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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. Other options
- Generate ANSI Standard C code using function prototypes. The default is
to use "classic" K&R C function declaration syntax.
- Generates C code that uses the
strncmp function to perform
string comparisons. The default action is to use
- 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.
- 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.
- Handle keywords whose key position sets hash to duplicate values.
Duplicate hash values occur for two reasons:
gperf does not backtrack it is possible for it to process
all your input keywords without finding a unique mapping for each word.
However, frequently only a very small number of duplicates occur, and
the majority of keys still require one probe into the table.
- Sometimes a set of keys may have the same names, but possess different
attributes. With the -D option
gperf treats all these keys as part of
an equivalence class and generates a perfect hash function with multiple
comparisons for duplicate keys. It is up to you to completely
disambiguate the keywords by modifying the generated C code. However,
gperf helps you out by organizing the output.
Option `-D' is extremely useful for certain large or highly
redundant keyword sets, i.e., 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.
- `-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.
- 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 (jjc at ai.mit.edu).
- `-f iteration amount'
- Generate the perfect hash function "fast." This decreases
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.
- Assume a GNU compiler, e.g.,
makes all generated routines use the "inline" keyword to remove the
cost of function calls. Note that `-g' does not imply
`-a', since other non-ANSI C compilers may have provisions for a
- Generate the static table of keywords as a static global variable,
rather than hiding it inside of the lookup function (which is the
- Prints a short summary on the meaning of each program option. Aborts
further program execution.
- `-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
- `-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 overriden 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
jumps by random amounts.
- `-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 '$' instructs the hash function to use the "final character"
of a key (this is the only way to use a character position greater than
For instance, the option `-k 1,2,4,6-10,'$'' 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.
- `-K key name'
- 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
- 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
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.
- `-L generated language name'
gperf to generate code in the language specified by the
option's argument. Languages handled are currently C++ and C. The
default is C.
- 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.
- `-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.
- 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
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
- Changes the return value of the generated function
from boolean (i.e., 0 or 1), to either type "pointer to
user-defined struct," (if the `-t' option is enabled), or simply
char *, if `-t' is not enabled. This option is most
useful when the `-t' option (allowing user-defined structs) is
used. For example, it is possible to automatically generate the GNU C
reserved word lookup routine with the options `-p' and `-t'.
- 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
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
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
- `-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.
- Allows you to include a
struct type declaration for generated
code. Any text before a pair of consecutive %% is consider part of the
type declaration. Key words 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, and G++, Pascal, and Modula 2 and 3
reserved words are distributed with this release.
- Prevents the transfer of the type declaration to the output file. Use
this option if the type is already defined elsewhere.
- Prints out the current version number.
- `-Z class name'
- Allow user to specify name of generated C++ class. Default name is
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