gperf.info [plain text]

This is gperf.info, produced by makeinfo version 4.0 from gperf.texi.

INFO-DIR-SECTION Programming Tools
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 2.7.2.

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 General Public License"
and 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.

* Concept Index::

High-Level Description of GNU `gperf'

* Input Format:: Input Format to `gperf'
* Output Format:: Output Format for Generated C Code with `gperf'
* Binary Strings:: Use of NUL characters

Input Format to `gperf'

* Declarations:: `struct' Declarations and C Code Inclusion.
* Keywords:: Format for Keyword Entries.
* Functions:: Including Additional C Functions.

Invoking `gperf'

* Input Details:: Options that affect Interpretation of the Input File
* Output Language:: Specifying the Language for the Output Code
* Output Details:: Fine tuning Details in the Output Code
* Algorithmic Details:: Changing the Algorithms employed by `gperf'
* Verbosity:: Informative Output

File: gperf.info, Node: Copying, Next: Contributors, Prev: Top, Up: Top

GNU GENERAL PUBLIC LICENSE
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Preamble
========

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(which makes passes at compilers) written
by James Hacker.

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Ty Coon, President of Vice

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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 both
`<bug-gnu-utils@gnu.org>' and `<gperf-bugs@lists.sourceforge.net>'.

* 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'.

* A testsuite was added by Bruno Haible. He also rewrote the output
routines for better reliability.

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 `ftp://ftp.gnu.org/pub/gnu/gperf/'. 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'
* Binary Strings:: Use of NUL characters

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 *' or `const char *' identifier called `name',
although it is possible to modify this field's name with the `-K'
option described below.

Here is a 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 keywords 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 can be given in two ways: as a simple name, i.e., without surrounding
string quotation marks, or as a string enclosed in double-quotes, in C
syntax, possibly with backslash escapes like `\"' or `\234' or `\xa8'.
In either case, it must start right at the beginning of the line,
without leading whitespace. 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, Next: Binary Strings, 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 their names 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:

- Function: unsigned int hash (const char * STR, unsigned 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::).

- Function: in_word_set (const char * STR, unsigned int LEN)
If STR is in the keyword set, returns a pointer to that keyword.
More exactly, if the option `-t' was given, it returns a pointer
to the matching keyword's structure. Otherwise it returns `NULL'.

If the option `-c' is not used, STR must be a NUL terminated string
of exactly length LEN. If `-c' is used, STR must simply be an array of
LEN characters and does not need to be NUL terminated.

The code generated for these two functions is affected by the
following options:

`-t'
`--struct-type'
Make use of the user-defined `struct'.

`-S TOTAL-SWITCH-STATEMENTS'
`--switch=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' and `-S' 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: Binary Strings, Prev: Output Format, Up: Description

Use of NUL characters
=====================

By default, the code generated by `gperf' operates on zero
terminated strings, the usual representation of strings in C. This means
that the keywords in the input file must not contain NUL characters,
and the STR argument passed to `hash' or `in_word_set' must be NUL
terminated and have exactly length LEN.

If option `-c' is used, then the STR argument does not need to be
NUL terminated. The code generated by `gperf' will only access the
first LEN, not LEN+1, bytes starting at STR. However, the keywords in
the input file still must not contain NUL characters.

If option `-l' is used, then the hash table performs binary
comparison. The keywords in the input file may contain NUL characters,
written in string syntax as `\000' or `\x00', and the code generated by
`gperf' will treat NUL like any other character. Also, in this case
the `-c' option is ignored.

File: gperf.info, Node: Options, Next: Bugs, Prev: Description, Up: Top

Invoking `gperf'
****************

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. Here is the complete list of
options.

* Menu:

File: gperf.info, Node: Input Details, Next: Output Language, Prev: Options, Up: Options

Options that affect Interpretation of the Input File
====================================================

`-e KEYWORD-DELIMITER-LIST'
`--delimiters=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.

`-t'
`--struct-type'
Allows you to include a `struct' type declaration for generated
code. Any text before a pair of consecutive `%%' is considered
part of the type declaration. Keywords 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, C++,
Pascal, Modula 2, Modula 3 and JavaScript reserved words are
distributed with this release.

File: gperf.info, Node: Output Language, Next: Output Details, Prev: Input Details, Up: Options

Options to specify the Language for the Output Code
===================================================

`-L GENERATED-LANGUAGE-NAME'
`--language=GENERATED-LANGUAGE-NAME'
Instructs `gperf' to generate code in the language specified by the
option's argument. Languages handled are currently:

`KR-C'
Old-style K&R C. This language is understood by old-style C
compilers and ANSI C compilers, but ANSI C compilers may flag
warnings (or even errors) because of lacking `const'.

`C'
Common C. This language is understood by ANSI C compilers,
and also by old-style C compilers, provided that you `#define
const' to empty for compilers which don't know about this
keyword.

`ANSI-C'
ANSI C. This language is understood by ANSI C compilers and
C++ compilers.

`C++'
C++. This language is understood by C++ compilers.

The default is C.

`-a'
This option is supported for compatibility with previous releases
of `gperf'. It does not do anything.

