path: root/doc/gperf.info

This is gperf.info, produced by makeinfo version 4.8 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 3.0.3.

   Copyright (C) 1989-2006 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 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::                  The purpose of `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.
* 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 bytes

Input Format to `gperf'

* Declarations::                Declarations.
* Keywords::                    Format for Keyword Entries.
* Functions::                   Including Additional C Functions.
* Controls for GNU indent::     Where to place directives for GNU `indent'.

Declarations

* User-supplied Struct::        Specifying keywords with attributes.
* Gperf Declarations::          Embedding command line options in the input.
* C Code Inclusion::            Including C declarations and definitions.

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
**************************

                         Version 2, June 1991

     Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
     59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.

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

Preamble
========

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                      END OF TERMS AND CONDITIONS
How to Apply These Terms to Your New Programs
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   This General Public License does not permit incorporating your
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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
     written in GNU C++ by Douglas C. Schmidt.  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 `<bug-gnu-gperf@gnu.org>'.

   * 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'.

   * Bruno Haible enhanced and optimized the search algorithm.  He also
     rewrote the input routines and the output routines for better
     reliability, and added a testsuite.


File: gperf.info,  Node: Motivation,  Next: Search Structures,  Prev: Contributors,  Up: Top

1 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-1.  If K = N-1 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 Java, GNU
Pascal, GNU Modula 3, and GNU indent.  Complete C++ source code for
`gperf' is available from `http://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 or from
`http://www.cs.wustl.edu/~schmidt/resume.html'.


File: gperf.info,  Node: Search Structures,  Next: Description,  Prev: Motivation,  Up: Top

2 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 Java, 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

3 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 bytes

   The perfect hash function generator `gperf' reads a set of
"keywords" from an input file (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 bytes 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

3.1 Input Format to `gperf'
===========================

You can control the input file 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', the declarations section and the
functions section are optional.  The following sections describe the
input format for each section.

* Menu:

* Declarations::                Declarations.
* Keywords::                    Format for Keyword Entries.
* Functions::                   Including Additional C Functions.
* Controls for GNU indent::     Where to place directives for GNU `indent'.

   It is possible to omit the declaration section entirely, if the `-t'
option is not given.  In this case the input file begins directly with
the first keyword line, e.g.:

     january
     february
     march
     april
     ...


File: gperf.info,  Node: Declarations,  Next: Keywords,  Prev: Input Format,  Up: Input Format

3.1.1 Declarations
------------------

The keyword input file optionally contains a section for including
arbitrary C declarations and definitions, `gperf' declarations that act
like command-line options, as well as for providing a user-supplied
`struct'.

* Menu:

* User-supplied Struct::        Specifying keywords with attributes.
* Gperf Declarations::          Embedding command line options in the input.
* C Code Inclusion::            Including C declarations and definitions.


File: gperf.info,  Node: User-supplied Struct,  Next: Gperf Declarations,  Prev: Declarations,  Up: Declarations

3.1.1.1 User-supplied `struct'
..............................

If the `-t' option (or, equivalently, the `%struct-type' declaration)
_is_ enabled, you _must_ provide a C `struct' as the last component in
the declaration section from the input file.  The first field in this
struct must be of type `char *' or `const char *' if the `-P' option is
not given, or of type `int' if the option `-P' (or, equivalently, the
`%pic' declaration) is enabled.  This first field must be called
`name', although it is possible to modify its name with the `-K' option
(or, equivalently, the `%define slot-name' declaration) described below.

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

     struct month { 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'.

   If the `struct' has already been declared in an include file, it can
be mentioned in an abbreviated form, like this:

     struct month;
     %%
     january,   1, 31, 31
     ...


File: gperf.info,  Node: Gperf Declarations,  Next: C Code Inclusion,  Prev: User-supplied Struct,  Up: Declarations

3.1.1.2 Gperf Declarations
..........................

The declaration section can contain `gperf' declarations.  They
influence the way `gperf' works, like command line options do.  In
fact, every such declaration is equivalent to a command line option.
There are three forms of declarations:

  1. Declarations without argument, like `%compare-lengths'.

  2. Declarations with an argument, like `%switch=COUNT'.

  3. Declarations of names of entities in the output file, like
     `%define lookup-function-name NAME'.

   When a declaration is given both in the input file and as a command
line option, the command-line option's value prevails.

   The following `gperf' declarations are available.

`%delimiters=DELIMITER-LIST'
     Allows you to provide a string containing delimiters used to
     separate keywords from their attributes.  The default is ",".  This
     option is essential if you want to use keywords that have embedded
     commas or newlines.

