path: root/gperf.texinfo

\input texinfo  @c -*-texinfo-*-
@settitle User's Guide to @code{gperf}
@setfilename gperf.info

@ifinfo
This file documents the features of the GNU Perfect Hash Function Generator

Copyright (C) 1989 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through @TeX{} and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
section entitled ``GNU General Public License'' is included exactly as
in the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the section entitled ``GNU @code{gperf} General Public License'' and
this permission notice may be included in translations approved by the
Free Software Foundation instead of in the original English.
@end ifinfo

@setchapternewpage odd

@titlepage
@center @titlefont{User's Guide}
@sp 2
@center @titlefont{for the}
@sp 2
@center @titlefont{GNU @code{gperf} Utility}
@sp 4
@center Douglas C. Schmidt
@sp 3
@center last updated 1 November 1989
@sp 1
@center for version 2.0
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1989 Free Software Foundation, Inc.


Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
section entitled ``GNU @code{gperf} General Public License'' is included exactly as
in the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that the section entitled ``GNU @code{gperf} General Public License'' may be
included in a translation approved by the author instead of in the original
English.
@end titlepage

@ifinfo
@node Top, Copying, , (DIR)
@ichapter Introduction

This manual documents the GNU @code{gperf} perfect hash function generator
utility, focusing on its features and how to use them, and how to report
bugs.

@end ifinfo
@menu
* Copying::         GNU @code{gperf} General Public License says
                    how you can copy and share @code{gperf}.
* Contributors::    People who have contributed to @code{gperf}.
* Motivation::      Introduction to @code{gperf}.
* Search Structures:: Static search structures and GNU GPERF.
* Description::     High-level discussion of how GPERF functions.
* Options::         A description of options to the program.
* Bugs::            Known bugs and limitations with GPERF.
* Projects::        Things still left to do.
* Implementation::  Implementation Details for GNU GPERF.
* Bibliography::    Material Referenced in this Report.
@end menu

@node Copying, Contributors, Top, Top
@unnumbered GNU GENERAL PUBLIC LICENSE
@center Version 1, February 1989

@display
Copyright @copyright{} 1989 Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display

@unnumberedsec Preamble

  The license agreements of most software companies try to keep users
at the mercy of those companies.  By contrast, our General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users.  The
General Public License applies to the Free Software Foundation's
software and to any other program whose authors commit to using it.
You can use it for your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Specifically, the General Public License is designed to make
sure that you have the freedom to give away or sell copies of free
software, that you receive source code or can get it if you want it,
that you can change the software or use pieces of it in new free
programs; and that you know you can do these things.

  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

  For example, if you distribute copies of a such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must tell them their rights.

  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

  The precise terms and conditions for copying, distribution and
modification follow.

@iftex
@unnumberedsec TERMS AND CONDITIONS
@end iftex
@ifinfo
@center TERMS AND CONDITIONS
@end ifinfo

@enumerate
@item
This License Agreement applies to any program or other work which
contains a notice placed by the copyright holder saying it may be
distributed under the terms of this General Public License.  The
``Program'', below, refers to any such program or work, and a ``work based
on the Program'' means either the Program or any work containing the
Program or a portion of it, either verbatim or with modifications.  Each
licensee is addressed as ``you''.

@item
You may copy and distribute verbatim copies of the Program's source
code as you receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice and
disclaimer of warranty; keep intact all the notices that refer to this
General Public License and to the absence of any warranty; and give any
other recipients of the Program a copy of this General Public License
along with the Program.  You may charge a fee for the physical act of
transferring a copy.

@item
You may modify your copy or copies of the Program or any portion of
it, and copy and distribute such modifications under the terms of Paragraph
1 above, provided that you also do the following:

@itemize @bullet
@item
cause the modified files to carry prominent notices stating that
you changed the files and the date of any change; and

@item
cause the whole of any work that you distribute or publish, that
in whole or in part contains the Program or any part thereof, either
with or without modifications, to be licensed at no charge to all
third parties under the terms of this General Public License (except
that you may choose to grant warranty protection to some or all
third parties, at your option).

