home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Languguage OS 2
/
Languguage OS II Version 10-94 (Knowledge Media)(1994).ISO
/
gnu
/
libg_261.zip
/
libg_261
/
libg++
/
gperf
/
gperf.info
(
.txt
)
< prev
next >
Wrap
GNU Info File
|
1994-10-24
|
52KB
|
925 lines
This is Info file gperf.info, produced by Makeinfo-1.55 from the input
file ./gperf.texi.
START-INFO-DIR-ENTRY
* Gperf: (gperf). Perfect Hash Function Generator.
END-INFO-DIR-ENTRY
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.
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 `gperf' General Public
License" an d this permission notice may be included in translations
approved by the Free Software Foundation instead of in the original
English.
File: gperf.info, Node: Top, Next: Copying, Prev: (dir), Up: (dir)
Introduction
************
This manual documents the GNU `gperf' perfect hash function generator
utility, focusing on its features and how to use them, and how to report
bugs.
* Menu:
* Copying:: GNU `gperf' General Public License says
how you can copy and share `gperf'.
* Contributors:: People who have contributed to `gperf'.
* Motivation:: Static search structures and GNU 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.
-- The Detailed Node Listing --
High-Level Description of GNU `gperf'
* Input Format:: Input Format to `gperf'
* Output Format:: Output Format for Generated C Code with `gperf'
Input Format to `gperf'
* Declarations:: `struct' Declarations and C Code Inclusion.
* Keywords:: Format for Keyword Entries.
* Functions:: Including Additional C Functions.
File: gperf.info, Node: Copying, Next: Contributors, Prev: Top, Up: Top
GNU GENERAL PUBLIC LICENSE
**************************
Version 1, February 1989
Copyright (C) 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.
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.
TERMS AND CONDITIONS
1. 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".
2. 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.
3. 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:
* cause the modified files to carry prominent notices stating
that you changed the files and the date of any change; and
* 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).
* 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.
* 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.
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.
4. 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:
* accompany it with the complete corresponding machine-readable
source code, which must be distributed under the terms of
Paragraphs 1 and 2 above; or,
* 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,
* 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.)
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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
NO WARRANTY
10. 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.
11. 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 OF TERMS AND CONDITIONS
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.
ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
Copyright (C) 19YY 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.
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:
Gnomovision version 69, Copyright (C) 19YY 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.
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:
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.
SIGNATURE OF TY COON, 1 April 1989
Ty Coon, President of Vice
That's all there is to it!
File: gperf.info, Node: Contributors, Next: Motivation, Prev: Copying, Up: Top
Contributors to GNU `gperf' Utility
***********************************
* The GNU `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.
* 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 `gperf'.
File: gperf.info, Node: Motivation, Next: Search Structures, Prev: Contributors, Up: Top
Introduction
************
`gperf' is a perfect hash function generator written in C++. It
transforms an *n* element user-specified keyword set *W* into a perfect
hash function *F*. *F* uniquely maps keywords in *W* onto the range
0..*k*, where *k* >= *n*. If *k = n* then *F* is a *minimal* perfect
hash function. `gperf' generates a 0..*k* element static lookup table
and a pair of C functions. These functions determine whether a given
character string *s* occurs in *W*, using at most one probe into the
lookup table.
`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 `gperf' is
available via anonymous ftp from ics.uci.edu. `gperf' also is
distributed along with the GNU libg++ library. A highly portable,
functionally equivalent K&R C version of `gperf' is archived in
comp.sources.unix, volume 20. Finally, a paper describing `gperf''s
design and implementation in greater detail is available in the Second
USENIX C++ Conference proceedings.
File: gperf.info, Node: Search Structures, Next: Description, Prev: Motivation, Up: Top
Static search structures and GNU `gperf'
****************************************
A "static search structure" is an Abstract Data Type with certain
fundamental operations, *e.g.*, *initialize*, *insert*, and *retrieve*.
Conceptually, all insertions occur before any retrievals. In
practice, `gperf' generates a `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 *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 "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, *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.
*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:
* It allows keyword recognition in a static search set using at most
*one* probe into the hash table. This represents the "perfect"
property.
* The actual memory allocated to store the keywords is precisely
large enough for the keyword set, and *no larger*. This is the
"minimal" property.
For most applications it is far easier to generate *perfect* hash
functions than *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. `gperf''s default behavior generates
*near-minimal* perfect hash functions for keyword sets. However,
`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 *once*, if it subsequently
receives heavy use multiple times. `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 `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 distribution. Each compiler utilizes `gperf'
to automatically generate static search structures that efficiently
identify their respective reserved keywords.
