Go forward to Bugs.
Go backward to Description.
Go up to Top.
Options to the `gperf' Utility
******************************
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 include:
`-a'
Generate ANSI Standard C code using function prototypes. The
default is to use "classic" K&R C function declaration syntax.
`-c'
Generates C code that uses the `strncmp' function to perform
string comparisons. The default action is to use `strcmp'.
`-C'
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.
`-d'
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.
`-D'
Handle keywords whose key position sets hash to duplicate
values. Duplicate hash values occur for two reasons:
* Since `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.
`-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.
`-g'
Assume a GNU compiler, *e.g.*, `g++' or `gcc'. This 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 function `inline' feature.
`-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).
`-h'
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 same file.
`-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 `gperf' 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 '$'
instructs the hash function to use the "final character" of a
key (this is the only way to use a character position greater
than 126, incidentally).
For instance, the option `-k 1,2,4,6-10,'$'' 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.
`-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
`struct'.
`-l'
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.
`-n'
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.
`-o'
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.
`-p'
Changes the return value of the generated function
`in_word_set' from boolean (*i.e.*, 0 or 1), to either type
"pointer to user-defined struct," (if the `-t' option is
enabled), or simply to `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'.
`-r'
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"
the maximum associated value range should be, in relationship
to the 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." If 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.
`-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.
`-t'
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.
`-T'
Prevents the transfer of the type declaration to the output
file. Use this option if the type is already defined
elsewhere.
`-v'
Prints out the current version number.