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Bruno Haible
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<HTML>
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<!-- This HTML file has been created by texi2html 1.51
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from gperf.texi on 15 April 1998 -->
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<TITLE>User's Guide to gperf - 4 Options to the gperf Utility</TITLE>
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</HEAD>
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<BODY>
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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>.
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<P><HR><P>
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<H1><A NAME="SEC14" HREF="gperf_toc.html#TOC14">4 Options to the <CODE>gperf</CODE> Utility</A></H1>
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<P>
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There are <EM>many</EM> options to <CODE>gperf</CODE>. They were added to make
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the program more convenient for use with real applications. "On-line"
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help is readily available via the <SAMP>`-h'</SAMP> option. Here is the complete
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list of options.
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</P>
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<H2><A NAME="SEC15" HREF="gperf_toc.html#TOC15">4.1 Options that affect Interpretation of the Input File</A></H2>
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<UL>
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<DL COMPACT>
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<DT><SAMP>`-e <VAR>keyword delimiter list</VAR>'</SAMP>
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<DD>
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Allows the user to provide a string containing delimiters used to
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separate keywords from their attributes. The default is ",\n". This
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option is essential if you want to use keywords that have embedded
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commas or newlines. One useful trick is to use -e'TAB', where TAB is
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the literal tab character.
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<DT><SAMP>`-t'</SAMP>
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<DD>
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Allows you to include a <CODE>struct</CODE> type declaration for generated
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code. Any text before a pair of consecutive %% is consider part of the
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type declaration. Key words and additional fields may follow this, one
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group of fields per line. A set of examples for generating perfect hash
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tables and functions for Ada, C, and G++, Pascal, and Modula 2 and 3
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reserved words are distributed with this release.
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</DL>
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</UL>
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<H2><A NAME="SEC16" HREF="gperf_toc.html#TOC16">4.2 Options to specify the Language for the Output Code</A></H2>
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<UL>
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<DL COMPACT>
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<DT><SAMP>`-L <VAR>generated language name</VAR>'</SAMP>
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<DD>
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Instructs <CODE>gperf</CODE> to generate code in the language specified by the
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option's argument. Languages handled are currently:
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<UL>
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<DL COMPACT>
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<DT><SAMP>`KR-C'</SAMP>
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<DD>
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Old-style K&R C. This language is understood by old-style C compilers and
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ANSI C compilers, but ANSI C compilers may flag warnings (or even errors)
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because of lacking <SAMP>`const'</SAMP>.
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<DT><SAMP>`C'</SAMP>
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<DD>
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Common C. This language is understood by ANSI C compilers, and also by
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old-style C compilers, provided that you <CODE>#define const</CODE> to empty
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for compilers which don't know about this keyword.
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<DT><SAMP>`ANSI-C'</SAMP>
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<DD>
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ANSI C. This language is understood by ANSI C compilers and C++ compilers.
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<DT><SAMP>`C++'</SAMP>
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<DD>
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C++. This language is understood by C++ compilers.
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</DL>
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</UL>
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The default is C.
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<DT><SAMP>`-a'</SAMP>
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<DD>
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This option is supported for compatibility with previous releases of
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<CODE>gperf</CODE>. It does not do anything.
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<DT><SAMP>`-g'</SAMP>
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<DD>
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This option is supported for compatibility with previous releases of
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<CODE>gperf</CODE>. It does not do anything.
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</DL>
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</UL>
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<H2><A NAME="SEC17" HREF="gperf_toc.html#TOC17">4.3 Options for fine tuning Details in the Output Code</A></H2>
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<UL>
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<DL COMPACT>
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<DT><SAMP>`-K <VAR>key name</VAR>'</SAMP>
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<DD>
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This option is only useful when option <SAMP>`-t'</SAMP> has been given.
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By default, the program assumes the structure component identifier for
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the keyword is <SAMP>`name'</SAMP>. This option allows an arbitrary choice of
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identifier for this component, although it still must occur as the first
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field in your supplied <CODE>struct</CODE>.
