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   28 .\" Footnote numbering [by Henry Spencer]
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   39 .ND
   40 .LP
   41 .TL
   42 \fIsdbm\fP \(em Substitute DBM
   43 .br
   44 or
   45 .br
   46 Berkeley \fIndbm\fP for Every UN*X\** Made Simple
   47 .AU
   48 Ozan (oz) Yigit
   49 .AI
   50 The Guild of PD Software Toolmakers
   51 Toronto - Canada
   52 .sp
   53 oz@nexus.yorku.ca
   54 .LP
   55 .FS
   56 UN*X is not a trademark of any (dis)organization.
   57 .FE
   58 .sp 2
   59 \fIImplementation is the sincerest form of flattery. \(em L. Peter Deutsch\fP
   60 .SH
   61 A The Clone of the \fIndbm\fP library
   62 .PP
   63 The sources accompanying this notice \(em \fIsdbm\fP \(em constitute
   64 the first public release (Dec. 1990) of a complete clone of
   65 the Berkeley UN*X \fIndbm\fP library. The \fIsdbm\fP library is meant to
   66 clone the proven functionality of \fIndbm\fP as closely as possible,
   67 including a few improvements. It is practical, easy to understand, and
   68 compatible.
   69 The \fIsdbm\fP library is not derived from any licensed, proprietary or
   70 copyrighted software.
   71 .PP
   72 The \fIsdbm\fP implementation is based on a 1978 algorithm
   73 [Lar78] by P.-A. (Paul) Larson known as "Dynamic Hashing".
   74 In the course of searching for a substitute for \fIndbm\fP, I
   75 prototyped three different external-hashing algorithms [Lar78, Fag79, Lit80]
   76 and ultimately chose Larson's algorithm as a basis of the \fIsdbm\fP
   77 implementation. The Bell Labs
   78 \fIdbm\fP (and therefore \fIndbm\fP) is based on an algorithm invented by
   79 Ken Thompson, [Tho90, Tor87] and predates Larson's work.
   80 .PP
   81 The \fIsdbm\fR programming interface is totally compatible
   82 with \fIndbm\fP and includes a slight improvement in database initialization.
   83 It is also expected to be binary-compatible under most UN*X versions that
   84 support the \fIndbm\fP library.
   85 .PP
   86 The \fIsdbm\fP implementation shares the shortcomings of the \fIndbm\fP
   87 library, as a side effect of various simplifications to the original Larson
   88 algorithm. It does produce \fIholes\fP in the page file as it writes
   89 pages past the end of file. (Larson's paper include a clever solution to
   90 this problem that is a result of using the hash value directly as a block
   91 address.) On the other hand, extensive tests seem to indicate that \fIsdbm\fP
   92 creates fewer holes in general, and the resulting pagefiles are
   93 smaller. The \fIsdbm\fP implementation is also faster than \fIndbm\fP
   94 in database creation.
   95 Unlike the \fIndbm\fP, the \fIsdbm\fP
   96 .CW store
   97 operation will not "wander away" trying to split its
   98 data pages to insert a datum that \fIcannot\fP (due to elaborate worst-case
   99 situations) be inserted. (It will fail after a pre-defined number of attempts.)
  100 .SH
  101 Important Compatibility Warning
  102 .PP
  103 The \fIsdbm\fP and \fIndbm\fP
  104 libraries \fIcannot\fP share databases: one cannot read the (dir/pag)
  105 database created by the other. This is due to the differences
  106 between the \fIndbm\fP and \fIsdbm\fP algorithms\**, 
  107 .FS
  108 Torek's discussion [Tor87]
  109 indicates that \fIdbm/ndbm\fP implementations use the hash
  110 value to traverse the radix trie differently than \fIsdbm\fP
  111 and as a result, the page indexes are generated in \fIdifferent\fP order.
  112 For more information, send e-mail to the author.
  113 .FE
  114 and the hash functions
  115 used.
  116 It is easy to convert between the \fIdbm/ndbm\fP databases and \fIsdbm\fP
  117 by ignoring the index completely: see
  118 .CW dbd ,
  119 .CW dbu
  120 etc.
  121 .R
  122 .LP
  123 .SH
  124 Notice of Intellectual Property
  125 .LP
  126 \fIThe entire\fP sdbm  \fIlibrary package, as authored by me,\fP Ozan S. Yigit,
  127 \fIis hereby placed in the public domain.\fP As such, the author is not
  128 responsible for the consequences of use of this software, no matter how
  129 awful, even if they arise from defects in it. There is no expressed or
  130 implied warranty for the \fIsdbm\fP library.
