.\"	$NetBSD: yacc.1,v 1.9 2020/07/27 00:34:56 uwe Exp $
.\"
.\" Copyright (c) 1989, 1990 The Regents of the University of California.
.\" All rights reserved.
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.\" This code is derived from software contributed to Berkeley by
.\" Robert Paul Corbett.
.\"
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.\" modification, are permitted provided that the following conditions
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.\"    may be used to endorse or promote products derived from this software
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.\"	from: @(#)yacc.1	5.7 (Berkeley) 7/30/91
.\"	from: Id: yacc.1,v 1.24 2014/10/06 00:03:48 tom Exp
.\"	$NetBSD: yacc.1,v 1.9 2020/07/27 00:34:56 uwe Exp $
.\"
.Dd October 5, 2014
.Dt YACC 1
.Os
.Sh NAME
.Nm yacc
.Nd an
.Tn LALR Ns (1)
parser generator
.Sh SYNOPSIS
.Nm
.Op Fl BdgilLPrtvVy
.Op Fl b Ar file_prefix
.Op Fl o Ar output_file
.Op Fl p Ar symbol_prefix
.Ar filename
.Sh DESCRIPTION
.Nm
reads the grammar specification in the file
.Ar filename
and generates an
.Tn LALR Ns (1)
parser for it.
The parsers consist of a set of
.Tn LALR Ns (1)
parsing tables and a driver routine
written in the C programming language.
.Nm
normally writes the parse tables and the driver routine to the file
.Pa y.tab.c .
.Pp
The following options are available:
.Bl -tag -width Fl
.It Fl b Ar file_prefix
The
.Fl b
option changes the prefix prepended to the output file names to
the string denoted by
.Ar file_prefix .
The default prefix is the character
.Ql y .
.It Fl B
Create a backtracking parser (compile-type configuration for
.Nm ) .
.It Fl d
The
.Fl d
option causes the header file
.Pa y.tab.h
to be written.
It contains
.No #define Ns 's
for the token identifiers.
.It Fl g
The
.Fl g
option causes a graphical description of the generated
.Tn LALR Ns (1)
parser to be written to the file
.Pa y.dot
in graphviz format, ready to be processed by
.Xr dot 1 .
.It Fl i
The
.Fl i
option causes a supplementary header file
.Pa y.tab.i
to be written.
It contains extern declarations
and supplementary
.No #define Ns 's
as needed to map the conventional
.Nm
.Va yy Ns \&-prefixed
names to whatever the
.Fl p
option may specify.
The code file, e.g.,
.Pa y.tab.c
is modified to
.No #include
this file as well as the
.Pa y.tab.h
file, enforcing consistent usage of the symbols defined in those files.
The supplementary header file makes it simpler to separate compilation
of lex- and yacc-files.
.It Fl l
If the
.Fl l
option is not specified,
.Nm
will insert
.No #line
directives in the generated code.
The
.No #line
directives let the C compiler relate errors in the
generated code to the user's original code.
If the
.Fl l
option is specified,
.Nm
will not insert the
.No #line
directives.
.No #line
directives specified by the user will be retained.
.It Fl L
Enable position processing, e.g.,
.Ql %locations
(compile-type configuration for
.Nm ) .
.It Fl o Ar output_file
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.
.Pa y.tab.c .
This overrides the
.Fl b
option.
.It Fl P
The
.Fl P
options instructs
.Nm
to create a reentrant parser, like
.Ql %pure-parser
does.
.It Fl p Ar symbol_prefix
The
.Fl p
option changes the prefix prepended to yacc-generated symbols to
the string denoted by
.Ar symbol_prefix .
The default prefix is the string
.Ql yy .
.It Fl r
The
.Fl r
option causes
.Nm
to produce separate files for code and tables.
The code file is named
.Pa y.code.c ,
and the tables file is named
.Pa y.tab.c .
The prefix
.Ql y
can be overridden using the
.Fl b
option.
.It Fl s
Suppress
.No #define
statements generated for string literals in a
.Ql %token
statement, to more closely match original
.Nm
behavior.
.Pp
Normally when
.Nm
sees a line such as
.Pp
.Dl %token OP_ADD \*qADD\*q
.Pp
it notices that the quoted
.Dq ADD
is a valid C identifier, and generates a
.No #define
not only for
.Dv OP_ADD ,
but for
.Dv ADD
as well,
e.g.,
.Bd -literal -offset indent
#define OP_ADD 257
#define ADD 258
.Ed
.Pp
The original
.Nm
does not generate the second
.No #define .