`%struct-type'
     Allows you to include a `struct' type declaration for generated
     code; see above for an example.

`%ignore-case'
     Consider upper and lower case ASCII characters as equivalent.  The
     string comparison will use a case insignificant character
     comparison.  Note that locale dependent case mappings are ignored.

`%language=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.

`%define slot-name NAME'
     This declaration is only useful when option `-t' (or,
     equivalently, the `%struct-type' declaration) 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'.

`%define initializer-suffix INITIALIZERS'
     This declaration is only useful when option `-t' (or,
     equivalently, the `%struct-type' declaration) has been given.  It
     permits to specify initializers for the structure members following
     SLOT-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 SLOT-NAME.

`%define hash-function-name 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.

`%define lookup-function-name NAME'
     Allows you to specify the name for the generated lookup function.
     Default name is `in_word_set'.  This option permits multiple
     generated hash functions to be used in the same application.

`%define class-name NAME'
     This option is only useful when option `-L C++' (or, equivalently,
     the `%language=C++' declaration) has been given.  It allows you to
     specify the name of generated C++ class.  Default name is
     `Perfect_Hash'.

`%7bit'
     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 (bytes in
     the range 0..127).  (Note that the ANSI C functions `isalnum' and
     `isgraph' do _not_ guarantee that a byte is in this range.  Only
     an explicit test like `c >= 'A' && c <= 'Z'' guarantees this.)

`%compare-lengths'
     Compare keyword lengths before trying a string comparison.  This
     option is mandatory for binary comparisons (*note Binary
     Strings::).  It also might cut down on the number of string
     comparisons made during the lookup, since keywords with different
     lengths are never compared via `strcmp'.  However, using
     `%compare-lengths' might greatly increase the size of the
     generated C code if the lookup table range is large (which implies
     that the switch option `-S' or `%switch' is not enabled), since
     the length table contains as many elements as there are entries in
     the lookup table.

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

`%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.

`%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>'.

`%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.

`%global-table'
     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).

`%pic'
     Optimize the generated table for inclusion in shared libraries.
     This reduces the startup time of programs using a shared library
     containing the generated code.  If the `%struct-type' declaration
     (or, equivalently, the option `-t') is also given, the first field
     of the user-defined struct must be of type `int', not `char *',
     because it will contain offsets into the string pool instead of
     actual strings.  To convert such an offset to a string, you can
     use the expression `stringpool + O', where O is the offset.  The
     string pool name can be changed through the `%define
     string-pool-name' declaration.

`%define string-pool-name NAME'
     Allows you to specify the name of the generated string pool
     created by the declaration `%pic' (or, equivalently, the option
     `-P').  The default name is `stringpool'.  This declaration
     permits the use of two hash tables in the same file, with `%pic'
     and even when the `%global-table' declaration (or, equivalently,
     the option `-G') is given.

`%null-strings'
     Use NULL strings instead of empty strings for empty keyword table
     entries.  This reduces the startup time of programs using a shared
     library containing the generated code (but not as much as the
     declaration `%pic'), at the expense of one more test-and-branch
     instruction at run time.

`%define word-array-name 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' (or, equivalently, the `%global-table' declaration) is given.

`%define length-table-name NAME'
     Allows you to specify the name for the generated array containing
     the length table.  Default name is `lengthtable'.  This option
     permits the use of two length tables in the same file, even when
     the option `-G' (or, equivalently, the `%global-table'
     declaration) is given.

`%switch=COUNT'
     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 input files.  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.

`%omit-struct-type'
     Prevents the transfer of the type declaration to the output file.
     Use this option if the type is already defined elsewhere.


File: gperf.info,  Node: C Code Inclusion,  Prev: Gperf Declarations,  Up: Declarations

3.1.1.3 C Code Inclusion
........................

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 month *months, int is_leap_year);
     %}
     struct month { char *name; int number; int days; int leap_days; };
     %%
     january,   1, 31, 31
     february,  2, 28, 29
     march,     3, 31, 31
     ...


File: gperf.info,  Node: Keywords,  Next: Functions,  Prev: Declarations,  Up: Input Format

3.1.2 Format for Keyword Entries
--------------------------------

The second input file 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.  A line
beginning with `%' in the first column is an option declaration and
must not occur within the keywords section.

   The first field of each non-comment line is always the keyword
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 (or, equivalently, the
`%struct-type' declaration) is _not_ enabled these fields are simply
ignored.  All previous examples except the last one contain keyword
attributes.


