GPERF(1) FSF GPERF(1)

# [NAME](#NAME)

gperf - generate a perfect hash function from a key set

# [SYNOPSIS](#SYNOPSIS)

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

# [DESCRIPTION](#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_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:](#Input_file_interpretation:)

[**-e**, **--delimiters**=*DELIMITER-LIST*](#e)

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

[**-t**, **--struct-type**](#t)

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**](#ignore)

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

## [Language for the output code:](#Language_for_the_output_code:)

[**-L**, **--language**=*LANGUAGE-NAME*](#L)

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:](#Details_in_the_output_code:)

[**-K**, **--slot-name**=*NAME*](#K)

Select name of the keyword component in the keyword structure.

[**-F**, **--initializer-suffix**=*INITIALIZERS*](#F)

Initializers for additional components in the keyword structure.

[**-H**, **--hash-function-name**=*NAME*](#H)

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

[**-N**, **--lookup-function-name**=*NAME*](#N)

Specify name of generated lookup function. Default name is 'in\_word\_set'.

[**-Z**, **--class-name**=*NAME*](#Z)

Specify name of generated C++ class. Default name is 'Perfect\_Hash'.

**-7**, **--seven-bit**

Assume 7-bit characters.

[**-l**, **--compare-lengths**](#l)

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**](#c)

Generate comparison code using strncmp rather than strcmp.

[**-C**, **--readonly-tables**](#C)

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

[**-E**, **--enum**](#E)

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

[**-I**, **--includes**](#I)

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

[**-G**, **--global-table**](#G)

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

[**-P**, **--pic**](#P)

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*](#Q)

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

[**--null-strings**](#null)

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

[**--constants-prefix**=*PREFIX*](#constants)

Specify prefix for the constants like TOTAL\_KEYWORDS.

[**-W**, **--word-array-name**=*NAME*](#W)

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

[**--length-table-name**=*NAME*](#length)

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

[**-S**, **--switch**=*COUNT*](#S)

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**](#T)

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:](#Algorithm_employed_by_gperf:)

[**-k**, **--key-positions**=*KEYS*](#k)

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**](#D)

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

[**-m**, **--multiple-iterations**=*ITERATIONS*](#m)

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*](#i)

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*](#j)

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**](#n)

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

[**-r**, **--random**](#r)

Utilizes randomness to initialize the associated values table.

[**-s**, **--size-multiple**=*N*](#s)

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:](#Informative_output:)

[**-h**, **--help**](#h)

Print this message.

[**-v**, **--version**](#v)

Print the gperf version number.

[**-d**, **--debug**](#d)

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

# [AUTHOR](#AUTHOR)

Written by Douglas C. Schmidt and Bruno Haible.

# [REPORTING BUGS](#REPORTING_BUGS)

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

# [COPYRIGHT](#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.

# [SEE ALSO](#SEE_ALSO)

The full documentation for **gperf** is maintained as a Texinfo manual. If the **info** and **gperf** programs are properly installed at your site, the command

**info gperf**

should give you access to the complete manual.

August 2026 GNU gperf 3.3

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Document: gperf(1) from gperf 3.3-r6 (Chainguard OS)
Source: https://git.savannah.gnu.org/git/gperf.git