructs in y.tab.c. Use this only after
the grammar and associated actions are fully debugged. [Tru64 UNIX] Provides yacc with extra storage for building its LALR tables, which
may be necessary when compiling very large grammars. The number should be larger than 40,000 when you use this option. Allows multiple
yacc parsers to be linked together. Use symbol_prefix instead of yy to prefix global symbols. [Tru64 UNIX] Specifies an alternative
parser (instead of /usr/ccs/lib/yaccpar). The pathname specifies the filename of the skeleton to be used in place of yaccpar). [Tru64
UNIX] Breaks the yyparse() function into several smaller functions. Because its size is somewhat proportional to that of the grammar, it
is possible for yyparse() to become too large to compile, optimize, or execute efficiently. Compiles run-time debugging code. By default,
this code is not included when y.tab.c is compiled. If YYDEBUG has a nonzero value, the C compiler (cc) includes the debugging code,
whether or not the -t option was used. Without compiling this code, yyparse() will run more quickly. Produces the y.output file, which
contains a readable description of the parsing tables and a report on conflicts generated by grammar ambiguities.
OPERANDS
The pathname of a file containing input instructions. The format of this file is described in the DESCRIPTION section.
DESCRIPTION
The yacc command converts a context-free grammar specification into a set of tables for a simple automaton that executes an LR(1) parsing
algorithm. The yacc grammar can be ambiguous; specified precedence rules are used to break ambiguities.
You must compile the y.tab.c output file with a C language compiler to produce the yyparse() function. This function must be loaded with a
yylex lexical analyzer function, as well as two routines that you must provide, main() and an error-handling routine, yyerror(). The lex
command is useful for creating lexical analyzers usable by yacc.
The yacc program reads its skeleton parser from the file /usr/ccs/lib/yaccpar. Use the environment variable YACCPAR to specify another
location for the yacc program to read from. If you use this environment variable, the -P option is ignored, if specified.
The general format of the yacc input file is as follows:
[definitions] %% rules [%% [user subroutines]]
where Is the section where you define the variables to be used later in the grammar, such as in the rules section. It is also where files
are included (#include) and processing conditions are defined. This section is optional. Is the section that contains grammar rules for
the parser. A yacc input file must have a rules section. Is the section that contains user-supplied subroutines that can be used by the
actions in the rules section. This section is optional.
Comments, in C syntax, can appear anywhere in the user subroutines section or the definitions section. In the rules section, comments can
appear wherever a symbol is allowed. Blank lines or lines consisting of white space can be inserted anywhere in the file, and are ignored.
The NULL character must not be used in grammar rules or literals.
Definitions Section of Input File
The definitions section of a yacc input file contains entries that perform the following functions: Includes standard I/O header file.
Defines global variables. Defines the list rule as the place to start processing. Defines the tokens used by the parser. Defines the
operators and their precedence.
Each line in the definitions section can be: When placed on lines by themselves, these enclose C code to be passed into the global defini-
tions of the output file. Such lines commonly include preprocessor directives and declarations of external variables and functions. Lists
tokens or terminal symbols to be used in the rest of the input file. This line is needed for tokens that do not appear in other % defini-
tions. If type is present, the C type for all tokens on this line is declared to be the type referenced by type. If a positive integer num-
ber follows a token, that value is assigned to the token. Indicates that each token is an operator, all tokens in this definition have
equal precedence, and a succession of the operators listed in this definition are evaluated left to right. Indicates that each token is an
operator, that all tokens in this definition have equal precedence, and that a succession of the operators listed in this definition are
evaluated right to left. Indicates that each token is an operator, and that the operators listed in this definition cannot appear in suc-
cession. Indicates that the token cannot be used associatively. Indicates the highest-level production rule to be reduced; in other words,
the rule where the parser can consider its work done and can terminate processing. If this definition is not included, the parser uses the
first production rule. The symbol must be non-terminal (not a token). Defines each symbol as data type type, to resolve ambiguities. If
this construct is present, yacc performs type checking and otherwise assumes all symbols to be of type integer. Defines the yylval global
variable as a union, where union-def is a standard C definition in the format: { type member ; [type member ; ...] }
At least one member should be an int. Any valid C data type can be defined, including structures. When you run yacc with the -d
option, the definition of yylval is placed in the <y.tab.h> file and can be referred to in a lex input file.
Every token (non-terminal symbol) must be listed in one of the preceding % definitions. Multiple tokens can be separated by white space or
commas. All the tokens in %left, %right, and %nonassoc definitions are assigned a precedence with tokens in later definitions having prece-
dence over those in earlier definitions.