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<DT><SAMP>`-H <VAR>hash function name</VAR>'</SAMP>
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<DD>
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Allows you to specify the name for the generated hash function. Default
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name is <SAMP>`hash'</SAMP>. This option permits the use of two hash tables in the
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same file.
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<DT><SAMP>`-N <VAR>lookup function name</VAR>'</SAMP>
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<DD>
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Allows you to specify the name for the generated lookup function.
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Default name is <SAMP>`in_word_set'</SAMP>. This option permits completely automatic
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generation of perfect hash functions, especially when multiple generated
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hash functions are used in the same application.
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<DT><SAMP>`-Z <VAR>class name</VAR>'</SAMP>
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<DD>
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This option is only useful when option <SAMP>`-L C++'</SAMP> has been given.
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It allows you to specify the name of generated C++ class. Default name is
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<CODE>Perfect_Hash</CODE>.
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<DT><SAMP>`-7'</SAMP>
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<DD>
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This option specifies that all strings that will be passed as arguments
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to the generated hash function and the generated lookup function will
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solely consist of 7-bit ASCII characters (characters in the range 0..127).
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(Note that the ANSI C functions <CODE>isalnum</CODE> and <CODE>isgraph</CODE> do
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<EM>not</EM> guarantee that a character is in this range. Only an explicit
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test like <SAMP>`c >= 'A' && c <= 'Z''</SAMP> guarantees this.) This was the
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default in earlier versions of <CODE>gperf</CODE>; now the default is to assume
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8-bit characters.
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<DT><SAMP>`-c'</SAMP>
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<DD>
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Generates C code that uses the <CODE>strncmp</CODE> function to perform
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string comparisons. The default action is to use <CODE>strcmp</CODE>.
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<DT><SAMP>`-C'</SAMP>
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<DD>
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Makes the contents of all generated lookup tables constant, i.e.,
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"readonly". Many compilers can generate more efficient code for this
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by putting the tables in readonly memory.
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<DT><SAMP>`-E'</SAMP>
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<DD>
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Define constant values using an enum local to the lookup function rather
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than with #defines. This also means that different lookup functions can
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reside in the same file. Thanks to James Clark <CODE><jjc@ai.mit.edu></CODE>.
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<DT><SAMP>`-I'</SAMP>
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<DD>
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Include the necessary system include file, <CODE><string.h></CODE>, at the
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beginning of the code. By default, this is not done; the user must
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include this header file himself to allow compilation of the code.
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<DT><SAMP>`-G'</SAMP>
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<DD>
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Generate the static table of keywords as a static global variable,
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rather than hiding it inside of the lookup function (which is the
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default behavior).
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<DT><SAMP>`-W <VAR>hash table array name</VAR>'</SAMP>
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<DD>
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Allows you to specify the name for the generated array containing the
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hash table. Default name is <SAMP>`wordlist'</SAMP>. This option permits the
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use of two hash tables in the same file, even when the option <SAMP>`-G'</SAMP>
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is given.
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<DT><SAMP>`-S <VAR>total switch statements</VAR>'</SAMP>
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<DD>
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Causes the generated C code to use a <CODE>switch</CODE> statement scheme,
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rather than an array lookup table. This can lead to a reduction in both
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time and space requirements for some keyfiles. The argument to this
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option determines how many <CODE>switch</CODE> statements are generated. A
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value of 1 generates 1 <CODE>switch</CODE> containing all the elements, a
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value of 2 generates 2 tables with 1/2 the elements in each
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<CODE>switch</CODE>, etc. This is useful since many C compilers cannot
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correctly generate code for large <CODE>switch</CODE> statements. This option
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was inspired in part by Keith Bostic's original C program.
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<DT><SAMP>`-T'</SAMP>
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<DD>
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Prevents the transfer of the type declaration to the output file. Use
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this option if the type is already defined elsewhere.
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<DT><SAMP>`-p'</SAMP>
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<DD>
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This option is supported for compatibility with previous releases of
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<CODE>gperf</CODE>. It does not do anything.