The
.Fl s
option suppresses this
.No #define .
.Pp
.St -p1003.1
documents only names and numbers for
.Ql %token ,
though the original
.Nm
and
.Xr bison 1
also accept string literals.
.It Fl t
The
.Fl t
option changes the preprocessor directives generated by
.Nm
so that debugging statements will be incorporated in the compiled code.
.It Fl V
The
.Fl V
option prints the version number to the standard output.
.It Fl v
The
.Fl v
option causes a human-readable description of the generated parser to
be written to the file
.Pa y.output .
.It Fl y
.Nm
ignores this option,
which
.Xr bison 1
supports for ostensible POSIX compatibility.
.El
.Sh EXTENSIONS
.Nm
provides some extensions for
compatibility with
.Xr bison 1
and other implementations of yacc.
The
.Ql %destructor
and
.Ql %locations
features are available only if
.Nm yacc
has been configured and compiled to support the back-tracking functionality.
The remaining features are always available:
.Bl -tag -width Ic
.It Ic %destructor { Ar code Ic } Ar symbol Ns +
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.
.Pp
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
.Vt YYSTYPE
value is, or contains, pointers to dynamically allocated memory.
.Pp
The bracketed
.Ar code
is invoked whenever the parser discards one of the symbols.
Within it
.Sq Li $$
or
.Sq Li $\*[Lt] Ns Ar tag Ns Li \*[Gt]$
designates the semantic value associated with the discarded symbol, and
.Sq Li @$
designates its location (see
.Ql %locations
directive).
.Pp
A per-symbol destructor is defined by listing a grammar symbol
in
.Ar symbol Ns + .
A per-type destructor is defined  by listing a semantic type tag (e.g.,
.Sq Li \*[Lt] Ns Ar some_tag Ns Li \*[Gt] )
in
.Ar symbol Ns + ;
in this case, the parser will invoke
.Ar code
whenever it discards any grammar symbol that has that semantic type tag,
unless that symbol has its own per-symbol destructor.
.Pp
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:
.Pp
The code for
.Sq Li \*[Lt]*\*[Gt]
is used
for grammar symbols that have an explicitly declared semantic type tag
(via
.Ql %type ) ;
.Pp
The code for
.Sq Li \*[Lt]\*[Gt]
is used for grammar symbols that have no declared semantic type tag.
.It Ic %expect Ar number
Tell
.Nm
the expected number of shift/reduce conflicts.
That makes it only report the number if it differs.
.It Ic %expect-rr Ar number
Tell
.Nm
the expected number of reduce/reduce conflicts.
That makes it only report the number if it differs.
This is, unlike
.Xr bison 1 ,
allowable in
.Tn LALR Ns (1)
parsers.
.It Ic %locations
Tell
.Nm
to enable  management of position information associated with each token,
provided by the lexer in the global variable
.Va yylloc ,
similar to management of semantic value information provided in
.Va yylval .
.Pp
As for semantic values, locations can be referenced within actions using
.Sq Li @$
to refer to the location of the left hand side symbol, and
.Sq Li @ Ns Ar N\|
.Ar ( N
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
.Sq Li @$
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
.Sq Li @$
in a rule action.
.Pp
The type of
.Va yylloc
is
.Vt YYLTYPE ,
which is defined by default as:
.Bd -literal -offset indent
typedef struct YYLTYPE {
    int first_line;
    int first_column;
    int last_line;
    int last_column;
} YYLTYPE;
.Ed
.Pp
.Vt YYLTYPE
can be redefined by the user
.Dv ( YYLTYPE_IS_DEFINED
must be defined, to inhibit the default)
in the declarations section of the specification file.
As in
.Xr bison 1 ,
the macro
.Dv YYLLOC_DEFAULT
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.
.Pp
This directive adds a
.Vt YYLTYPE
parameter to
.Fn yyerror .
If the
.Ql %pure-parser
directive is present,
a
.Vt YYLTYPE
parameter is added to
.Fn yylex
calls.
.It Ic %lex-param { Ar argument-declaration Ic }
By default, the lexer accepts no parameters, e.g.,
.Fn yylex .
Use this directive to add parameter declarations for your customized lexer.
.It Ic %parse-param { Ar argument-declaration Ic }
By default, the parser accepts no parameters, e.g.,
.Fn yyparse .
