User's Guide to gperf 2.7.2

The GNU Perfect Hash Function Generator

Edition 2.7.2, 26 September 2000

Douglas C. Schmidt

Table of Contents


Version 2, June 1991

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Contributors to GNU gperf Utility

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. If k = n 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 Pascal, GNU Modula 3, and GNU indent. Complete C++ source code for gperf is available via anonymous ftp from A paper describing gperf's design and implementation in greater detail is available in the Second USENIX C++ Conference proceedings.

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:

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 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.

3 High-Level Description of GNU gperf

The perfect hash function generator gperf reads a set of "keywords" from a keyfile (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 characters 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.

3.1 Input Format to gperf

You can control the input keyfile 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:


Unlike flex or bison, all sections of gperf's input are optional. The following sections describe the input format for each section.

3.1.1 struct Declarations and C Code Inclusion

The keyword input file optionally contains a section for including arbitrary C declarations and definitions, as well as provisions for providing a user-supplied struct. If the `-t' option is enabled, you must provide a C struct as the last component in the declaration section from the keyfile file. The first field in this struct must be a char * or const char * identifier called `name', although it is possible to modify this field's name with the `-K' option described below.

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

struct months { 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.

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

It is possible to omit the declaration section entirely. In this case the keyfile begins directly with the first keyword line, e.g.:

january,   1, 31, 31
february,  2, 28, 29
march,     3, 31, 31
april,     4, 30, 30

3.1.2 Format for Keyword Entries

The second keyfile 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.

The first field of each non-comment line is always the key 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.

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 is not enabled these fields are simply ignored. All previous examples except the last one contain keyword attributes.

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.

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 function 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 key 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'; its meaning and properties are described below (see section 7 Implementation Details of GNU gperf). The relevant key positions are specified via the `-k' option when running gperf, as detailed in the Options section below(see section 4 Invoking gperf).

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' was given, it returns a pointer to the matching keyword's structure. Otherwise it returns NULL.

If the option `-c' is not used, str must be a NUL terminated string of exactly length len. If `-c' is used, str must simply be an array of len characters and does not need to be NUL terminated.

The code generated for these two functions is affected by the following options:

Make use of the user-defined struct.
`-S 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 are omitted, the default action is to generate a char * array containing the keys, together with additional null 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.

3.3 Use of NUL characters

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 characters, and the str argument passed to hash or in_word_set must be NUL terminated and have exactly length len.

If option `-c' 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 characters.

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

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

4.1 Options that affect Interpretation of the Input File

`-e keyword-delimiter-list'
Allows the user to provide a string containing delimiters used to separate keywords from their attributes. The default is ",\n". 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.
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.

4.2 Options to specify the Language for the Output Code

`-L generated-language-name'
Instructs gperf to generate code in the language specified by the option's argument. Languages handled are currently:
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'.
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. This language is understood by ANSI C compilers and C++ compilers.
C++. This language is understood by C++ compilers.
The default is C.
This option is supported for compatibility with previous releases of gperf. It does not do anything.
This option is supported for compatibility with previous releases of gperf. It does not do anything.

4.3 Options for fine tuning Details in the Output Code

`-K key-name'
This option is only useful when option `-t' 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'
This option is only useful when option `-t' has been given. It permits to specify initializers for the structure members following key 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 key name.
`-H 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'
Allows you to specify the name for the generated lookup function. Default name is `in_word_set'. This option permits completely automatic generation of perfect hash functions, especially when multiple generated hash functions are used in the same application.
`-Z class-name'
This option is only useful when option `-L C++' has been given. It allows you to specify the name of generated C++ class. Default name is Perfect_Hash.
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 (characters in the range 0..127). (Note that the ANSI C functions isalnum and isgraph do not guarantee that a character 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 assume 8-bit characters.
Generates C code that uses the strncmp function to perform string comparisons. The default action is to use strcmp.
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.
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 <>.
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.
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).
`-W 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' is given.
`-S 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 keyfiles. 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.
Prevents the transfer of the type declaration to the output file. Use this option if the type is already defined elsewhere.
This option is supported for compatibility with previous releases of gperf. It does not do anything.