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</DL>
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</UL>
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<H2><A NAME="SEC18" HREF="gperf_toc.html#TOC18">4.4 Options for changing the Algorithms employed by <CODE>gperf</CODE></A></H2>
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<UL>
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<DL COMPACT>
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<DT><SAMP>`-k <VAR>keys</VAR>'</SAMP>
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<DD>
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Allows selection of the character key positions used in the keywords'
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hash function. The allowable choices range between 1-126, inclusive.
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The positions are separated by commas, e.g., <SAMP>`-k 9,4,13,14'</SAMP>;
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ranges may be used, e.g., <SAMP>`-k 2-7'</SAMP>; and positions may occur
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in any order. Furthermore, the meta-character '*' causes the generated
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hash function to consider <STRONG>all</STRONG> character positions in each key,
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whereas '$' instructs the hash function to use the "final character"
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of a key (this is the only way to use a character position greater than
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126, incidentally).
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For instance, the option <SAMP>`-k 1,2,4,6-10,'$''</SAMP> generates a hash
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function that considers positions 1,2,4,6,7,8,9,10, plus the last
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character in each key (which may differ for each key, obviously). Keys
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with length less than the indicated key positions work properly, since
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selected key positions exceeding the key length are simply not
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referenced in the hash function.
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<DT><SAMP>`-l'</SAMP>
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<DD>
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Compare key lengths before trying a string comparison. This might cut
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down on the number of string comparisons made during the lookup, since
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keys with different lengths are never compared via <CODE>strcmp</CODE>.
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However, using <SAMP>`-l'</SAMP> might greatly increase the size of the
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generated C code if the lookup table range is large (which implies that
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the switch option <SAMP>`-S'</SAMP> is not enabled), since the length table
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contains as many elements as there are entries in the lookup table.
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<DT><SAMP>`-D'</SAMP>
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<DD>
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Handle keywords whose key position sets hash to duplicate values.
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Duplicate hash values occur for two reasons:
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<UL>
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<LI>
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Since <CODE>gperf</CODE> does not backtrack it is possible for it to process
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all your input keywords without finding a unique mapping for each word.
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However, frequently only a very small number of duplicates occur, and
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the majority of keys still require one probe into the table.
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<LI>
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Sometimes a set of keys may have the same names, but possess different
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attributes. With the -D option <CODE>gperf</CODE> treats all these keys as part of
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an equivalence class and generates a perfect hash function with multiple
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comparisons for duplicate keys. It is up to you to completely
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disambiguate the keywords by modifying the generated C code. However,
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<CODE>gperf</CODE> helps you out by organizing the output.
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</UL>
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Option <SAMP>`-D'</SAMP> is extremely useful for certain large or highly
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redundant keyword sets, e.g., assembler instruction opcodes.
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Using this option usually means that the generated hash function is no
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longer perfect. On the other hand, it permits <CODE>gperf</CODE> to work on
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keyword sets that it otherwise could not handle.
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<DT><SAMP>`-f <VAR>iteration amount</VAR>'</SAMP>
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<DD>
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Generate the perfect hash function "fast". This decreases <CODE>gperf</CODE>'s
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running time at the cost of minimizing generated table-size. The
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iteration amount represents the number of times to iterate when
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resolving a collision. `0' means iterate by the number of keywords.
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This option is probably most useful when used in conjunction with options
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<SAMP>`-D'</SAMP> and/or <SAMP>`-S'</SAMP> for <EM>large</EM> keyword sets.
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<DT><SAMP>`-i <VAR>initial value</VAR>'</SAMP>
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<DD>
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Provides an initial <VAR>value</VAR> for the associate values array. Default
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is 0. Increasing the initial value helps inflate the final table size,
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possibly leading to more time efficient keyword lookups. Note that this
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option is not particularly useful when <SAMP>`-S'</SAMP> is used. Also,
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<SAMP>`-i'</SAMP> is overriden when the <SAMP>`-r'</SAMP> option is used.
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<DT><SAMP>`-j <VAR>jump value</VAR>'</SAMP>
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<DD>
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Affects the "jump value", i.e., how far to advance the
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associated character value upon collisions. <VAR>Jump value</VAR> is rounded
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up to an odd number, the default is 5. If the <VAR>jump value</VAR> is 0
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<CODE>gperf</CODE> jumps by random amounts.