@item
If the modified program normally reads commands interactively when
run, you must cause it, when started running for such interactive use
in the simplest and most usual way, to print or display an
announcement including an appropriate copyright notice and a notice
that there is no warranty (or else, saying that you provide a
warranty) and that users may redistribute the program under these
conditions, and telling the user how to view a copy of this General
Public License.

@item
You may charge a fee for the physical act of transferring a
copy, and you may at your option offer warranty protection in
exchange for a fee.
@end itemize

Mere aggregation of another independent work with the Program (or its
derivative) on a volume of a storage or distribution medium does not bring
the other work under the scope of these terms.

@item
You may copy and distribute the Program (or a portion or derivative of
it, under Paragraph 2) in object code or executable form under the terms of
Paragraphs 1 and 2 above provided that you also do one of the following:

@itemize @bullet
@item
accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of
Paragraphs 1 and 2 above; or,

@item
accompany it with a written offer, valid for at least three
years, to give any third party free (except for a nominal charge
for the cost of distribution) a complete machine-readable copy of the
corresponding source code, to be distributed under the terms of
Paragraphs 1 and 2 above; or,

@item
accompany it with the information you received as to where the
corresponding source code may be obtained.  (This alternative is
allowed only for noncommercial distribution and only if you
received the program in object code or executable form alone.)
@end itemize

Source code for a work means the preferred form of the work for making
modifications to it.  For an executable file, complete source code means
all the source code for all modules it contains; but, as a special
exception, it need not include source code for modules which are standard
libraries that accompany the operating system on which the executable
file runs, or for standard header files or definitions files that
accompany that operating system.

@item
You may not copy, modify, sublicense, distribute or transfer the
Program except as expressly provided under this General Public License.
Any attempt otherwise to copy, modify, sublicense, distribute or transfer
the Program is void, and will automatically terminate your rights to use
the Program under this License.  However, parties who have received
copies, or rights to use copies, from you under this General Public
License will not have their licenses terminated so long as such parties
remain in full compliance.

@item
By copying, distributing or modifying the Program (or any work based
on the Program) you indicate your acceptance of this license to do so,
and all its terms and conditions.

@item
Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the original
licensor to copy, distribute or modify the Program subject to these
terms and conditions.  You may not impose any further restrictions on the
recipients' exercise of the rights granted herein.

@item
The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
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 @code{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 @code{schmidt@@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 @code{ics.uci.edu}.  @code{gperf} also is
distributed along with the GNU libg++ library.  Finally, a highly
portable, functionally equivalent K&R C version of @code{gperf} is
archived in @code{comp.sources.unix}, volume 20.

@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.@footnote{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 $n$ element
sorted array is space efficient, though the average-case time
complexity for retrieval operations using binary search is
proportional to $\log n$.  Conversely, hash table implementations
often locate a table entry in constant time, but typically impose
additional memory overhead and exhibit poor worst case performance
@cite{aho, etc.}.

@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 @cite{many bozos}.
Moreover, non-minimal perfect hash functions frequently execute faster
than minimal ones in practice @cite{cichelli}.  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.  @footnote{The latter two
compilers are not yet part of the official GNU distribution.} 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, Declarations, 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

The keyword input file optionally contains a section for including
arbitrary C declarations and definitions, as well as provisions for
providing a user-supplied @code{struct}.  If the @samp{-t} option
@emph{is} enabled, you @emph{must} provide a C @code{struct} as the last
component in the declaration section from the keyfile file.  The first
field in this struct must be a @code{char *} identifier called ``name,''
although it is possible to modify this field's name with the @samp{-K}
option described below.

Here is simple example, using months of the year and their attributes as
input:

@group
@example
struct months @{ char *name; int number; int days; int leap_days; @};
%%
january,   1, 31, 31
february,  2, 28, 29
march,     3, 31, 31
april,     4, 30, 30
may,       5, 31, 31
june,      6, 30, 30
july,      7, 31, 31
august,    8, 31, 31
september, 9, 30, 30
october,  10, 31, 31
november, 11, 30, 30
december, 12, 31, 31
@end example
@end group

Separating the @code{struct} declaration from the list of key words and
other fields are a pair of consecutive percent signs, @code{%%},
appearing left justified in the first column, as in the UNIX utility
@code{lex}.