File: gperf.info,  Node: Functions,  Next: Controls for GNU indent,  Prev: Keywords,  Up: Input Format

3.1.3 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: Controls for GNU indent,  Prev: Functions,  Up: Input Format

3.1.4 Where to place directives for GNU `indent'.
-------------------------------------------------

If you want to invoke GNU `indent' on a `gperf' input file, you will
see that GNU `indent' doesn't understand the `%%', `%{' and `%}'
directives that control `gperf''s interpretation of the input file.
Therefore you have to insert some directives for GNU `indent'.  More
precisely, assuming the most general input file structure

     declarations part 1
     %{
     verbatim code
     %}
     declarations part 2
     %%
     keywords
     %%
     functions

you would insert `*INDENT-OFF*' and `*INDENT-ON*' comments as follows:

     /* *INDENT-OFF* */
     declarations part 1
     %{
     /* *INDENT-ON* */
     verbatim code
     /* *INDENT-OFF* */
     %}
     declarations part 2
     %%
     keywords
     %%
     /* *INDENT-ON* */
     functions


File: gperf.info,  Node: Output Format,  Next: Binary Strings,  Prev: Input Format,  Up: Description

3.2 Output Format for Generated C Code with `gperf'
===================================================

Several options control how the generated C code appears on the standard
output.  Two C functions 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 byte 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'.  The relevant selected positions (i.e. indices into
     STR) 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' (or, equivalently, the
     `%struct-type' declaration) was given, it returns a pointer to the
     matching keyword's structure.  Otherwise it returns `NULL'.

   If the option `-c' (or, equivalently, the `%compare-strncmp'
declaration) is not used, STR must be a NUL terminated string of
exactly length LEN.  If `-c' (or, equivalently, the `%compare-strncmp'
declaration) is used, STR must simply be an array of LEN bytes 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 (or, equivalently, the `%struct-type'
and `%switch' declarations) are omitted, the default action is to
generate a `char *' array containing the keywords, together with
additional empty 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

3.3 Use of NUL bytes
====================

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 bytes, and the STR
argument passed to `hash' or `in_word_set' must be NUL terminated and
have exactly length LEN.

   If option `-c' (or, equivalently, the `%compare-strncmp'
declaration) 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 bytes.

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


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

4 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 `--help' option.  Here is the complete
list of options.

* Menu:

* Output File::                 Specifying the Location of the Output File
* 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: Output File,  Next: Input Details,  Prev: Options,  Up: Options

4.1 Specifying the Location of the Output File
==============================================

`--output-file=FILE'
     Allows you to specify the name of the file to which the output is
     written to.

   The results are written to standard output if no output file is
specified or if it is `-'.


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

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

These options are also available as declarations in the input file
(*note Gperf Declarations::).

`-e KEYWORD-DELIMITER-LIST'
`--delimiters=KEYWORD-DELIMITER-LIST'
     Allows you to provide a string containing delimiters used to
     separate keywords from their attributes.  The default is ",".  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.

`--ignore-case'
     Consider upper and lower case ASCII characters as equivalent.  The
     string comparison will use a case insignificant character
     comparison.  Note that locale dependent case mappings are ignored.
     This option is therefore not suitable if a properly
     internationalized or locale aware case mapping should be used.
     (For example, in a Turkish locale, the upper case equivalent of
     the lowercase ASCII letter `i' is the non-ASCII character `capital
     i with dot above'.)  For this case, it is better to apply an
     uppercase or lowercase conversion on the string before passing it
     to the `gperf' generated function.


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

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

These options are also available as declarations in the input file
(*note Gperf Declarations::).

`-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

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

Most of these options are also available as declarations in the input
file (*note Gperf Declarations::).

`-K SLOT-NAME'
`--slot-name=SLOT-NAME'
     This option is only useful when option `-t' (or, equivalently, the
     `%struct-type' declaration) 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' (or, equivalently, the
     `%struct-type' declaration) has been given.  It permits to specify
     initializers for the structure members following SLOT-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 SLOT-NAME.

`-H HASH-FUNCTION-NAME'
`--hash-function-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-function-name=LOOKUP-FUNCTION-NAME'
     Allows you to specify the name for the generated lookup function.
     Default name is `in_word_set'.  This option permits multiple
     generated hash functions to be used in the same application.

`-Z CLASS-NAME'
`--class-name=CLASS-NAME'
     This option is only useful when option `-L C++' (or, equivalently,
     the `%language=C++' declaration) 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 (bytes in
     the range 0..127).  (Note that the ANSI C functions `isalnum' and
     `isgraph' do _not_ guarantee that a byte 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 support 8-bit and multibyte characters.