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<DT><SAMP>`-n'</SAMP>
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<DD>
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Instructs the generator not to include the length of a keyword when
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computing its hash value. This may save a few assembly instructions in
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the generated lookup table.
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<DT><SAMP>`-o'</SAMP>
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<DD>
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Reorders the keywords by sorting the keywords so that frequently
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occuring key position set components appear first. A second reordering
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pass follows so that keys with "already determined values" are placed
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towards the front of the keylist. This may decrease the time required
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to generate a perfect hash function for many keyword sets, and also
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produce more minimal perfect hash functions. The reason for this is
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that the reordering helps prune the search time by handling inevitable
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collisions early in the search process. On the other hand, if the
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number of keywords is <EM>very</EM> large using <SAMP>`-o'</SAMP> may
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<EM>increase</EM> <CODE>gperf</CODE>'s execution time, since collisions will begin
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earlier and continue throughout the remainder of keyword processing.
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See Cichelli's paper from the January 1980 Communications of the ACM for
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details.
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<DT><SAMP>`-r'</SAMP>
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<DD>
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Utilizes randomness to initialize the associated values table. This
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frequently generates solutions faster than using deterministic
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initialization (which starts all associated values at 0). Furthermore,
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using the randomization option generally increases the size of the
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table. If <CODE>gperf</CODE> has difficultly with a certain keyword set try using
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<SAMP>`-r'</SAMP> or <SAMP>`-D'</SAMP>.
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<DT><SAMP>`-s <VAR>size-multiple</VAR>'</SAMP>
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<DD>
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Affects the size of the generated hash table. The numeric argument for
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this option indicates "how many times larger or smaller" the maximum
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associated value range should be, in relationship to the number of keys.
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If the <VAR>size-multiple</VAR> is negative the maximum associated value is
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calculated by <EM>dividing</EM> it into the total number of keys. For
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example, a value of 3 means "allow the maximum associated value to be
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about 3 times larger than the number of input keys".
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Conversely, a value of -3 means "allow the maximum associated value to
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be about 3 times smaller than the number of input keys". Negative
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values are useful for limiting the overall size of the generated hash
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table, though this usually increases the number of duplicate hash
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values.
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If `generate switch' option <SAMP>`-S'</SAMP> is <EM>not</EM> enabled, the maximum
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associated value influences the static array table size, and a larger
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table should decrease the time required for an unsuccessful search, at
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the expense of extra table space.
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The default value is 1, thus the default maximum associated value about
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the same size as the number of keys (for efficiency, the maximum
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associated value is always rounded up to a power of 2). The actual
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table size may vary somewhat, since this technique is essentially a
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heuristic. In particular, setting this value too high slows down
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<CODE>gperf</CODE>'s runtime, since it must search through a much larger range
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of values. Judicious use of the <SAMP>`-f'</SAMP> option helps alleviate this
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overhead, however.
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</DL>
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</UL>
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<H2><A NAME="SEC19" HREF="gperf_toc.html#TOC19">4.5 Informative Output</A></H2>
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||||
<UL>
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||||
<DL COMPACT>
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||||
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<DT><SAMP>`-h'</SAMP>
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<DD>
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Prints a short summary on the meaning of each program option. Aborts
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further program execution.
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<DT><SAMP>`-v'</SAMP>
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<DD>
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Prints out the current version number.
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<DT><SAMP>`-d'</SAMP>
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<DD>
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Enables the debugging option. This produces verbose diagnostics to
|
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"standard error" when <CODE>gperf</CODE> is executing. It is useful both for
|
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maintaining the program and for determining whether a given set of
|
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options is actually speeding up the search for a solution. Some useful
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||||
information is dumped at the end of the program when the <SAMP>`-d'</SAMP>
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option is enabled.
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</DL>
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||||
</UL>
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||||
<P><HR><P>
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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>.
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</BODY>
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||||
</HTML>
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||||
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