File: gperf.info, Node: Description, Next: Options, Prev: Search Structures, Up: Top
High-Level Description of GNU `gperf'
*************************************
* Menu:
* Input Format:: Input Format to `gperf'
* Output Format:: Output Format for Generated C Code with `gperf'
The perfect hash function generator `gperf' reads a set of
"keywords" from a "keyfile" (or from the standard input by default).
It attempts to derive a perfect hashing function that recognizes a
member of the "static keyword set" with at most a single probe into the
lookup table. If `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 `gperf'.
By default, `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 `gperf' to utilize a C `switch' statement
scheme that minimizes data space storage size. Furthermore, using a C
`switch' may actually speed up the keyword retrieval time somewhat.
Actual results depend on your C compiler, of course.
In general, `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 `gperf' to find and generate a perfect hash function.
Experimentation is the key to getting the most from `gperf'.
File: gperf.info, Node: Input Format, Next: Output Format, Prev: Description, Up: Description
Input Format to `gperf'
=======================
You can control the input keyfile format by varying certain
command-line arguments, in particular the `-t' option. The input's
appearance is similar to GNU utilities `flex' and `bison' (or UNIX
utilities `lex' and `yacc'). Here's an outline of the general format:
declarations
%%
keywords
%%
functions
*Unlike* `flex' or `bison', all sections of `gperf''s input are
optional. The following sections describe the input format for each
section.
* Menu:
* Declarations:: `struct' Declarations and C Code Inclusion.
* Keywords:: Format for Keyword Entries.
* Functions:: Including Additional C Functions.
File: gperf.info, Node: Declarations, Next: Keywords, Prev: Input Format, Up: Input Format
`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 `struct'. If the `-t' option *is* enabled,
you *must* provide a C `struct' as the last component in the
declaration section from the keyfile file. The first field in this
struct must be a `char *' identifier called "name," although it is
possible to modify this field's name with the `-K' option described
below.
Here is simple example, using months of the year and their
attributes as input:
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
Separating the `struct' declaration from the list of key words and
other fields are a pair of consecutive percent signs, `%%', appearing
left justified in the first column, as in the UNIX utility `lex'.
Using a syntax similar to GNU utilities `flex' and `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 `%{', `%}' pairs. Here is an input
fragment based on the previous example that illustrates this feature:
%{
#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
...
It is possible to omit the declaration section entirely. In this
case the keyfile begins directly with the first keyword line, *e.g.*:
january, 1, 31, 31
february, 2, 28, 29
march, 3, 31, 31
april, 4, 30, 30
...
File: gperf.info, Node: Keywords, Next: Functions, Prev: Declarations, Up: Input Format
Format for Keyword Entries
--------------------------
The second keyfile format section contains lines of keywords and any
associated attributes you might supply. A line beginning with `#' in
the first column is considered a comment. Everything following the `#'
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, *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:
# These are a few C reserved words, see the c.`gperf' file
# for a complete list of ANSI C reserved words.
unsigned
sizeof
switch
signed
if
default
for
while
return
Note that unlike `flex' or `bison' the first `%%' 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 `struct' provided by you in the
declaration section. If the `-t' option is *not* enabled these fields
are simply ignored. All previous examples except the last one contain
keyword attributes.
File: gperf.info, Node: Functions, Prev: Keywords, Up: Input Format
Including Additional C Functions
--------------------------------
The optional third section also corresponds closely with conventions
found in `flex' and `bison'. All text in this section, starting at the
final `%%' 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.
File: gperf.info, Node: Output Format, Prev: Input Format, Up: Description
Output Format for Generated C Code with `gperf'
===============================================
Several options control how the generated C code appears on the
standard output. Two C function are generated. They are called `hash'
and `in_word_set', although you may modify the name for `in_word_set'
with a command-line option. Both functions require two arguments, a
string, `char *' STR, and a length parameter, `int' LEN. Their default
function prototypes are as follows:
static int hash (char *str, int len);
int in_word_set (char *str, int len);
By default, the generated `hash' function returns an integer value
created by adding LEN to several user-specified STR key positions
indexed into an "associated values" table stored in a local static
array. The associated values table is constructed internally by
`gperf' and later output as a static local C array called HASH_TABLE;
its meaning and properties are described below. *Note
Implementation::. The relevant key positions are specified via the `-k'
option when running `gperf', as detailed in the *Options* section
below. *Note Options::.
Two options, `-g' (assume you are compiling with GNU C and its
`inline' feature) and `-a' (assume ANSI C-style function prototypes),
alter the content of both the generated `hash' and `in_word_set'
routines. However, function `in_word_set' may be modified more
extensively, in response to your option settings. The options that
affect the `in_word_set' structure are:
`-p'
Have function `in_word_set' return a pointer rather than a
boolean.