4.4 Options for changing the Algorithms employed by gperf

`-k keys'
Allows selection of the character key positions used in the keywords' hash function. The allowable choices range between 1-126, 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 meta-character '*' causes the generated hash function to consider all character positions in each key, whereas '$' instructs the hash function to use the "final character" of a key (this is the only way to use a character position greater than 126, incidentally). For instance, the option `-k 1,2,4,6-10,'$'' generates a hash function that considers positions 1,2,4,6,7,8,9,10, plus the last character in each key (which may differ for each key, obviously). Keys with length less than the indicated key positions work properly, since selected key positions exceeding the key length are simply not referenced in the hash function.
Compare key lengths before trying a string comparison. This might cut down on the number of string comparisons made during the lookup, since keys 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' is not enabled), since the length table contains as many elements as there are entries in the lookup table. This option is mandatory for binary comparisons (see section 3.3 Use of NUL characters).
Handle keywords whose key position sets hash to duplicate values. Duplicate hash values occur for two reasons: Option `-D' is extremely useful for certain large or highly redundant keyword sets, e.g., assembler instruction opcodes. 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.
`-f iteration-amount'
Generate the perfect hash function "fast". This decreases gperf's running time at the cost of minimizing generated table-size. The iteration amount represents the number of times to iterate when resolving a collision. `0' means iterate by the number of keywords. This option is probably most useful when used in conjunction with options `-D' and/or `-S' for large keyword sets.
`-i 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' is used. Also, `-i' is overridden when the `-r' option is used.
`-j jump-value'
Affects the "jump value", i.e., how far to advance the associated character 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.
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.
Reorders the keywords by sorting the keywords so that frequently occuring key position set components appear first. A second reordering pass follows so that keys with "already determined values" are placed towards the front of the keylist. This may decrease the time required to generate a perfect hash function for many keyword sets, and also produce more minimal perfect hash functions. The reason for this is that the reordering helps prune the search time by handling inevitable collisions early in the search process. On the other hand, if the number of keywords is very large using `-o' may increase gperf's execution time, since collisions will begin earlier and continue throughout the remainder of keyword processing. See Cichelli's paper from the January 1980 Communications of the ACM for details.
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. If gperf has difficultly with a certain keyword set try using `-r' or `-D'.
`-s 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 keys. If the size-multiple is negative the maximum associated value is calculated by dividing it into the total number of keys. For example, 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 -3 means "allow the maximum associated value to be about 3 times smaller than the number of input keys". Negative values are useful for limiting the overall size of the generated hash table, though this usually increases the number of duplicate hash values. If `generate switch' option `-S' 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 keys (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. In particular, setting this value too high slows down gperf's runtime, since it must search through a much larger range of values. Judicious use of the `-f' option helps alleviate this overhead, however.

4.5 Informative Output

Prints a short summary on the meaning of each program option. Aborts further program execution.
Prints out the current version number.
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.

5 Known Bugs and Limitations with gperf

The following are some limitations with the current release of gperf:

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:

7 Implementation Details of GNU gperf

A paper describing the high-level description of the data structures and algorithms used to implement gperf will soon be available. This paper is useful not only from a maintenance and enhancement perspective, but also because they demonstrate several clever and useful programming techniques, e.g., `Iteration Number' boolean arrays, double hashing, a "safe" and efficient method for reading arbitrarily long input from a file, and a provably optimal algorithm for simultaneously determining both the minimum and maximum elements in a list.

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

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

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

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

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

Concept Index


  • `%%'
  • `%{'
  • `%}'
  • a

  • Array name
  • b

  • Bugs
  • c

  • Class name
  • d

  • Declaration section
  • Delimiters
  • Duplicates
  • f

  • Format
  • Functions section
  • h

  • hash
  • hash table
  • i

  • in_word_set
  • Initializers
  • j

  • Jump value
  • k

  • Keywords section
  • m

  • Minimal perfect hash functions
  • n

  • NUL
  • s

  • Slot name
  • Static search structure
  • switch, switch

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