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.TH YACC 1 "October 5, 2014" "Berkeley Yacc" "User Commands"
.SH NAME
Yacc \- an LALR(1) parser generator
.SH SYNOPSIS
.B yacc [ -BdgilLPrtvVy ] [ \-b
.I file_prefix
.B ] [ \-o
.I output_file
.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 \-B
create a backtracking parser (compile-time configuration for \fBbtyacc\fP).
.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
.B \-L
enable position processing, e.g., \*(``%locations\*('' (compile-time configuration for \fBbtyacc\fP).
.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-b\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"
.XE
.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
.XE
.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.
The \fB%destructor\fP and \fB%locations\fP features are available
only if \fByacc\fP has been configured and compiled to support the
back-tracking (\fBbtyacc\fP) functionality.
The remaining features are always available:
.TP
\fB %destructor\fP { \fIcode\fP } \fIsymbol+\fP
defines code that is invoked when a symbol is automatically
discarded during error recovery.
This code can be used to
reclaim dynamically allocated memory associated with the corresponding
semantic value for cases where user actions cannot manage the memory
explicitly.
.IP
On encountering a parse error, the generated parser
discards symbols on the stack and input tokens until it reaches a state
that will allow parsing to continue.
This error recovery approach results in a memory leak
if the \fBYYSTYPE\fP value is, or contains,
pointers to dynamically allocated memory.
.IP
The bracketed \fIcode\fP is invoked whenever the parser discards one of
the symbols. Within \fIcode\fP, \*(``\fB$$\fP\*('' or
\*(``\fB$<tag>$\fP\*('' designates the semantic value associated with the
discarded symbol, and  \*(``\fB@$\fP\*('' designates its location (see
\fB%locations\fP directive).
.IP
A per-symbol destructor is defined by listing a grammar symbol
in \fIsymbol+\fP.  A per-type destructor is defined  by listing
a semantic type tag (e.g., \*(``<some_tag>\*('') in \fIsymbol+\fP; in this
case, the parser will invoke \fIcode\fP whenever it discards any grammar
symbol that has that semantic type tag, unless that symbol has its own
per-symbol destructor.
.IP
Two categories of default destructor are supported that are
invoked when discarding any grammar symbol that has no per-symbol and no
per-type destructor:
.RS
.bP
the code for \*(``\fB<*>\fP\*('' is used
for grammar symbols that have an explicitly declared semantic type tag
(via \*(``\fB%type\fP\*('');
.bP
the code for \*(``\fB<>\fP\*('' is used
for grammar symbols that have no declared semantic type tag.
.RE
.TP
\fB %expect\fP \fInumber\fP
tells \fByacc\fP the expected number of shift/reduce conflicts.
That makes it only report the number if it differs.
.TP
\fB %expect-rr\fP \fInumber\fP
tell \fByacc\fP the expected number of reduce/reduce conflicts.
That makes it only report the number if it differs.
This is (unlike bison) allowable in LALR parsers.
.TP
\fB %locations\fP
tells \fByacc\fP to enable  management of position information associated
with each token, provided by the lexer in the global variable \fByylloc\fP,
similar to management of semantic value information provided in \fByylval\fP.
.IP
As for semantic values, locations can be referenced within actions using
\fB@$\fP to refer to the location of the left hand side symbol, and \fB@N\fP
(\fBN\fP an integer) to refer to the location of one of the right hand side
symbols. Also as for semantic values, when a rule is matched, a default
action is used the compute the location represented by \fB@$\fP as the
beginning of the first symbol and the end of the last symbol in the right
hand side of the rule. This default computation can be overridden by
explicit assignment to \fB@$\fP in a rule action.
.IP
The type of \fByylloc\fP is \fBYYLTYPE\fP, which is defined by default as:
.ES
typedef struct YYLTYPE {
    int first_line;
    int first_column;
    int last_line;
    int last_column;
} YYLTYPE;
.XE
.IP
\fBYYLTYPE\fP can be redefined by the user
(\fBYYLTYPE_IS_DEFINED\fP must be defined, to inhibit the default)
in the declarations section of the specification file.
As in bison, the macro \fBYYLLOC_DEFAULT\fP is invoked
each time a rule is matched to calculate a position for the left hand side of
the rule, before the associated action is executed; this macro can be
redefined by the user.
.IP
This directive adds a \fBYYLTYPE\fP parameter to \fByyerror()\fP.
If the \fB%pure-parser\fP directive is present,
a \fBYYLTYPE\fP parameter is added to \fByylex()\fP calls.
.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.
.TP
\fB %token-table\fP
Make the parser's names for tokens available in the \fByytname\fP array.
However,
.B yacc
does not predefine \*(``$end\*('', \*(``$error\*(''
or \*(``$undefined\*('' in this array.
.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.
.XE
.PP
The rationale in
.ES
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
.XE
.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 grammar 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();
		}
.XE
.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.
.bP
\fBYacc\fP accepts multiple parameters with \fB%lex-param\fP and \fB%parse-param\fP
in two forms
.ES
{type1 name1} {type2 name2} ...
{type1 name1,  type2 name2 ...}
.XE
.IP
Bison accepts the latter (though undocumented), but depending on the
release may generate bad code.
.bP
Like bison, \fByacc\fP will add parameters specified via \fB%parse-param\fP
to \fByyparse\fP, \fByyerror\fP and (if configured for back-tracking)
to the destructor declared using \fB%destructor\fP.
Bison puts the additional parameters \fIfirst\fP for
\fByyparse\fP and \fByyerror\fP but \fIlast\fP for destructors.
\fBYacc\fP matches this behavior.
.
.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.