  131 .PP
  132 Since the \fIsdbm\fP
  133 library package is in the public domain, this \fIoriginal\fP
  134 release or any additional public-domain releases of the modified original
  135 cannot possibly (by definition) be withheld from you. Also by definition,
  136 You (singular) have all the rights to this code (including the right to
  137 sell without permission, the right to hoard\**
  138 .FS
  139 You cannot really hoard something that is available to the public at
  140 large, but try if it makes you feel any better.
  141 .FE
  142 and the right to do other icky things as
  143 you see fit) but those rights are also granted to everyone else.
  144 .PP
  145 Please note that all previous distributions of this software contained
  146 a copyright (which is now dropped) to protect its
  147 origins and its current public domain status against any possible claims
  148 and/or challenges.
  149 .SH
  150 Acknowledgments
  151 .PP
  152 Many people have been very helpful and supportive.  A partial list would
  153 necessarily include Rayan Zacherissen (who contributed the man page,
  154 and also hacked a MMAP version of \fIsdbm\fP),
  155 Arnold Robbins, Chris Lewis,
  156 Bill Davidsen, Henry Spencer, Geoff Collyer, Rich Salz (who got me started
  157 in the first place), Johannes Ruschein
  158 (who did the minix port) and David Tilbrook. I thank you all.
  159 .SH
  160 Distribution Manifest and Notes
  161 .LP
  162 This distribution of \fIsdbm\fP includes (at least) the following:
  163 .P1
  164     CHANGES     change log
  165     README      this file.
  166     biblio      a small bibliography on external hashing
  167     dba.c       a crude (n/s)dbm page file analyzer
  168     dbd.c       a crude (n/s)dbm page file dumper (for conversion)
  169     dbe.1       man page for dbe.c
  170     dbe.c       Janick's database editor
  171     dbm.c       a dbm library emulation wrapper for ndbm/sdbm
  172     dbm.h       header file for the above
  173     dbu.c       a crude db management utility
  174     hash.c      hashing function
  175     makefile    guess.
  176     pair.c      page-level routines (posted earlier)
  177     pair.h      header file for the above
  178     readme.ms   troff source for the README file
  179     sdbm.3      man page
  180     sdbm.c      the real thing
  181     sdbm.h      header file for the above
  182     tune.h      place for tuning & portability thingies
  183     util.c      miscellaneous
  184 .P2
  185 .PP
  186 .CW dbu
  187 is a simple database manipulation program\** that tries to look
  188 .FS
  189 The 
  190 .CW dbd ,
  191 .CW dba ,
  192 .CW dbu
  193 utilities are quick hacks and are not fit for production use. They were
  194 developed late one night, just to test out \fIsdbm\fP, and convert some
  195 databases.
  196 .FE
  197 like Bell Labs'
  198 .CW cbt
  199 utility. It is currently incomplete in functionality.
  200 I use
  201 .CW dbu
  202 to test out the routines: it takes (from stdin) tab separated
  203 key/value pairs for commands like
  204 .CW build
  205 or
  206 .CW insert
  207 or takes keys for
  208 commands like
  209 .CW delete
  210 or
  211 .CW look .
  212 .P1
  213     dbu <build|creat|look|insert|cat|delete> dbmfile
  214 .P2
  215 .PP
  216 .CW dba
  217 is a crude analyzer of \fIdbm/sdbm/ndbm\fP
  218 page files. It scans the entire
  219 page file, reporting page level statistics, and totals at the end.
  220 .PP
  221 .CW dbd
  222 is a crude dump program for \fIdbm/ndbm/sdbm\fP
  223 databases. It ignores the
  224 bitmap, and dumps the data pages in sequence. It can be used to create
  225 input for the
  226 .CW dbu 
  227 utility.
  228 Note that
  229 .CW dbd
  230 will skip any NULLs in the key and data
  231 fields, thus is unsuitable to convert some peculiar databases that
  232 insist in including the terminating null.
  233 .PP
  234 I have also included a copy of the
  235 .CW dbe
  236 (\fIndbm\fP DataBase Editor) by Janick Bergeron [janick@bnr.ca] for
  237 your pleasure. You may find it more useful than the little
  238 .CW dbu
  239 utility.
  240 .PP
  241 .CW dbm.[ch]
  242 is a \fIdbm\fP library emulation on top of \fIndbm\fP
  243 (and hence suitable for \fIsdbm\fP). Written by Robert Elz.
