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.TH YACC 1 "September 7, 2011" "Berkeley Yacc" "User Commands"
.SH NAME
Yacc \- an LALR(1) parser generator
.SH SYNOPSIS
.B yacc [ -dgilrtv ] [ \-b
.I file_prefix
.B ] [ \-p
.I symbol_prefix
.B ]
.I filename
.SH DESCRIPTION
.B Yacc
reads the grammar specification in the file
.I filename
and generates an LALR(1) parser for it.
The parsers consist of a set of LALR(1) parsing tables and a driver routine
written in the C programming language.
.B Yacc
normally writes the parse tables and the driver routine to the file
.IR y.tab.c.
.PP
The following options are available:
.TP 5
\fB\-b \fP\fIfile_prefix\fR
The
.B \-b
option changes the prefix prepended to the output file names to
the string denoted by
.IR file_prefix.
The default prefix is the character
.IR y.
.TP
.B \-d
The \fB-d\fR option causes the header file
.BR y.tab.h
to be written.
It contains #define's for the token identifiers.
.TP
.B \-g
The
.B \-g
option causes a graphical description of the generated LALR(1) parser to
be written to the file
.BR y.dot
in graphviz format, ready to be processed by dot(1).
.TP
.B \-i
The \fB-i\fR option causes a supplementary header file
.BR y.tab.i
to be written.
It contains extern declarations
and supplementary #define's as needed to map the conventional \fIyacc\fP
\fByy\fP-prefixed names to whatever the \fB-p\fP option may specify.
The code file, e.g., \fBy.tab.c\fP is modified to #include this file
as well as the \fBy.tab.h\fP file, enforcing consistent usage of the
symbols defined in those files.
.IP
The supplementary header file makes it simpler to separate compilation
of lex- and yacc-files.
.TP
.B \-l
If the
.B \-l
option is not specified,
.B yacc
will insert \fI#line\fP directives in the generated code.
The \fI#line\fP directives let the C compiler relate errors in the
generated code to the user's original code.
If the \fB-l\fR option is specified,
.B yacc
will not insert the \fI#line\fP directives.
\&\fI#line\fP directives specified by the user will be retained.
.TP
\fB\-o \fP\fIoutput_file\fR
specify the filename for the parser file.
If this option is not given, the output filename is
the file prefix concatenated with the file suffix, e.g., \fBy.tab.c\fP.
This overrides the \fB-p\fP option.
.TP
\fB\-p \fP\fIsymbol_prefix\fR
The
.B \-p
option changes the prefix prepended to yacc-generated symbols to
the string denoted by
.IR symbol_prefix.
The default prefix is the string
.BR yy.
.TP
.B \-P
create a reentrant parser, e.g., "%pure-parser".
.TP
.B \-r
The
.B \-r
option causes
.B yacc
to produce separate files for code and tables.  The code file
is named
.IR y.code.c,
and the tables file is named
.IR y.tab.c.
The prefix "\fIy.\fP" can be overridden using the \fB\-b\fP option.
.TP
.B \-s
suppress "\fB#define\fP" statements generated for string literals in
a "\fB%token\fP" statement, to more closely match original \fByacc\fP behavior.
.IP
Normally when \fByacc\fP sees a line such as
.ES
%token OP_ADD "ADD"
.EE
.IP
it notices that the quoted "ADD" is a valid C identifier,
and generates a #define not only for OP_ADD,
but for ADD as well,
e.g.,
.ES
#define OP_ADD 257
.br
#define ADD 258
.EE
.IP
The original \fByacc\fP does not generate the second "\fB#define\fP".
The \fB\-s\fP option suppresses this "\fB#define\fP".
.IP
POSIX (IEEE 1003.1 2004) documents only names and numbers for "\fB%token\fP",
though original \fByacc\fP and bison also accept string literals.
.TP
.B \-t
The
.B \-t
option changes the preprocessor directives generated by
.B yacc
so that debugging statements will be incorporated in the compiled code.
.TP
.B \-v
The
.B \-v
option causes a human-readable description of the generated parser to
be written to the file
.IR y.output.
.TP
.B \-V
print the version number to the standard output.
.TP
.B \-y
\fByacc\fP ignores this option,
which bison supports for ostensible POSIX compatibility.
.SH EXTENSIONS
.B yacc
provides some extensions for compatibility with bison and other implementations
of yacc:
.TP
\fB %expect\fP \fInumber\fP
tell \fByacc\fP the expected number of shift/reduce conflicts.
That makes it only report the number if it differs.
.TP
\fB %expect-rr\fP \fInumber\fP
tell \fByacc\fP the expected number of reduce/reduce conflicts.
That makes it only report the number if it differs.
This is (unlike bison) allowable in LALR parsers.
.TP
\fB %lex-param\fP { \fIargument-declaration\fP }
By default, the lexer accepts no parameters, e.g., \fByylex()\fP.
Use this directive to add parameter declarations for your customized lexer.
.TP
\fB %parse-param\fP { \fIargument-declaration\fP }
By default, the parser accepts no parameters, e.g., \fByyparse()\fP.
Use this directive to add parameter declarations for your customized parser.
.TP
\fB %pure-parser\fP
Most variables (other than \fByydebug\fP and \fByynerrs\fP) are
allocated on the stack within \fByyparse\fP, making the parser reasonably
reentrant.
.SH PORTABILITY
According to Robert Corbett,
.ES
    Berkeley Yacc is an LALR(1) parser generator.  Berkeley Yacc has been made
as compatible as possible with AT&T Yacc.  Berkeley Yacc can accept any input
specification that conforms to the AT&T Yacc documentation.  Specifications
that take advantage of undocumented features of AT&T Yacc will probably be
rejected.
.EE
.PP
The rationale in
.ES
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
.EE
.PP
documents some features of AT&T yacc which are no longer required for POSIX
compliance.
.PP
That said, you may be interested in reusing grammary files with some
other implementation which is not strictly compatible with AT&T yacc.
For instance, there is bison.
Here are a few differences:
.bP
\fBYacc\fP accepts an equals mark preceding the left curly brace
of an action (as in the original grammar file \fBftp.y\fP):
.ES
	|	STAT CRLF
		= {
			statcmd();
		}
.EE
.bP
\fBYacc\fP and bison emit code in different order, and in particular bison
makes forward reference to common functions such as yylex, yyparse and
yyerror without providing prototypes.
.bP
Bison's support for "%expect" is broken in more than one release.
For best results using bison, delete that directive.
.bP
Bison has no equivalent for some of \fByacc\fP's commmand-line options,
relying on directives embedded in the grammar file.
.bP
Bison's "\fB\-y\fP" option does not affect bison's lack of support for
features of AT&T yacc which were deemed obsolescent.
.
.SH DIAGNOSTICS
If there are rules that are never reduced, the number of such rules is
reported on standard error.
If there are any LALR(1) conflicts, the number of conflicts is reported
on standard error.