gperf.texinfo

早期freebsd实现

specifies a version number of the license which applies to it and ``any
later version'', you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation.  If the Program does not specify a version number of
the license, you may choose any version ever published by the Free Software
Foundation.

@item
If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission.  For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

@iftex
@heading NO WARRANTY
@end iftex
@ifinfo
@center NO WARRANTY
@end ifinfo

@item
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.

@item
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL
ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES
ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT
LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES
SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE
WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
@end enumerate

@iftex
@heading END OF TERMS AND CONDITIONS
@end iftex
@ifinfo
@center END OF TERMS AND CONDITIONS
@end ifinfo

@page
@unnumberedsec Appendix: How to Apply These Terms to Your New Programs

  If you develop a new program, and you want it to be of the greatest
possible use to humanity, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.

  To do so, attach the following notices to the program.  It is safest to
attach them to the start of each source file to most effectively convey
the exclusion of warranty; and each file should have at least the
``copyright'' line and a pointer to where the full notice is found.

@smallexample
@var{one line to give the program's name and a brief idea of what it does.}
Copyright (C) 19@var{yy}  @var{name of author}

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 1, or (at your option)
any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
@end smallexample

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

@smallexample
Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author}
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
@end smallexample

The hypothetical commands `show w' and `show c' should show the
appropriate parts of the General Public License.  Of course, the
commands you use may be called something other than `show w' and `show
c'; they could even be mouse-clicks or menu items---whatever suits your
program.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a ``copyright disclaimer'' for the program, if
necessary.  Here a sample; alter the names:

@example
Yoyodyne, Inc., hereby disclaims all copyright interest in the
program `Gnomovision' (a program to direct compilers to make passes
at assemblers) written by James Hacker.

@var{signature of Ty Coon}, 1 April 1989
Ty Coon, President of Vice
@end example

That's all there is to it!

@node Contributors, Motivation, Copying, Top
@unnumbered Contributors to GNU @code{gperf} Utility

@itemize @bullet
@item
The GNU @code{gperf} perfect hash function generator utility was
originally written in GNU C++ by Douglas C. Schmidt.  It is now also
available in a highly-portable ``old-style'' C version.  The general
idea for the perfect hash function generator was inspired by Keith
Bostic's algorithm written in C, and distributed to net.sources around
1984.  The current program is a heavily modified, enhanced, and extended
implementation of Keith's basic idea, created at the University of
California, Irvine.  Bugs, patches, and suggestions should be reported
to schmidt at ics.uci.edu.

@item
Special thanks is extended to Michael Tiemann and Doug Lea, for
providing a useful compiler, and for giving me a forum to exhibit my
creation.

In addition, Adam de Boor and Nels Olson provided many tips and insights
that greatly helped improve the quality and functionality of @code{gperf}.
@end itemize

@node Motivation, Search Structures, Contributors, Top
@chapter Introduction

@code{gperf} is a perfect hash function generator written in C++.  It
transforms an @emph{n} element user-specified keyword set @emph{W} into
a perfect hash function @emph{F}.  @emph{F} uniquely maps keywords in
@emph{W} onto the range 0..@emph{k}, where @emph{k} >= @emph{n}.  If
@emph{k = n} then @emph{F} is a @emph{minimal} perfect hash function.
@code{gperf} generates a 0..@emph{k} element static lookup table and a
pair of C functions.  These functions determine whether a given
character string @emph{s} occurs in @emph{W}, using at most one probe
into the lookup table.

@code{gperf} currently generates the reserved keyword recognizer for
lexical analyzers in several production and research compilers and
language processing tools, including GNU C, GNU C++, GNU Pascal, GNU
Modula 3, and GNU indent.  Complete C++ source code for @code{gperf} is
available via anonymous ftp from ics.uci.edu.  @code{gperf} also is
distributed along with the GNU libg++ library.  A highly portable,
functionally equivalent K&R C version of @code{gperf} is archived in
comp.sources.unix, volume 20.  Finally, a paper describing
@code{gperf}'s design and implementation in greater detail is available
in the Second USENIX C++ Conference proceedings.

@node Search Structures, Description, Motivation, Top
@chapter Static search structures and GNU @code{gperf}

A @dfn{static search structure} is an Abstract Data Type with certain
fundamental operations, @emph{e.g.}, @emph{initialize}, @emph{insert},
and @emph{retrieve}.  Conceptually, all insertions occur before any
retrievals.  In practice, @code{gperf} generates a @code{static} array
containing search set keywords and any associated attributes specified
by the user.  Thus, there is essentially no execution-time cost for the
insertions.  It is a useful data structure for representing @emph{static
search sets}.  Static search sets occur frequently in software system
applications.  Typical static search sets include compiler reserved
words, assembler instruction opcodes, and built-in shell interpreter
commands.  Search set members, called @dfn{keywords}, are inserted into
the structure only once, usually during program initialization, and are
not generally modified at run-time.