`-l'
`--compare-lengths'
     Compare keyword lengths before trying a string comparison.  This
     option is mandatory for binary comparisons (*note Binary
     Strings::).  It also might cut down on the number of string
     comparisons made during the lookup, since keywords 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' or `%switch' is not enabled), since the length table contains
     as many elements as there are entries in the lookup table.

`-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-table'
     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).

`-P'
`--pic'
     Optimize the generated table for inclusion in shared libraries.
     This reduces the startup time of programs using a shared library
     containing the generated code.  If the option `-t' (or,
     equivalently, the `%struct-type' declaration) is also given, the
     first field of the user-defined struct must be of type `int', not
     `char *', because it will contain offsets into the string pool
     instead of actual strings.  To convert such an offset to a string,
     you can use the expression `stringpool + O', where O is the
     offset.  The string pool name can be changed through the option
     `--string-pool-name'.

`-Q STRING-POOL-NAME'
`--string-pool-name=STRING-POOL-NAME'
     Allows you to specify the name of the generated string pool
     created by option `-P'.  The default name is `stringpool'.  This
     option permits the use of two hash tables in the same file, with
     `-P' and even when the option `-G' (or, equivalently, the
     `%global-table' declaration) is given.

`--null-strings'
     Use NULL strings instead of empty strings for empty keyword table
     entries.  This reduces the startup time of programs using a shared
     library containing the generated code (but not as much as option
     `-P'), at the expense of one more test-and-branch instruction at
     run time.

`-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' (or, equivalently, the `%global-table' declaration) is given.

`--length-table-name=LENGTH-TABLE-ARRAY-NAME'
     Allows you to specify the name for the generated array containing
     the length table.  Default name is `lengthtable'.  This option
     permits the use of two length tables in the same file, even when
     the option `-G' (or, equivalently, the `%global-table'
     declaration) 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 input files.  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

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

`-k SELECTED-BYTE-POSITIONS'
`--key-positions=SELECTED-BYTE-POSITIONS'
     Allows selection of the byte positions used in the keywords' hash
     function.  The allowable choices range between 1-255, 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 wildcard '*' causes the generated hash
     function to consider *all* byte positions in each keyword, whereas
     '$' instructs the hash function to use the "final byte" of a
     keyword (this is the only way to use a byte position greater than
     255, 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
     byte in each keyword (which may be at a different position for each
     keyword, obviously).  Keywords with length less than the indicated
     byte positions work properly, since selected byte positions
     exceeding the keyword length are simply not referenced in the hash
     function.

     This option is not normally needed since version 2.8 of `gperf';
     the default byte positions are computed depending on the keyword
     set, through a search that minimizes the number of byte positions.

`-D'
`--duplicates'
     Handle keywords whose selected byte sets hash to duplicate values.
     Duplicate hash values can occur if a set of keywords has the same
     names, but possesses different attributes, or if the selected byte
     positions are not well chosen.  With the -D option `gperf' treats
     all these keywords as part of an equivalence class and generates a
     perfect hash function with multiple comparisons for duplicate
     keywords.  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.

     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.

`-m ITERATIONS'
`--multiple-iterations=ITERATIONS'
     Perform multiple choices of the `-i' and `-j' values, and choose
     the best results.  This increases the running time by a factor of
     ITERATIONS but does a good job minimizing the generated table size.

`-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' (or,
     equivalently, `%switch') 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
     byte 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.

`-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.

`-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 keywords.  It can be written as an
     integer, a floating-point number or a fraction.  For example, a
     value of 3 means "allow the maximum associated value to be about 3
     times larger than the number of input keywords".  Conversely, a
     value of 1/3 means "allow the maximum associated value to be about
     3 times smaller than the number of input keywords".  Values
     smaller than 1 are useful for limiting the overall size of the
     generated hash table, though the option `-m' is better at this
     purpose.

     If `generate switch' option `-S' (or, equivalently, `%switch') 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 keywords (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.


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

4.6 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

5 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.

   * 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
     generate 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 selected byte positions has an arbitrary
     limit of 255.  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: Bibliography,  Prev: Bugs,  Up: Top

6 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:

   * 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).  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 an Ada package as the code output, in addition to the
     current C and C++ routines.


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

7 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] Schmidt, Douglas C. GPERF: A Perfect Hash Function Generator
C++ Report, SIGS 10 10 (November/December 1998).

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

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

   [13] Stallman, Richard M. Using and Porting GNU CC Free Software
Foundation, 1988.

   [14] Stroustrup, Bjarne The C++ Programming Language.
Addison-Wesley, 1986.

   [15] Tiemann, Michael D. User's Guide to GNU C++ Free Software
Foundation, 1989.


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