Using a syntax similar to GNU utilities @code{flex} and @code{bison}, it
is possible to directly include C source text and comments verbatim into
the generated output file.  This is accomplished by enclosing the region
inside left-justified surrounding @code{%@{}, @code{%@}} pairs.  Here is
an input fragment based on the previous example that illustrates this
feature:

@group
@example
%@{
#include <assert.h>
/* This section of code is inserted directly into the output. */
int return_month_days (struct months *months, int is_leap_year);
%@}
struct months @{ char *name; int number; int days; int leap_days; @};
%%
january,   1, 31, 31
february,  2, 28, 29
march,     3, 31, 31
...
@end example
@end group

It is possible to omit the declaration section entirely.  In this case
the keyfile begins directly with the first keyword line, @emph{e.g.}:

@group
@example
january,   1, 31, 31
february,  2, 28, 29
march,     3, 31, 31
april,     4, 30, 30
...
@end example
@end group

@node Keywords, Functions, Declarations, Input Format
@subsection Format for Keyword Entries

The second keyfile format section contains lines of keywords and any
associated attributes you might supply.  A line beginning with @samp{#}
in the first column is considered a comment.  Everything following the
@samp{#} is ignored, up to and including the following newline.

The first field of each non-comment line is always the key itself.  It
should be given as a simple name, @emph{i.e.}, without surrounding
string quotation marks, and be left-justified flush against the first
column.  In this context, a ``field'' is considered to extend up to, but
not include, the first blank, comma, or newline.  Here is a simple
example taken from a partial list of C reserved words:

@group
@example
# These are a few C reserved words, see the c.@code{gperf} file 
# for a complete list of ANSI C reserved words.
unsigned
sizeof
switch
signed
if
default
for
while
return
@end example
@end group

Note that unlike @code{flex} or @code{bison} the first @code{%%} marker
may be elided if the declaration section is empty.

Additional fields may optionally follow the leading keyword.  Fields
should be separated by commas, and terminate at the end of line.  What
these fields mean is entirely up to you; they are used to initialize the
elements of the user-defined @code{struct} provided by you in the
declaration section.  If the @samp{-t} option is @emph{not} enabled
these fields are simply ignored.  All previous examples except the last
one contain keyword attributes.

@node Functions, Output Format, Keywords, Input Format
@subsection Including Additional C Functions

The optional third section also corresponds closely with conventions
found in @code{flex} and @code{bison}.  All text in this section,
starting at the final @code{%%} and extending to the end of the input
file, is included verbatim into the generated output file.  Naturally,
it is your responsibility to ensure that the code contained in this
section is valid C.

@node Output Format, , Functions, Description
@section Output Format for Generated C Code with @code{gperf}

Several options control how the generated C code appears on the standard
output.  Two C function are generated.  They are called @code{hash} and
@code{in_word_set}, although you may modify the name for
@code{in_word_set} with a command-line option.  Both functions require
two arguments, a string, @code{char *} @var{str}, and a length
parameter, @code{int} @var{len}.  Their default function prototypes are
as follows:

@group
@example
static int hash (char *str, int len);
int in_word_set (char *str, int len);
@end example
@end group

By default, the generated @code{hash} function returns an integer value
created by adding @var{len} to several user-specified @var{str} key
positions indexed into an @dfn{associated values} table stored in a
local static array.  The associated values table is constructed
internally by @code{gperf} and later output as a static local C array called
@var{hash_table}; its meaning and properties are described below.
@xref{Implementation}. The relevant key positions are specified via the
@samp{-k} option when running @code{gperf}, as detailed in the @emph{Options}
section below. @xref{Options}.

Two options, @samp{-g} (assume you are compiling with GNU C and its
@code{inline} feature) and @samp{-a} (assume ANSI C-style function
prototypes), alter the content of both the generated @code{hash} and
@code{in_word_set} routines.  However, function @code{in_word_set} may
be modified more extensively, in response to your option settings.  The
options that affect the @code{in_word_set} structure are:

@itemize @bullet
@table @samp
@item -p
Have function @code{in_word_set} return a pointer rather than a boolean.