`-g'
This option is supported for compatibility with previous releases
of `gperf'. It does not do anything.

File: gperf.info, Node: Output Details, Next: Algorithmic Details, Prev: Output Language, Up: Options

Options for fine tuning Details in the Output Code
==================================================

`-K KEY-NAME'
`--slot-name=KEY-NAME'
This option is only useful when option `-t' has been given. 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'.

`-F INITIALIZERS'
`--initializer-suffix=INITIALIZERS'
This option is only useful when option `-t' has been given. It
permits to specify initializers for the structure members following
KEY NAME in empty hash table entries. The list of initializers
should start with a comma. By default, the emitted code will
zero-initialize structure members following KEY NAME.

`-H HASH-FUNCTION-NAME'
`--hash-fn-name=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.

`-N LOOKUP-FUNCTION-NAME'
`--lookup-fn-name=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.

`-Z CLASS-NAME'
`--class-name=CLASS-NAME'
This option is only useful when option `-L C++' has been given. It
allows you to specify the name of generated C++ class. Default
name is `Perfect_Hash'.

`-7'
`--seven-bit'
This option specifies that all strings that will be passed as
arguments to the generated hash function and the generated lookup
function will solely consist of 7-bit ASCII characters (characters
in the range 0..127). (Note that the ANSI C functions `isalnum'
and `isgraph' do _not_ guarantee that a character is in this
range. Only an explicit test like `c >= 'A' && c <= 'Z''
guarantees this.) This was the default in versions of `gperf'
earlier than 2.7; now the default is to assume 8-bit characters.

`-c'
`--compare-strncmp'
Generates C code that uses the `strncmp' function to perform
string comparisons. The default action is to use `strcmp'.

`-C'
`--readonly-tables'
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.

`-E'
`--enum'
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@ai.mit.edu>'.

`-I'
`--includes'
Include the necessary system include file, `<string.h>', at the
beginning of the code. By default, this is not done; the user must
include this header file himself to allow compilation of the code.

`-G'
`--global'
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).

`-W HASH-TABLE-ARRAY-NAME'
`--word-array-name=HASH-TABLE-ARRAY-NAME'
Allows you to specify the name for the generated array containing
the hash table. Default name is `wordlist'. This option permits
the use of two hash tables in the same file, even when the option
`-G' is given.

`-S TOTAL-SWITCH-STATEMENTS'
`--switch=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'
`--omit-struct-type'
Prevents the transfer of the type declaration to the output file.
Use this option if the type is already defined elsewhere.

`-p'
This option is supported for compatibility with previous releases
of `gperf'. It does not do anything.

File: gperf.info, Node: Algorithmic Details, Next: Verbosity, Prev: Output Details, Up: Options

Options for changing the Algorithms employed by `gperf'
=======================================================

`-k KEYS'
`--key-positions=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.

`-l'
`--compare-strlen'
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. This option is mandatory for binary comparisons
(*note Binary Strings::).

`-D'
`--duplicates'
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, e.g., 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.

`-f ITERATION-AMOUNT'
`--fast=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.

`-i INITIAL-VALUE'
`--initial-asso=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 overridden when the `-r' option is used.

`-j JUMP-VALUE'
`--jump=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 `gperf'
jumps by random amounts.

`-n'
`--no-strlen'
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.

`-o'
`--occurrence-sort'
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.

`-r'
`--random'
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'
`--size-multiple=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.

File: gperf.info, Node: Verbosity, Prev: Algorithmic Details, Up: Options

Informative Output
==================

`-h'
`--help'
Prints a short summary on the meaning of each program option.
Aborts further program execution.

`-v'
`--version'
Prints out the current version number.

`-d'
`--debug'
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.

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.

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, Next: Concept Index, 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
Perfect 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.

File: gperf.info, Node: Concept Index, Prev: Bibliography, Up: Top

Concept Index
*************

* Menu:

* %%: Declarations.
* %{: Declarations.
* %}: Declarations.
* Array name: Output Details.
* Bugs: Contributors.
* Class name: Output Details.
* Declaration section: Input Format.
* Delimiters: Input Details.
* Duplicates: Algorithmic Details.
* Format: Input Format.
* Functions section: Input Format.
* hash: Output Format.
* hash table: Output Format.
* in_word_set: Output Format.
* Initializers: Output Details.
* Jump value: Algorithmic Details.
* Keywords section: Input Format.
* Minimal perfect hash functions: Search Structures.
* NUL: Binary Strings.
* Slot name: Output Details.
* Static search structure: Search Structures.
* switch <1>: Output Details.
* switch: Output Format.

Tag Table:
Node: Top1236
Node: Copying3130
Node: Contributors22321
Node: Motivation23580
Node: Search Structures24656
Node: Description28201
Node: Input Format30102
Node: Declarations30944
Node: Keywords33268
Node: Functions35023
Node: Output Format35517
Node: Binary Strings38113
Node: Options39119
Node: Input Details39825
Node: Output Language40890
Node: Output Details42194
Node: Algorithmic Details46842
Node: Verbosity54284
Node: Bugs54987
Node: Projects57215
Node: Implementation58792
Node: Bibliography59509
Node: Concept Index61452

End Tag Table