gperf - Man Page

generate a perfect hash function from a key set


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


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.


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'.


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


Specify prefix for the constants like TOTAL_KEYWORDS.

-W,  --word-array-name=NAME

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


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).


Written by Douglas C. Schmidt and Bruno Haible.

Reporting Bugs

Report bugs to <>.

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.


January 2017 GNU gperf 3.1 FSF