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| author | Bruno Haible <bruno@clisp.org> | 2000-08-20 18:44:23 +0000 |
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| committer | Bruno Haible <bruno@clisp.org> | 2000-08-20 18:44:23 +0000 |
| commit | 745d7fc2fb5401bddae806a6a889cc8c9e5646c2 (patch) | |
| tree | 483a8e4f1fd293938bdfc9e547ed8675ca95c46f /doc/gperf_7.html | |
| parent | 4bdae6102758a6e3805a058fc638a6c882c08e81 (diff) | |
| download | gperf-745d7fc2fb5401bddae806a6a889cc8c9e5646c2.tar.gz | |
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| -rw-r--r-- | doc/gperf_7.html | 403 |
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diff --git a/doc/gperf_7.html b/doc/gperf_7.html index 0516c37..fa36cd4 100644 --- a/doc/gperf_7.html +++ b/doc/gperf_7.html @@ -1,390 +1,63 @@ <HTML> <HEAD> <!-- This HTML file has been created by texi2html 1.51 - from gperf.texi on 15 April 1998 --> + from gperf.texi on 20 August 2000 --> -<TITLE>User's Guide to gperf - 4 Options to the gperf Utility</TITLE> +<TITLE>Perfect Hash Function Generator - 5 Known Bugs and Limitations with gperf</TITLE> </HEAD> <BODY> Go to the <A HREF="gperf_1.html">first</A>, <A HREF="gperf_6.html">previous</A>, <A HREF="gperf_8.html">next</A>, <A HREF="gperf_11.html">last</A> section, <A HREF="gperf_toc.html">table of contents</A>. <P><HR><P> -<H1><A NAME="SEC14" HREF="gperf_toc.html#TOC14">4 Options to the <CODE>gperf</CODE> Utility</A></H1> +<H1><A NAME="SEC20" HREF="gperf_toc.html#TOC20">5 Known Bugs and Limitations with <CODE>gperf</CODE></A></H1> <P> -There are <EM>many</EM> options to <CODE>gperf</CODE>. They were added to make -the program more convenient for use with real applications. "On-line" -help is readily available via the <SAMP>`-h'</SAMP> option. Here is the complete -list of options. +The following are some limitations with the current release of +<CODE>gperf</CODE>: </P> - - -<H2><A NAME="SEC15" HREF="gperf_toc.html#TOC15">4.1 Options that affect Interpretation of the Input File</A></H2> - - -<UL> -<DL COMPACT> - -<DT><SAMP>`-e <VAR>keyword delimiter list</VAR>'</SAMP> -<DD> -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. - -<DT><SAMP>`-t'</SAMP> -<DD> -Allows you to include a <CODE>struct</CODE> 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. -</DL> -</UL> - - - -<H2><A NAME="SEC16" HREF="gperf_toc.html#TOC16">4.2 Options to specify the Language for the Output Code</A></H2> - - -<UL> -<DL COMPACT> - -<DT><SAMP>`-L <VAR>generated language name</VAR>'</SAMP> -<DD> -Instructs <CODE>gperf</CODE> to generate code in the language specified by the -option's argument. Languages handled are currently: - - -<UL> -<DL COMPACT> - -<DT><SAMP>`KR-C'</SAMP> -<DD> -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 <SAMP>`const'</SAMP>. - -<DT><SAMP>`C'</SAMP> -<DD> -Common C. This language is understood by ANSI C compilers, and also by -old-style C compilers, provided that you <CODE>#define const</CODE> to empty -for compilers which don't know about this keyword. - -<DT><SAMP>`ANSI-C'</SAMP> -<DD> -ANSI C. This language is understood by ANSI C compilers and C++ compilers. - -<DT><SAMP>`C++'</SAMP> -<DD> -C++. This language is understood by C++ compilers. -</DL> -</UL> - -The default is C. - -<DT><SAMP>`-a'</SAMP> -<DD> -This option is supported for compatibility with previous releases of -<CODE>gperf</CODE>. It does not do anything. - -<DT><SAMP>`-g'</SAMP> -<DD> -This option is supported for compatibility with previous releases of -<CODE>gperf</CODE>. It does not do anything. -</DL> -</UL> - - - -<H2><A NAME="SEC17" HREF="gperf_toc.html#TOC17">4.3 Options for fine tuning Details in the Output Code</A></H2> - - -<UL> -<DL COMPACT> - -<DT><SAMP>`-K <VAR>key name</VAR>'</SAMP> -<DD> -This option is only useful when option <SAMP>`-t'</SAMP> has been given. -By default, the program assumes the structure component identifier for -the keyword is <SAMP>`name'</SAMP>. This option allows an arbitrary choice of -identifier for this component, although it still must occur as the first -field in your supplied <CODE>struct</CODE>. - -<DT><SAMP>`-H <VAR>hash function name</VAR>'</SAMP> -<DD> -Allows you to specify the name for the generated hash function. Default -name is <SAMP>`hash'</SAMP>. This option permits the use of two hash tables in the -same file. - -<DT><SAMP>`-N <VAR>lookup function name</VAR>'</SAMP> -<DD> -Allows you to specify the name for the generated lookup function. -Default name is <SAMP>`in_word_set'</SAMP>. This option permits completely automatic -generation of perfect hash functions, especially when multiple generated -hash functions are used in the same application. - -<DT><SAMP>`-Z <VAR>class name</VAR>'</SAMP> -<DD> -This option is only useful when option <SAMP>`-L C++'</SAMP> has been given. -It allows you to specify the name of generated C++ class. Default name is -<CODE>Perfect_Hash</CODE>. - -<DT><SAMP>`-7'</SAMP> -<DD> -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 <CODE>isalnum</CODE> and <CODE>isgraph</CODE> do -<EM>not</EM> guarantee that a character is in this range. Only an explicit -test like <SAMP>`c >= 'A' && c <= 'Z''</SAMP> guarantees this.) This was the -default in earlier versions of <CODE>gperf</CODE>; now the default is to assume -8-bit characters. - -<DT><SAMP>`-c'</SAMP> -<DD> -Generates C code that uses the <CODE>strncmp</CODE> function to perform -string comparisons. The default action is to use <CODE>strcmp</CODE>. - -<DT><SAMP>`-C'</SAMP> -<DD> -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. - -<DT><SAMP>`-E'</SAMP> -<DD> -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 <CODE><jjc@ai.mit.edu></CODE>. - -<DT><SAMP>`-I'</SAMP> -<DD> -Include the necessary system include file, <CODE><string.h></CODE>, 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. - -<DT><SAMP>`-G'</SAMP> -<DD> -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). - -<DT><SAMP>`-W <VAR>hash table array name</VAR>'</SAMP> -<DD> -Allows you to specify the name for the generated array containing the -hash table. Default name is <SAMP>`wordlist'</SAMP>. This option permits the -use of two hash tables in the same file, even when the option <SAMP>`-G'</SAMP> -is given. - -<DT><SAMP>`-S <VAR>total switch statements</VAR>'</SAMP> -<DD> -Causes the generated C code to use a <CODE>switch</CODE> 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 <CODE>switch</CODE> statements are generated. A -value of 1 generates 1 <CODE>switch</CODE> containing all the elements, a -value of 2 generates 2 tables with 1/2 the elements in each -<CODE>switch</CODE>, etc. This is useful since many C compilers cannot -correctly generate code for large <CODE>switch</CODE> statements. This option -was inspired in part by Keith Bostic's original C program. - -<DT><SAMP>`-T'</SAMP> -<DD> -Prevents the transfer of the type declaration to the output file. Use -this option if the type is already defined elsewhere. - -<DT><SAMP>`-p'</SAMP> -<DD> -This option is supported for compatibility with previous releases of -<CODE>gperf</CODE>. It does not do anything. -</DL> -</UL> - - - -<H2><A NAME="SEC18" HREF="gperf_toc.html#TOC18">4.4 Options for changing the Algorithms employed by <CODE>gperf</CODE></A></H2> - - -<UL> -<DL COMPACT> - -<DT><SAMP>`-k <VAR>keys</VAR>'</SAMP> -<DD> -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., <SAMP>`-k 9,4,13,14'</SAMP>; -ranges may be used, e.g., <SAMP>`-k 2-7'</SAMP>; and positions may occur -in any order. Furthermore, the meta-character '*' causes the generated -hash function to consider <STRONG>all</STRONG> character positions in each key, -whereas '$' instructs the hash function to use the "final character" -of a key (this is the only way to use a character position greater than -126, incidentally). - -For instance, the option <SAMP>`-k 1,2,4,6-10,'$''</SAMP> 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. - -<DT><SAMP>`-l'</SAMP> -<DD> -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 <CODE>strcmp</CODE>. -However, using <SAMP>`-l'</SAMP> might greatly increase the size of the -generated C code if the lookup table range is large (which implies that -the switch option <SAMP>`-S'</SAMP> is not enabled), since the length table -contains as many elements as there are entries in the lookup table. - -<DT><SAMP>`-D'</SAMP> -<DD> -Handle keywords whose key position sets hash to duplicate values. -Duplicate hash values occur for two reasons: - - <UL> <LI> -Since <CODE>gperf</CODE> 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. -<LI> +The <CODE>gperf</CODE> utility is tuned to execute quickly, and works quickly +for small to medium size data sets (around 1000 keywords). It is +extremely useful for maintaining perfect hash functions for compiler +keyword sets. Several recent enhancements now enable <CODE>gperf</CODE> to +work efficiently on much larger keyword sets (over 15,000 keywords). +When processing large keyword sets it helps greatly to have