Numerous static search structure implementations exist, @emph{e.g.},
arrays, linked lists, binary search trees, digital search tries, and
hash tables.  Different approaches offer trade-offs between space
utilization and search time efficiency.  For example, an @emph{n} element
sorted array is space efficient, though the average-case time
complexity for retrieval operations using binary search is
proportional to log @emph{n}.  Conversely, hash table implementations
often locate a table entry in constant time, but typically impose
additional memory overhead and exhibit poor worst case performance.


@emph{Minimal perfect hash functions} provide an optimal solution for a
particular class of static search sets.  A minimal perfect hash
function is defined by two properties:

@itemize @bullet
@item 
It allows keyword recognition in a static search set using at most
@emph{one} probe into the hash table.  This represents the ``perfect''
property.
@item 
The actual memory allocated to store the keywords is precisely large
enough for the keyword set, and @emph{no larger}.  This is the
``minimal'' property.
@end itemize

For most applications it is far easier to generate @emph{perfect} hash
functions than @emph{minimal perfect} hash functions.  Moreover,
non-minimal perfect hash functions frequently execute faster than
minimal ones in practice.  This phenomena occurs since searching a
sparse keyword table increases the probability of locating a ``null''
entry, thereby reducing string comparisons.  @code{gperf}'s default
behavior generates @emph{near-minimal} perfect hash functions for
keyword sets.  However, @code{gperf} provides many options that permit
user control over the degree of minimality and perfection.

Static search sets often exhibit relative stability over time.  For
example, Ada's 63 reserved words have remained constant for nearly a
decade.  It is therefore frequently worthwhile to expend concerted
effort building an optimal search structure @emph{once}, if it
subsequently receives heavy use multiple times.  @code{gperf} removes
the drudgery associated with constructing time- and space-efficient
search structures by hand.  It has proven a useful and practical tool
for serious programming projects.  Output from @code{gperf} is currently
used in several production and research compilers, including GNU C, GNU
C++, GNU Pascal, and GNU Modula 3.  The latter two compilers are not yet
part of the official GNU distr ibution.  Each compiler utilizes
@code{gperf} to automatically generate static search structures that
efficiently identify their respective reserved keywords.

@node Description, Options, Search Structures, Top
@chapter High-Level Description of GNU @code{gperf}

@menu
* Input Format::		Input Format to @code{gperf}
* Output Format::		Output Format for Generated C Code with @code{gperf}
@end menu

The perfect hash function generator @code{gperf} reads a set of
``keywords'' from a @dfn{keyfile} (or from the standard input by
default).  It attempts to derive a perfect hashing function that
recognizes a member of the @dfn{static keyword set} with at most a
single probe into the lookup table.  If @code{gperf} succeeds in
generating such a function it produces a pair of C source code routines
that perform hashing and table lookup recognition.  All generated C code
is directed to the standard output.  Command-line options described
below allow you to modify the input and output format to @code{gperf}.

By default, @code{gperf} attempts to produce time-efficient code, with
less emphasis on efficient space utilization.  However, several options
exist that permit trading-off execution time for storage space and vice
versa.  In particular, expanding the generated table size produces a
sparse search structure, generally yielding faster searches.
Conversely, you can direct @code{gperf} to utilize a C @code{switch}
statement scheme that minimizes data space storage size.  Furthermore,
using a C @code{switch} may actually speed up the keyword retrieval time
somewhat.  Actual results depend on your C compiler, of course.

In general, @code{gperf} assigns values to the characters it is using
for hashing until some set of values gives each keyword a unique value.
A helpful heuristic is that the larger the hash value range, the easier
it is for @code{gperf} to find and generate a perfect hash function.
Experimentation is the key to getting the most from @code{gperf}.

@node Input Format, Output Format, Description, Description
@section Input Format to @code{gperf}

You can control the input keyfile format by varying certain command-line
arguments, in particular the @samp{-t} option.  The input's appearance
is similar to GNU utilities @code{flex} and @code{bison} (or UNIX
utilities @code{lex} and @code{yacc}).  Here's an outline of the general
format:

@group
@example
declarations
%%
keywords
%%
functions
@end example
@end group

@emph{Unlike} @code{flex} or @code{bison}, all sections of @code{gperf}'s input
are optional.  The following sections describe the input format for each
section.

@menu
* Declarations::		@code{struct} Declarations and C Code Inclusion.
* Keywords::			Format for Keyword Entries.
* Functions::			Including Additional C Functions.
@end menu

@node Declarations, Keywords, Input Format, Input Format
@subsection @code{struct} Declarations and C Code Inclusion