@item -t
Make use of the user-defined @code{struct}.

@item -S @var{total switch statements}
Generate 1 or more C @code{switch} statement rather than use a large,
(and potentially sparse) static array.  Although the exact time and
space savings of this approach vary according to your C compiler's
degree of optimization, this method often results in smaller and faster
code.
@end table
@end itemize

If the @samp{-t}, @samp{-S}, and @samp{-p} options are omitted the
default action is to generate a @code{char *} array containing the keys,
together with additional null strings used for padding the array.  By
experimenting with the various input and output options, and timing the
resulting C code, you can determine the best option choices for
different keyword set characteristics.

@node Options, Bugs, Description, Top
@chapter Options to the @code{gperf} Utility

There are @emph{many} options to @code{gperf}.  They were added to make
the program more convenient for use with real applications.  ``On-line''
help is readily available via the @samp{-h} option.  Other options
include:

@itemize @bullet
@table @samp
@item -a
Generate ANSI Standard C code using function prototypes.  The default is
to use ``classic'' K&R C function declaration syntax.

@item -c
Generates C code that uses the @code{strncmp} function to perform
string comparisons.  The default action is to use @code{strcmp}.

@item -C
Makes the contents of all generated lookup tables constant, @emph{i.e.},
``readonly.''  Many compilers can generate more efficient code for this
by putting the tables in readonly memory.

@item -d
Enables the debugging option.  This produces verbose diagnostics to
``standard error'' when @code{gperf} is executing.  It is useful both for
maintaining the program and for determining whether a given set of
options is actually speeding up the search for a solution.  Some useful
information is dumped at the end of the program when the @samp{-d}
option is enabled.

@item -D
Handle keywords whose key position sets hash to duplicate values.
Duplicate hash values occur for two reasons:

@itemize @bullet
@item
Since @code{gperf} does not backtrack it is possible for it to process
all your input keywords without finding a unique mapping for each word.
However, frequently only a very small number of duplicates occur, and 
the majority of keys still require one probe into the table.
@item
Sometimes a set of keys may have the same names, but possess different
attributes.  With the -D option @code{gperf} treats all these keys as part of
an equivalence class and generates a perfect hash function with multiple
comparisons for duplicate keys.  It is up to you to completely
disambiguate the keywords by modifying the generated C code.  However,
@code{gperf} helps you out by organizing the output.
@end itemize

Option @samp{-D} is extremely useful for certain large or highly
redundant keyword sets, @emph{i.e.}, assembler instruction opcodes.
Using this option usually means that the generated hash function is no
longer perfect.  On the other hand, it permits @code{gperf} to work on keyword
sets that it otherwise could not handle.

@item -e @var{keyword delimiter list}
Allows the user to provide a string containing delimiters used to
separate keywords from their attributes.  The default is ",\n".  This
option is essential if you want to use keywords that have embedded
commas or newlines.  One useful trick is to use -e'TAB', where TAB is
the literal tab character.

@item -f @var{iteration amount}
Generate the perfect hash function ``fast.''  This decreases @code{gperf}'s
running time at the cost of minimizing generated table-size.  The
iteration amount represents the number of times to iterate when
resolving a collision.  `0' means `iterate by the number of keywords.
This option is probably most useful when used in conjunction with options
@samp{-D} and/or @samp{-S} for @emph{large} keyword sets.

@item -g
Assume a GNU compiler, @emph{e.g.}, @code{g++} or @code{gcc}.  This
makes all generated routines use the ``inline'' keyword to remove the
cost of function calls.  Note that @samp{-g} does @emph{not} imply
@samp{-a}, since other non-ANSI C compilers may have provisions for a
function @code{inline} feature.

@item -G
Generate the static table of keywords as a static global variable,
rather than hiding it inside of the lookup function (which is the
default behavior).

@item -h
Prints a short summary on the meaning of each program option.  Aborts
further program execution.

