GPERF(1) FSF GPERF(1)
gperf - 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:
Allow user to provide a string containing delimiters used to separate keywords from
their attributes. Default is ",".
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:
Generates code in the specified language. Languages handled are currently C++,
ANSI-C, C, and KR-C. The default is C.
Details in the output code:
Select name of the keyword component in the keyword structure.
Initializers for additional components in the keyword structure.
Specify name of generated hash function. Default is 'hash'.
Specify name of generated lookup function. Default name is 'in_word_set'.
Specify name of generated C++ class. Default name is 'Perfect_Hash'.
Assume 7-bit characters.
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 compar-
isons made during the lookup.
Generate comparison code using strncmp rather than strcmp.
Make the contents of generated lookup tables constant, i.e., readonly.
Define constant values using an enum local to the lookup function rather than with
Include the necessary system include file <string.h> at the beginning of the code.
Generate the static table of keywords as a static global variable, rather than hid-
ing it inside of the lookup function (which is the default behavior).
Optimize the generated table for inclusion in shared libraries. This reduces the
startup time of programs using a shared library containing the generated code.
Specify name of string pool generated by option --pic. Default name is 'string-
Use NULL strings instead of empty strings for empty keyword table entries.
Specify name of word list array. Default name is 'wordlist'.
Specify name of length table array. Default name is 'lengthtable'.
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 gener-
ated. 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.
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:
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.
Handle keywords that hash to duplicate values. This is useful for certain highly
redundant keyword sets.
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.
Provide an initial value for the associate values array. Default is 0. Setting this
value larger helps inflate the size of the final table.
Affects the "jump value", i.e., how far to advance the associated character value
upon collisions. Must be an odd number, default is 5.
Do not include the length of the keyword when computing the hash function.
Utilizes randomness to initialize the associated values table.
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 unsuccess-
ful search, at the expense of extra table space. Default value is 1.
Print this message.
Print the gperf version number.
Enables the debugging option (produces verbose output to the standard error).
Written by Douglas C. Schmidt and Bruno Haible.
Report bugs to <email@example.com>.
Copyright (C) 1989-1998, 2000-2004, 2006-2007, 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO warranty; not
even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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
should give you access to the complete manual.
GNU gperf 3.0.4 February 2009 GPERF(1)