In addition to symbols, a token can be literal character enclosed in single quotes. (Multibyte characters are recognized by the lexical
analyzer and returned as tokens.) The following special characters can be used, just as in C programs: Alert Newline Tab Vertical tab Car-
riage Return Backspace Form Feed Backslash Single Quote Question mark One or more octal digits specifying the integer value of the charac-
ter
Rules Section of Input File
The rules section of a yacc input file defines the rules that parse the input stream. It consists of a series of production rules that the
parser tries to reduce. The format of each production rule is:
symbol : symbol-sequence [action] [| symbol-sequence [action] ...] ;
A symbol-sequence consists of zero or more symbols separated by white space. The first symbol must be the first character of the line, but
newlines and other white space can appear anywhere else in the rule. All terminal symbols must be declared in %token definitions.
Each symbol-sequence represents an alternative way of reducing the rule. A symbol can appear recursively in its own rule. Always use left-
recursion (where the recursive symbol appears before the terminating case in symbol-sequence).
The following sequence indicates that the current sequence of symbols is to be preferred over others, at the level of precedence assigned
to token in the definitions section of the input file:
%prec token
The specially defined token error matches any unrecognized sequence of input. This token causes the parser to invoke the yyerror function.
By default, the parser tries to synchronize with the input and continue processing it by reading and discarding all input up to the symbol
following error. (You can override this behavior through the yyerrok action.) If no error token appears in the yacc input file, the parser
exits with an error message upon encountering unrecognized input.
The parser always executes action after encountering the symbol that precedes it. Thus, an action can appear in the middle of a symbol-
sequence, after each symbol-sequence, or after multiple instances of symbol-sequence. In the last case, action is executed when the parser
matches any of the sequences.
The action consists of standard C code within braces and can also take the following values, variables, and keywords. If the token
returned by the yylex function is associated with a significant value, yylex should place the value in this global variable. By default,
yylval is of type long. The definitions section can include a %union definition to associate with other data types, including structures.
If you run yacc with the -d option, the full yylval definition is passed into the <y.tab.h> file for access by lex. Causes the parser to
start parsing tokens immediately after an erroneous sequence, instead of performing the default action of reading and discarding tokens up
to a synchronization token. The yyerrok action should appear immediately after the error token. Refers to symbol n, a token index in the
production, counting from the beginning of the production rule, where the first symbol after the colon is $1. The type variable is the name
of one of the union lines listed in the %union directive in the declaration section. The <type> syntax (non-standard) allows the value to
be cast to a specific data type. Note that you will rarely need to use the type syntax. Refers to the value returned by the matched sym-
bol-sequence and used for the matched symbol when reducing other rules. The symbol-sequence generally assigns a value to $$. The type vari-
able is the name of one of the union lines listed in the %union directive in the declaration section. The <type> syntax (non-standard)
allows the value to be cast to a specific data type. Note that you will rarely need to use the type syntax.
User Subroutines Section of Input File
The user subroutines section of the yacc input file contains user-supplied functions. Because these functions are included in this file,
you do not need to use the yacc library when processing this file. If you supply a lexical analyzer (yylex) to the parser, it must be con-
tained in the user subroutines section.
The following functions, which are contained in the user subroutines section, are invoked within the yyparse function generated by yacc.
The lexical analyzer called by yyparse to recognize each token of input. Usually this function is created by lex. yylex reads input, rec-
ognizes expressions within the input, and returns a token number representing the kind of token read. The function returns an int value. A
return value of 0 (zero) means the end of input.
If the parser and yylex do not agree on these token numbers, reliable communication between them cannot occur. For one-character
literals, the token is simply the numeric value of the character in the current character set. The numbers for other tokens can be
chosen by either yacc or the user. In either case, the #define construct of C is used to allow yylex() to return these numbers sym-
bolically. The #define statements are put into the code file, and into the header file if that file is requested. The set of charac-
ters permitted by yacc in an identifier is larger than that permitted by C. Token names found to contain such characters will not be
included in the #define declarations.
If the token numbers are chosen by yacc, those tokens other than literals are 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 declaration section with a number.
Names and literals not defined in this way retain their default definition. All assigned token numbers are unique and distinct from
the token numbers used for literals. If duplicate token numbers cause conflicts in parser generation, yacc reports an error; other-
wise, 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 that has a token number that is zero or negative. All lexical
analyzers return zero or negative as a token number upon reaching the end of their input. If the tokens up to, but not excluding,
the endmarker form a structure that matches the start symbol, the parser accepts the input. If the endmarker is seen in any other
context, it is considered an error. The function that the parser calls upon encountering an input error. The default function,
defined in liby.a, simply prints string to the standard error. The user can redefine the function. The function's type is void. The
wrap-up routine that returns a value of 1 when the end of input occurs.
The liby.a library contains default main() and yyerror() functions. (main() is the required main program that calls yyparse() to start the
program.) These routines look like the following, respectively:
main() {
setlocale(LC_ALL, );
(void) yyparse();
return(0); }
int yyerror(s);
char *s; {
fprintf(stderr,"%s
",s);
return (0); }
NOTES
The LANG and LC_* variables affect the execution of the yacc command as stated. The main() function defined by yacc issues the following
call:
setlocale(LC_ALL, )
As a result, the program generated by yacc will also be affected by the contents of these variables at run time.