`-t'
Make use of the user-defined `struct'.
`-S TOTAL SWITCH STATEMENTS'
Generate 1 or more C `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.
If the `-t', `-S', and `-p' options are omitted the default action
is to generate a `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.
File: gperf.info, Node: Options, Next: Bugs, Prev: Description, Up: Top
Options to the `gperf' Utility
******************************
There are *many* options to `gperf'. They were added to make the
program more convenient for use with real applications. "On-line" help
is readily available via the `-h' option. Other options include:
`-a'
Generate ANSI Standard C code using function prototypes. The
default is to use "classic" K&R C function declaration syntax.
`-c'
Generates C code that uses the `strncmp' function to perform
string comparisons. The default action is to use `strcmp'.
`-C'
Makes the contents of all generated lookup tables constant,
*i.e.*, "readonly." Many compilers can generate more
efficient code for this by putting the tables in readonly
memory.
`-d'
Enables the debugging option. This produces verbose
diagnostics to "standard error" when `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 `-d'
option is enabled.
`-D'
Handle keywords whose key position sets hash to duplicate
values. Duplicate hash values occur for two reasons:
* Since `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.
* Sometimes a set of keys may have the same names, but
possess different attributes. With the -D option
`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, `gperf' helps you out by
organizing the output.
Option `-D' is extremely useful for certain large or highly
redundant keyword sets, *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
`gperf' to work on keyword sets that it otherwise could not
handle.
`-e 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.
`-E'
Define constant values using an enum local to the lookup
function rather than with #defines. This also means that
different lookup functions can reside in the same file.
Thanks to James Clark (jjc at ai.mit.edu).
`-f ITERATION AMOUNT'
Generate the perfect hash function "fast." This decreases
`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 `-D' and/or
`-S' for *large* keyword sets.
`-g'
Assume a GNU compiler, *e.g.*, `g++' or `gcc'. This makes
all generated routines use the "inline" keyword to remove the
cost of function calls. Note that `-g' does *not* imply
`-a', since other non-ANSI C compilers may have provisions
for a function `inline' feature.
`-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).
`-h'
Prints a short summary on the meaning of each program option.
Aborts further program execution.
`-H 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.
`-i INITIAL VALUE'
Provides an initial 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 `-S' is used. Also, `-i' is overriden when the
`-r' option is used.
`-j JUMP VALUE'
Affects the "jump value," *i.e.*, how far to advance the
associated character value upon collisions. JUMP VALUE is
rounded up to an odd number, the default is 5. If the JUMP
VALUE is 0 `gper f' jumps by random amounts.
`-k 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,
*e.g.*, `-k 9,4,13,14'; ranges may be used, *e.g.*, `-k 2-7';
and positions may occur in any order. Furthermore, the
meta-character '*' causes the generated hash function to
consider *all* character positions in each key, whereas '$'
instructs the hash function to use the "final character" of a
key (this is the only way to use a character position greater
than 126, incidentally).
For instance, the option `-k 1,2,4,6-10,'$'' 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.
`-K 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
`struct'.
`-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 `strcmp'. However, using `-l' might
greatly increase the size of the generated C code if the
lookup table range is large (which implies that the switch
option `-S' is not enabled), since the length table contains
as many elements as there are entries in the lookup table.
`-L GENERATED LANGUAGE NAME'
Instructs `gperf' to generate code in the language specified
by the option's argument. Languages handled are currently
C++ and C. The default is C.
`-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.
`-N 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.
`-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 *very* large using
`-o' may *increase* `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.
`-p'
Changes the return value of the generated function
`in_word_set' from boolean (*i.e.*, 0 or 1), to either type
"pointer to user-defined struct," (if the `-t' option is
enabled), or simply to `char *', if `-t' is not enabled.
This option is most useful when the `-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 `-p' and `-t'.
`-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 `gperf' has
difficultly with a certain keyword set try using `-r' or `-D'.
`-s SIZE-MULTIPLE'
Affects the size of the generated hash table. The numeric
argument for this option indicates "how many times larger or
smaller" the maximum associated value range should be, in
relationship to the number of keys. If the SIZE-MULTIPLE is
negative the maximum associated value is calculated by
*dividing* it into the total 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."
Conversely, a value of -3 means "allow the maximum associated
value to be about 3 times smaller than the number of input
keys." Negative values are useful for limiting the overall
size of the generated hash table, though this usually
increases the number of duplicate hash values.