@item -H @var{hash function name}
Allows you to specify the name for the generated hash function.  Default
name is `hash.'  This option permits the use of two hash tables in the
same file.

@item -i @var{initial value}
Provides an initial @var{value} for the associate values array.  Default
is 0.  Increasing the initial value helps inflate the final table size,
possibly leading to more time efficient keyword lookups.  Note that this
option is not particularly useful when @samp{-S} is used.  Also,
@samp{-i} is overriden when the @samp{-r} option is used.

@item -j @var{jump value}
Affects the ``jump value,'' @emph{i.e.}, how far to advance the
associated character value upon collisions.  @var{Jump value} is rounded
up to an odd number, the default is 5.  If the @var{jump value} is 0 @code{gperf}
jumps by random amounts.

@item -k @var{keys}
Allows selection of the character key positions used in the keywords'
hash function. The allowable choices range between 1-126, inclusive.
The positions are separated by commas, @emph{e.g.}, @samp{-k 9,4,13,14};
ranges may be used, @emph{e.g.}, @samp{-k 2-7}; and positions may occur
in any order.  Furthermore, the meta-character '*' causes the generated
hash function to consider @strong{all} character positions in each key,
whereas '$' instructs the hash function to use the ``final character''
of a key (this is the only way to use a character position greater than
126, incidentally).

For instance, the option @samp{-k 1,2,4,6-10,'$'} generates a hash
function that considers positions 1,2,4,6,7,8,9,10, plus the last
character in each key (which may differ for each key, obviously).  Keys
with length less than the indicated key positions work properly, since
selected key positions exceeding the key length are simply not
referenced in the hash function.

@item -K @var{key name}
By default, the program assumes the structure component identifier for
the keyword is ``name.''  This option allows an arbitrary choice of
identifier for this component, although it still must occur as the first
field in your supplied @code{struct}.

@item -l
Compare key lengths before trying a string comparison.  This might cut
down on the number of string comparisons made during the lookup, since
keys with different lengths are never compared via @code{strcmp}.
However, using @samp{-l} might greatly increase the size of the
generated C code if the lookup table range is large (which implies that
the switch option @samp{-S} is not enabled), since the length table
contains as many elements as there are entries in the lookup table.

@item -n
Instructs the generator not to include the length of a keyword when
computing its hash value.  This may save a few assembly instructions in
the generated lookup table.

@item -N @var{lookup function name}
Allows you to specify the name for the generated lookup function.
Default name is `in_word_set.'  This option permits completely automatic
generation of perfect hash functions, especially when multiple generated
hash functions are used in the same application.

@item -o
Reorders the keywords by sorting the keywords so that frequently
occuring key position set components appear first.  A second reordering
pass follows so that keys with ``already determined values'' are placed
towards the front of the keylist.  This may decrease the time required
to generate a perfect hash function for many keyword sets, and also
produce more minimal perfect hash functions.  The reason for this is
that the reordering helps prune the search time by handling inevitable
collisions early in the search process.  On the other hand, if the
number of keywords is @emph{very} large using @samp{-o} may
@emph{increase} @code{gperf}'s execution time, since collisions will begin
earlier and continue throughout the remainder of keyword processing.
See Cichelli's paper from the January 1980 Communications of the ACM for
details.

@item -p
Changes the return value of the generated function @code{in_word_set}
from boolean (@emph{i.e.}, 0 or 1), to either type ``pointer to
user-defined struct,'' (if the @samp{-t} option is enabled), or simply
to @code{char *}, if @samp{-t} is not enabled.  This option is most
useful when the @samp{-t} option (allowing user-defined structs) is
used.  For example, it is possible to automatically generate the GNU C
reserved word lookup routine with the options @samp{-p} and @samp{-t}.

@item -r
Utilizes randomness to initialize the associated values table.  This
frequently generates solutions faster than using deterministic
initialization (which starts all associated values at 0).  Furthermore,
using the randomization option generally increases the size of the
table.  If @code{gperf} has difficultly with a certain keyword set try using
@samp{-r} or @samp{-D}.