The lex program can be compiled as a C program with -std0, -std, or -std1 mode. It can also be compiled as a C++ program. If YY_NOPROTO is
defined on the compilation command line, function prototypes are not generated.
EXIT STATUS
The following exit values are returned: Successful completion. An error occurred.
EXAMPLES
This section describes the example programs for the lex and yacc commands, which together create a simple desk calculator program that per-
forms addition, subtraction, multiplication, and division operations. The calculator program also allows you to assign values to variables
(each designated by a single lowercase ASCII letter), and then use the variables in calculations. The files that contain the program are as
follows: The lex specification file that defines the lexical analysis rules. The yacc grammar file that defines the parsing rules and
calls the yylex() function created by lex to provide input.
The remaining text expects that the current directory is the directory that contains the lex and yacc example program files.
Compiling the Example Program
Perform the following steps to create the example program using lex and yacc: Process the yacc grammar file using the -d option. The -d
option tells yacc to create a file that defines the tokens it uses in addition to creating the C language source code file.
yacc -d calc.y
The following files are created: The C language source file that yacc created for the parser. A header file containing #define
statements for the tokens used by the parser.
(The *.o files are created temporarily and then removed.) Process the lex specification file:
lex calc.l
The following file is created: The C language source file that lex created for the lexical analyzer. Compile and link the two C
language source files:
cc -o calc y.tab.c lex.yy.c
The following files are created: The object file for y.tab.c. The object file for lex.yy.c. The executable program file.
You can then run the program directly by entering: calc
Then, enter numbers and operators in calculator fashion. After you press <Return>, the program displays the result of the operation. If
you assign a value to a variable as follows, the cursor moves to the next line:
m=4 <Return> _
You can then use the variable in calculations and it will have the value assigned to it:
m+5 <Return> 9
The Parser Source Code
The file calc.y has entries in all three of the sections of a yacc grammar file--declarations, rules, and user subroutines. It contains the
following source code:
%{ #include <stdio.h>
int regs[26]; int base;
%}
%start list
%token DIGIT LETTER
%left '|' %left '&' %left '+' '-' %left '*' '/' '%' %left UMINUS /*supplies precedence for unary minus */
%% /* beginning of rules section */
list : /*empty */
| list stat '
'
| list error '
'
{ yyerrok; }
;
stat : expr
{ printf("%d
",$1); }
| LETTER '=' expr
{ regs[$1] = $3; }
;
expr : '(' expr ')'
{ $$ = $2; }
| expr '*' expr
{ $$ = $1 * $3; }
| expr '/' expr
{ $$ = $1 / $3; }
| expr '%' expr
{ $$ = $1 % $3; }
| expr '+' expr
{ $$ = $1 + $3; }
| expr '-' expr
{ $$ = $1 - $3; }
| expr '&' expr
{ $$ = $1 & $3; }
| expr '|' expr
{ $$ = $1 | $3; }
| '-' expr %prec UMINUS
{ $$ = -$2; }
| LETTER
{ $$ = regs[$1]; }
| number
;
number : DIGIT
{ $$ = $1; base = ($1==0) ? 8:10; }
| number DIGIT
{ $$ = base * $1 + $2; }
;
%% /* beginning of user subroutines section */ main() {
return(yyparse()); }
yyerror(s) char *s; {
fprintf(stderr,"%s
",s); }
yywrap() {
return(1); }
The Lexical Analyzer Source Code
The file calc.l contains the lexical analyzer source code. It contains the rules used to generate the tokens from the input stream. It
also contains include statements for standard input and output, as well as for the <y.tab.h> file. The yacc program generates the <y.tab.h>
file from the yacc grammar file information, if you use the -d option with the yacc command. The file <y.tab.h> contains definitions for
the tokens that the parser program uses.
Contents of calc.1: %{
#include <stdio.h> #include "y.tab.h" int c; #if !defined (YYSTYPE) #define YYSTYPE long #endif extern YYSTYPE yylval; %} %% " " ; [a-
z] {
c = yytext[0];
yylval = c - 'a';
return(LETTER);
} [0-9] {
c = yytext[0];
yylval = c - '0';
return(DIGIT);
} [^a-z 0-9] {
c = yytext[0];
return(c);
}
ENVIRONMENT VARIABLES
The following environment variables affect the execution of yacc: Provides a default value for the internationalization variables that are
unset or null. If LANG is unset or null, the corresponding value from the default locale is used. If any of the internationalization vari-
ables contain an invalid setting, the utility behaves as if none of the variables had been defined. If set to a non-empty string value,
overrides the values of all the other internationalization variables. Determines 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). Determines the
locale for the format and contents of diagnostic messages written to standard error. Determines the location of message catalogs for the
processing of LC_MESSAGES.
FILES
A readable description of parsing tables and a report on conflicts generated by grammar ambiguities Output file Definitions for token names
Temporary file Temporary file Temporary file Default skeleton parser for C programs The yacc library
SEE ALSO
Commands: lex(1)
Standards: standards(5)
Programming Support Tools
yacc(1)