If `generate switch' option `-S' is *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 `gperf''s
runtime, since it must search through a much larger range of
values. Judicious use of the `-f' option helps alleviate this
overhead, however.
`-S TOTAL SWITCH STATEMENTS'
Causes the generated C code to use a `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
`switch' statements are generated. A value of 1 generates 1
`switch' containing all the elements, a value of 2 generates
2 tables with 1/2 the elements in each `switch', etc. This
is useful since many C compilers cannot correctly generate
code for large `switch' statements. This option was inspired
in part by Keith Bostic's original C program.
`-t'
Allows you to include a `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.
`-T'
Prevents the transfer of the type declaration to the output
file. Use this option if the type is already defined
elsewhere.
`-v'
Prints out the current version number.
`-Z CLASS NAME'
Allow user to specify name of generated C++ class. Default
name is `Perfect_Hash'.
File: gperf.info, Node: Bugs, Next: Projects, Prev: Options, Up: Top
Known Bugs and Limitations with `gperf'
***************************************
The following are some limitations with the current release of
`gperf':
* The `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
`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 `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 `-r' option, and also try changing the default
arguments to the `-s' and `-j' options. To *guarantee* a
solution, use the `-D' and `-S' options, although the final
results are not likely to be a *perfect* hash function anymore!
Finally, use the `-f' option if you want `gperf' to generate the
perfect hash function *fast*, with less emphasis on making it
minimal.
* The size of the generate static keyword array can get *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 *greatly* inflates the object code size. If
this situation occurs, consider using the `-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 -S option with an appropriate numerical argument that
controls the number of switch statements generated.
* 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.
* 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
`gperf' uses ALLOCA in several places. Send mail to schmidt at
ics.uci.edu for information.
File: gperf.info, Node: Projects, Next: Implementation, Prev: Bugs, Up: Top
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:
* Make the algorithm more robust. At present, the program halts
with an error diagnostic if it can't find a direct solution and
the `-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.
* 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 `-S'
`switch' option can minimize the data size, at the expense of
slightly longer lookups (note that the gcc compiler generally
produces good code for `switch' statements, reducing the need for
more complex schemes).
* 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.
File: gperf.info, Node: Implementation, Next: Bibliography, Prev: Projects, Up: Top
Implementation Details of GNU `gperf'
*************************************
A paper describing the high-level description of the data structures
and algorithms used to implement `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, *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.
File: gperf.info, Node: Bibliography, Prev: Implementation, Up: Top
Bibliography
************
[1] Chang, C.C.: A Scheme for Constructing Ordered Minimal Perfect
Hashing Functions Information Sciences 39(1986), 187-195.
[2] Cichelli, Richard J. Author's Response to "On Cichelli's Minimal
Perfec t Hash Functions Method" Communications of the ACM, 23,
12(December 1980), 729.
[3] Cichelli, Richard J. Minimal Perfect Hash Functions Made Simple
Communications of the ACM, 23, 1(January 1980), 17-19.
[4] Cook, C. R. and Oldehoeft, R.R. 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.
Practical Perfect Hashing Computer Journal, 28, 1(January 1985), 54-58.
[6] Jaeschke, G. 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. On Cichelli's Minimal Perfect
Hash Functions Method Communications of the ACM, 23, 12(December 1980),
728-729.
[8] Sager, Thomas J. A Polynomial Time Generator for Minimal Perfect
Hash Functions Communications of the ACM, 28, 5(December 1985), 523-532
[9] Schmidt, Douglas C. GPERF: A Perfect Hash Function Generator
Second USENIX C++ Conference Proceedings, April 1990.
[10] Sebesta, R.W. and Taylor, M.A. Minimal Perfect Hash Functions
for Reserved Word Lists SIGPLAN Notices, 20, 12(September 1985), 47-53.
[11] Sprugnoli, R. Perfect Hashing Functions: A Single Probe
Retrieving Method for Static Sets Communications of the ACM, 20
11(November 1977), 841-850.
[12] Stallman, Richard M. Using and Porting GNU CC Free Software
Foundation, 1988.
[13] Stroustrup, Bjarne The C++ Programming Language.
Addison-Wesley, 1986.
[14] Tiemann, Michael D. User's Guide to GNU C++ Free Software
Foundation, 1989.
Tag Table:
Node: Top
Node: Copying
Node: Contributors
15742
Node: Motivation
16842
Node: Search Structures
18109
Node: Description
21662
Node: Input Format
23482
Node: Declarations
24277
Node: Keywords
26584
Node: Functions
28175
Node: Output Format
28669
Node: Options
31139
Node: Bugs
44509
Node: Projects
47196
Node: Implementation
48773
Node: Bibliography
49492
End Tag Table