@item -s @var{size-multiple}
Affects the size of the generated hash table.  The numeric argument for
this option indicates ``how many times larger'' the maximum associated
value range should be, in relationship to the number of keys.  For
example, a value of 3 means ``allow the maximum associated value to be
about 3 times larger than the number of input keys.''  If option
@samp{-S} is @emph{not} enabled, the maximum associated value influences
the static array table size, and a larger table should decrease the time
required for an unsuccessful search, at the expense of extra table
space.

The default value is 1, thus the default maximum associated value about
the same size as the number of keys ( for efficiency, the maximum
associated value is always rounded up to a power of 2).  The actual
table size may vary somewhat, since this technique is essentially a
heuristic.  In particular, setting this value too high slows down
@code{gperf}'s runtime, since it must search through a much larger range of
values.  Judicious use of the @samp{-f} option helps alleviate this
overhead, however.

@item -S @var{total switch statements}
Causes the generated C code to use a @code{switch} statement scheme,
rather than an array lookup table.  This can lead to a reduction in both
time and space requirements for some keyfiles.  The argument to this
option determines how many @code{switch} statements are generated. A
value of 1 generates 1 @code{switch} containing all the elements, a
value of 2 generates 2 tables with 1/2 the elements in each
@code{switch}, etc.  This is useful since many C compilers cannot
correctly generate code for large @code{switch} statements. This option
was inspired in part by Keith Bostic's original C program.

@item -t
Allows you to include a @code{struct} type declaration for generated
code.  Any text before a pair of consecutive %% is consider part of the
type declaration.  Key words and additional fields may follow this, one
group of fields per line.  A set of examples for generating perfect hash
tables and functions for Ada, C, and G++, Pascal, and Modula 2 and 3
reserved words are distributed with this release.

@item -T
Prevents the transfer of the type declaration to the output file.  Use
this option if the type is already defined elsewhere.

@item -v
Prints out the current version number.
@end table
@end itemize

@node Bugs, Projects, Options, Top
@chapter Known Bugs and Limitations with @code{gperf}

The following are some limitations with the current release of
@code{gperf}:

@itemize @bullet
@item
The @code{gperf} utility is tuned to execute quickly, and works quickly
for small to medium size data sets (around 1000 keywords).  It is
extremely useful for maintaining perfect hash functions for compiler
keyword sets.  Several recent enhancements now enable @code{gperf} to
work efficiently on much larger keyword sets (over 15,000 keywords).
When processing large keyword sets it helps greatly to have over 8 megs
of RAM.

However, since @code{gperf} does not backtrack no guaranteed solution
occurs on every run.  On the other hand, it is usually easy to obtain a
solution by varying the option parameters.  In particular, try the
@samp{-r} option, and also try changing the default arguments to the
@samp{-s} and @samp{-j} options.  To @emph{guarantee} a solution, use
the @samp{-D} and @samp{-S} options, although the final results are not
likely to be a @emph{perfect} hash function anymore!  Finally, use the
@samp{-f} option if you want @code{gperf} to generate the perfect hash
function @emph{fast}, with less emphasis on making it minimal.

@item 
The size of the generate static keyword array can get @emph{extremely}
large if the input keyword file is large or if the keywords are quite
similar.  This tends to slow down the compilation of the generated C
code, and @emph{greatly} inflates the object code size.  If this
situation occurs, consider using the @samp{-S} option to reduce data
size, potentially increasing keyword recognition time a negligible
amount.  Since many C compilers cannot correctly generated code for
large switch statements it is important to qualify the @var{-S} option
with an appropriate numerical argument that controls the number of
switch statements generated.

@item 
The maximum number of key positions selected for a given key has an
arbitrary limit of 126.  This restriction should be removed, and if
anyone considers this a problem write me and let me know so I can remove
the constraint.