Use this directive to add parameter declarations for your customized parser.
.It Ic %pure-parser
Most variables (other than
.Va yydebug
and
.Va yynerrs )
are allocated on the stack within
.Fn yyparse ,
making the parser reasonably reentrant.
.It Ic %token-table
Make the parser's names for tokens available in the
.Va yytname
array.
However,
.Nm
yacc
does not predefine
.Dq $end ,
.Dq $error
or
.Dq $undefined
in this array.
.El
.Sh PORTABILITY
According to Robert Corbett:
.Bd -filled -offset indent
Berkeley Yacc is an
.Tn LALR Ns (1)
parser generator.
Berkeley Yacc has been made as compatible as possible with
.Tn AT\*[Am]T
Yacc.
Berkeley Yacc can accept any input specification that conforms to the
.Tn AT\*[Am]T
Yacc documentation.
Specifications that take advantage of undocumented features of
.Tn AT\*[Am]T
Yacc will probably be rejected.
.Ed
.Pp
The rationale in
.%U http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html
documents some features of
.Tn AT\*[Am]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
.Tn AT\*[Am]T
yacc.
For instance, there is
.Xr bison 1 .
Here are a few differences:
.Pp
.Nm
accepts an equals mark preceding the left curly brace
of an action (as in the original grammar file
.Dv ftp.y ) :
.Bd -literal -offset indent
    |	STAT CRLF
	= {
		statcmd();
	}
.Ed
.Pp
.Nm
and
.Xr bison 1
emit code in different order, and in particular
.Xr bison 1
makes forward reference to common functions such as
.Fn yylex ,
.Fn yyparse
and
.Fn yyerror
without providing prototypes.
.Pp
.Xr bison 1
support for
.Ql %expect
is broken in more than one release.
For best results using
.Xr bison 1 ,
delete that directive.
.Pp
.Xr bison 1
has no equivalent for some of
.Nm Ns 's
command-line options, relying on directives embedded in the grammar file.
.Pp
.Xr bison 1
.Fl y
option does not affect bison's lack of support for
features of AT\*[Am]T yacc which were deemed obsolescent.
.Pp
.Nm
accepts multiple parameters with
.Ql %lex-param
and
.Ql %parse-param
in two forms
.Bd -literal -offset indent
{type1 name1} {type2 name2} ...
{type1 name1,  type2 name2 ...}
.Ed
.Pp
.Xr bison 1
accepts the latter (though undocumented), but depending on the
release may generate bad code.
.Pp
Like
.Xr bison 1 ,
.Nm
will add parameters specified via
.Ql %parse-param
to
.Fn yyparse ,
.Fn yyerror
and (if configured for back-tracking)
to the destructor declared using
.Ql %destructor .
.Pp
.Xr bison 1
puts the additional parameters
.Em first
for
.Fn yyparse
and
.Fn yyerror
but
.Em last
for destructors.
.Nm
matches this behavior.
.Sh ENVIRONMENT
The following environment variable is referenced by
.Nm :
.Bl -tag -width TMPDIR
.It Ev TMPDIR
If the environment variable
.Ev TMPDIR
is set, the string denoted by
.Ev TMPDIR
will be used as the name of the directory where the temporary
files are created.
.El
.Sh TABLES
The names of the tables generated by this version of
.Nm
are
.Va yylhs ,
.Va yylen ,
.Va yydefred ,
.Va yydgoto ,
.Va yysindex ,
.Va yyrindex ,
.Va yygindex ,
.Va yytable ,
and
.Va yycheck .
Two additional tables,
.Va yyname
and
.Va yyrule ,
are created if
.Dv YYDEBUG
is defined and non-zero.
.Sh FILES
.Bl -tag -compact
.It Pa y.code.c
.It Pa y.tab.c
.It Pa y.tab.h
.It Pa y.output
.It Pa /tmp/yacc.aXXXXXX
.It Pa /tmp/yacc.tXXXXXX
.It Pa /tmp/yacc.uXXXXXX
.El
.Sh DIAGNOSTICS
If there are rules that are never reduced, the number of such rules is
written to the standard error.
If there are any
.Tn LALR Ns (1)
conflicts, the number of conflicts is also written
to the standard error.
.\" .Sh SEE ALSO
.\" .Xr yyfix 1
.Sh STANDARDS
The
.Nm
utility conforms to
.St -p1003.2 .