over 8 megs +of RAM. + +However, since <CODE>gperf</CODE> does not backtrack no guaranteed solution +occurs on every run. On the other hand, it is usually easy to obtain a +solution by varying the option parameters. In particular, try the +<SAMP>`-r'</SAMP> option, and also try changing the default arguments to the +<SAMP>`-s'</SAMP> and <SAMP>`-j'</SAMP> options. To <EM>guarantee</EM> a solution, use +the <SAMP>`-D'</SAMP> and <SAMP>`-S'</SAMP> options, although the final results are not +likely to be a <EM>perfect</EM> hash function anymore! Finally, use the +<SAMP>`-f'</SAMP> option if you want <CODE>gperf</CODE> to generate the perfect hash +function <EM>fast</EM>, with less emphasis on making it minimal. -Sometimes a set of keys may have the same names, but possess different -attributes. With the -D option <CODE>gperf</CODE> 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, -<CODE>gperf</CODE> helps you out by organizing the output. -</UL> - -Option <SAMP>`-D'</SAMP> 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 <CODE>gperf</CODE> to work on -keyword sets that it otherwise could not handle. - -<DT><SAMP>`-f <VAR>iteration amount</VAR>'</SAMP> -<DD> -Generate the perfect hash function "fast". This decreases <CODE>gperf</CODE>'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 -<SAMP>`-D'</SAMP> and/or <SAMP>`-S'</SAMP> for <EM>large</EM> keyword sets. - -<DT><SAMP>`-i <VAR>initial value</VAR>'</SAMP> -<DD> -Provides an initial <VAR>value</VAR> 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 <SAMP>`-S'</SAMP> is used. Also, -<SAMP>`-i'</SAMP> is overriden when the <SAMP>`-r'</SAMP> option is used. - -<DT><SAMP>`-j <VAR>jump value</VAR>'</SAMP> -<DD> -Affects the "jump value", i.e., how far to advance the -associated character value upon collisions. <VAR>Jump value</VAR> is rounded -up to an odd number, the default is 5. If the <VAR>jump value</VAR> is 0 -<CODE>gperf</CODE> jumps by random amounts. - -<DT><SAMP>`-n'</SAMP> -<DD> -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. - -<DT><SAMP>`-o'</SAMP> -<DD> -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 <EM>very</EM> large using <SAMP>`-o'</SAMP> may -<EM>increase</EM> <CODE>gperf</CODE>'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. - -<DT><SAMP>`-r'</SAMP> -<DD> -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 <CODE>gperf</CODE> has difficultly with a certain keyword set try using -<SAMP>`-r'</SAMP> or <SAMP>`-D'</SAMP>. - -<DT><SAMP>`-s <VAR>size-multiple</VAR>'</SAMP> -<DD> -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 <VAR>size-multiple</VAR> is negative the maximum associated value is -calculated by <EM>dividing</EM> 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 <SAMP>`-S'</SAMP> is <EM>not</EM> 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 -<CODE>gperf</CODE>'s runtime, since it must search through a much larger range -of values. Judicious use of the <SAMP>`-f'</SAMP> option helps alleviate this -overhead, however. -</DL> -</UL> - - - -<H2><A NAME="SEC19" HREF="gperf_toc.html#TOC19">4.5 Informative Output</A></H2> - - -<UL> -<DL COMPACT> +<LI> -<DT><SAMP>`-h'</SAMP> -<DD> -Prints a short summary on the meaning of each program option. Aborts -further program execution. +The size of the generate static keyword array can get <EM>extremely</EM> +large if the input keyword file is large or if the keywords are quite +similar. This tends to slow down the compilation of the generated C +code, and <EM>greatly</EM> inflates the object code size. If this +situation occurs, consider using the <SAMP>`-S'</SAMP> option to reduce data +size, potentially increasing keyword recognition time a negligible +amount. Since many C compilers cannot correctly generated code for +large switch statements it is important to qualify the <VAR>-S</VAR> option +with an appropriate numerical argument that controls the number of +switch statements generated. -<DT><SAMP>`-v'</SAMP> -<DD> -Prints out the current version number. +<LI> -<DT><SAMP>`-d'</SAMP> -<DD> -Enables the debugging option. This produces verbose diagnostics to -"standard error" when <CODE>gperf</CODE> 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 <SAMP>`-d'</SAMP> -option is enabled. -</DL> +The maximum number of key positions selected for a given key has an +arbitrary limit of 126. This restriction should be removed, and if +anyone considers this a problem write me and let me know so I can remove +the constraint. </UL> <P><HR><P> |