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Manual

GPERF

NAME
SYNOPSIS
DESCRIPTION
Output file location:
Input file interpretation:
Language for the output code:
Details in the output code:
Algorithm employed by gperf:
Informative output:
AUTHOR
REPORTING BUGS
COPYRIGHT
SEE ALSO

NAME

gperf - generate a perfect hash function from a key set

SYNOPSIS

gperf [ OPTION ]... [ INPUT-FILE ]

DESCRIPTION

GNU ’gperf’ generates perfect hash functions.

If a long option shows an argument as mandatory, then it is mandatory for the equivalent short option also.

Output file location:

--output-file = FILE Write output to specified file.

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

Input file interpretation:

-e , --delimiters = DELIMITER-LIST

Allow user to provide a string containing delimiters used to separate keywords from their attributes. Default is ",".

-t , --struct-type

Allows the user to include a structured type declaration for generated code. Any text before %% is considered part of the type declaration. Key words and additional fields may follow this, one group of fields per line.

--ignore-case

Consider upper and lower case ASCII characters as equivalent. Note that locale dependent case mappings are ignored.

Language for the output code:

-L , --language = LANGUAGE-NAME

Generates code in the specified language. Languages handled are currently C++, ANSI-C, C, and KR-C. The default is ANSI-C.

Details in the output code:

-K , --slot-name = NAME

Select name of the keyword component in the keyword structure.

-F , --initializer-suffix = INITIALIZERS

Initializers for additional components in the keyword structure.

-H , --hash-function-name = NAME

Specify name of generated hash function. Default is ’hash’.

-N , --lookup-function-name = NAME

Specify name of generated lookup function. Default name is ’in_word_set’.

-Z , --class-name = NAME

Specify name of generated C++ class. Default name is ’Perfect_Hash’.

-7 , --seven-bit

Assume 7-bit characters.

-l , --compare-lengths

Compare key lengths before trying a string comparison. This is necessary if the keywords contain NUL bytes. It also helps cut down on the number of string comparisons made during the lookup.

-c , --compare-strncmp

Generate comparison code using strncmp rather than strcmp.

-C , --readonly-tables

Make the contents of generated lookup tables constant, i.e., readonly.

-E , --enum

Define constant values using an enum local to the lookup function rather than with defines.

-I , --includes

Include the necessary system include file <string.h> at the beginning 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.

-Q , --string-pool-name = NAME

Specify name of string pool generated by option --pic . Default name is ’stringpool’.

--null-strings

Use NULL strings instead of empty strings for empty keyword table entries.

--constants-prefix = PREFIX

Specify prefix for the constants like TOTAL_KEYWORDS.

-W , --word-array-name = NAME

Specify name of word list array. Default name is ’wordlist’.

--length-table-name = NAME

Specify name of length table array. Default name is ’lengthtable’.

-S , --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 keyfiles. The COUNT argument 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 table, etc. If COUNT is very large, say 1000000, the generated C code does a binary search.

-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.

Algorithm employed by gperf:

-k , --key-positions = KEYS

Select the key positions used in the hash function. The allowable choices range between 1-255, inclusive. The positions are separated by commas, ranges may be used, and key positions may occur in any order. Also, the meta-character ’*’ causes the generated hash function to consider ALL key positions, and $ indicates the "final character" of a key, e.g., $,1,2,4,6-10.

-D , --duplicates

Handle keywords that hash to duplicate values. This is useful for certain highly redundant keyword sets.

-m , --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-asso = N

Provide an initial value for the associate values array. Default is 0. Setting this value larger helps inflate the size of the final table.

-j , --jump = JUMP-VALUE

Affects the "jump value", i.e., how far to advance the associated character value upon collisions. Must be an odd number, default is 5.

-n , --no-strlen

Do not include the length of the keyword when computing the hash function.

-r , --random

Utilizes randomness to initialize the associated values table.

-s , --size-multiple = N

Affects the size of the generated hash table. The numeric argument N indicates "how many times larger or smaller" the associated value range should be, in relationship to the number of keys, e.g. 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 1/3 means "make the maximum associated value about 3 times smaller than the number of input keys". A larger table should decrease the time required for an unsuccessful search, at the expense of extra table space. Default value is 1.

Informative output:

-h , --help

Print this message.

-v , --version

Print the gperf version number.

-d , --debug

Enables the debugging option (produces verbose output to the standard error).

AUTHOR

Written by Douglas C. Schmidt and Bruno Haible.

REPORTING BUGS

Report bugs to <bug-gperf@gnu.org>.

COPYRIGHT

Copyright © 2025 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.