@item
The C++ source code only compiles correctly with GNU G++, version 1.36
(and hopefully later versions).  Porting to AT&T cfront would be
tedious, but possible (and desirable).  There is also a K&R C version
available now.  This should compile without change on most BSD systems,
but may require a bit of work to run on SYSV, since @code{gperf} uses
@var{alloca} in several places.  Send mail to @code{schmidt@@ics.uci.edu} for
information.
@end itemize

@node Projects, Implementation, Bugs, Top
@chapter Things Still Left to Do

It should be ``relatively'' easy to replace the current perfect hash
function algorithm with a more exhaustive approach; the perfect hash
module is essential independent from other program modules.  Additional
worthwhile improvements include:

@itemize @bullet
@item 
Make the algorithm more robust.  At present, the program halts with an
error diagnostic if it can't find a direct solution and the @samp{-D}
option is not enabled.  A more comprehensive, albeit computationally
expensive, approach would employ backtracking or enable alternative
options and retry.  It's not clear how helpful this would be, in
general, since most search sets are rather small in practice.

@item 
Another useful extension involves modifying the program to generate
``minimal'' perfect hash functions (under certain circumstances, the
current version can be rather extravagant in the generated table size).
Again, this is mostly of theoretical interest, since a sparse table
often produces faster lookups, and use of the @samp{-S} @code{switch}
option can minimize the data size, at the expense of slightly longer
lookups (note that the gcc compiler generally produces good code for
@code{switch} statements, reducing the need for more complex schemes).

@item
In addition to improving the algorithm, it would also be useful to
generate a C++ class or Ada package as the code output, in addition to
the current C routines.
@end itemize

@node Implementation, Bibliography, Projects, Top
@chapter Implementation Details of GNU @code{gperf}

A paper describing the high-level description of the data structures and
algorithms used to implement @code{gperf} will soon be available.  This
paper is useful not only from a maintenance and enhancement perspective,
but also because they demonstrate several clever and useful programming
techniques, @emph{e.g.}, `Iteration Number' boolean arrays, double
hashing, a ``safe'' and efficient method for reading arbitrarily long
input from a file, and a provably optimal algorithm for simultaneously
determining both the minimum and maximum elements in a list.

@page

@node Bibliography, , Implementation, Top
@chapter Bibliography

[1] Chang, C.C.: @i{A Scheme for Constructing Ordered Minimal Perfect
Hashing Functions} Information Sciences 39(1986), 187-195.
       
[2] Cichelli, Richard J. @i{Author's Response to ``On Cichelli's Minimal Perfect Hash
Functions Method''} Communications of the ACM, 23, 12(December 1980), 729.
    
[3] Cichelli, Richard J. @i{Minimal Perfect Hash Functions Made Simple}
Communications of the ACM, 23, 1(January 1980), 17-19.
           
[4] Cook, C. R. and Oldehoeft, R.R. @i{A Letter Oriented Minimal
Perfect Hashing Function} SIGPLAN Notices, 17, 9(September 1982), 18-27.

[5] Cormack, G. V. and Horspool, R. N. S. and Kaiserwerth, M.
@i{Practical Perfect Hashing} Computer Journal, 28, 1(January 1985), 54-58.
    
[6] Jaeschke, G. @i{Reciprocal Hashing: A Method for Generating Minimal
Perfect Hashing Functions} Communications of the ACM, 24, 12(December
1981), 829-833.

[7] Jaeschke, G. and Osterburg, G. @i{On Cichelli's Minimal Perfect
Hash Functions Method} Communications of the ACM, 23, 12(December 1980),
728-729.

[8] Sager, Thomas J. @i{A Polynomial Time Generator for Minimal Perfect
Hash Functions} Communications of the ACM, 28, 5(December 1985), 523-532

[9] Sebesta, R.W. and Taylor, M.A. @i{Minimal Perfect Hash Functions
for Reserved Word Lists}  SIGPLAN Notices, 20, 12(September 1985), 47-53.
@contents

[10] Sprugnoli, R. @i{Perfect Hashing Functions: A Single Probe
Retrieving Method for Static Sets} Communications of the ACM, 20
11(November 1977), 841-850.

[11] Stallman, Richard M. @i{Using and Porting GNU CC} Free Software Foundation,
1988.

[12] Stroustrup, Bjarne @i{The C++ Programming Language.} Addison-Wesley, 1986.

[13] Tiemann, Michael D. @i{User's Guide to GNU C++} Free Software
Foundation, 1989.
@bye