  244 .PP
  245 The \fIsdbm\fP
  246 library has been around in beta test for quite a long time, and from whatever
  247 little feedback I received (maybe no news is good news), I believe it has been
  248 functioning without any significant problems. I would, of course, appreciate
  249 all fixes and/or improvements. Portability enhancements would especially be
  250 useful.
  251 .SH
  252 Implementation Issues
  253 .PP
  254 Hash functions:
  255 The algorithm behind \fIsdbm\fP implementation needs a good bit-scrambling
  256 hash function to be effective. I ran into a set of constants for a simple
  257 hash function that seem to help \fIsdbm\fP perform better than \fIndbm\fP
  258 for various inputs:
  259 .P1
  260     /*
  261      * polynomial conversion ignoring overflows
  262      * 65599 nice. 65587 even better.
  263      */
  264     long
  265     dbm_hash(char *str, int len) {
  266         unsigned long n = 0;
  268         while (len--)
  269             n = n * 65599 + *str++;
  270         return n;
  271     }
  272 .P2
  273 .PP
  274 There may be better hash functions for the purposes of dynamic hashing.
  275 Try your favorite, and check the pagefile. If it contains too many pages
  276 with too many holes, (in relation to this one for example) or if
  277 \fIsdbm\fP
  278 simply stops working (fails after 
  280 attempts to split) when you feed your
  281 NEWS 
  282 .CW history
  283 file to it, you probably do not have a good hashing function.
  284 If you do better (for different types of input), I would like to know
  285 about the function you use.
  286 .PP
  287 Block sizes: It seems (from various tests on a few machines) that a page
  288 file block size
  290 of 1024 is by far the best for performance, but
  291 this also happens to limit the size of a key/value pair. Depending on your
  292 needs, you may wish to increase the page size, and also adjust
  294 (the maximum size of a key/value pair allowed: should always be at least
  295 three words smaller than
  296 .CW PBLKSIZ .)
  297 accordingly. The system-wide version of the library
  298 should probably be
  299 configured with 1024 (distribution default), as this appears to be sufficient
  300 for most common uses of \fIsdbm\fP.
  301 .SH
  302 Portability
  303 .PP
  304 This package has been tested in many different UN*Xes even including minix,
  305 and appears to be reasonably portable. This does not mean it will port
  306 easily to non-UN*X systems.
  307 .SH
  308 Notes and Miscellaneous
  309 .PP
  310 The \fIsdbm\fP is not a very complicated package, at least not after you
  311 familiarize yourself with the literature on external hashing. There are
  312 other interesting algorithms in existence that ensure (approximately)
  313 single-read access to a data value associated with any key. These are
  314 directory-less schemes such as \fIlinear hashing\fP [Lit80] (+ Larson
  315 variations), \fIspiral storage\fP [Mar79] or directory schemes such as
  316 \fIextensible hashing\fP [Fag79] by Fagin et al. I do hope these sources
  317 provide a reasonable playground for experimentation with other algorithms.
  318 See the June 1988 issue of ACM Computing Surveys [Enb88] for an
  319 excellent overview of the field. 
  320 .PG
  321 .SH
  322 References
  323 .LP
  324 .IP [Lar78] 4m
  325 P.-A. Larson,
  326 "Dynamic Hashing", \fIBIT\fP, vol.  18,  pp. 184-201, 1978.
  327 .IP [Tho90] 4m
  328 Ken Thompson, \fIprivate communication\fP, Nov. 1990
  329 .IP [Lit80] 4m
  330 W. Litwin,
  331 "Linear Hashing: A new tool  for  file  and table addressing",
  332 \fIProceedings of the 6th Conference on Very Large  Dabatases  (Montreal)\fP,
  333 pp.  212-223,  Very Large Database Foundation, Saratoga, Calif., 1980.
  334 .IP [Fag79] 4m
  335 R. Fagin, J.  Nievergelt,  N.  Pippinger,  and  H.  R. Strong,
  336 "Extendible Hashing - A Fast Access Method for Dynamic Files",
  337 \fIACM Trans. Database Syst.\fP, vol. 4,  no.3, pp. 315-344, Sept. 1979.
  338 .IP [Wal84] 4m
  339 Rich Wales,
  340 "Discussion of 'dbm' data base system", \fIUSENET newsgroup unix.wizards\fP,
  341 Jan. 1984.
  342 .IP [Tor87] 4m
  343 Chris Torek,
  344 "Re:  dbm.a  and  ndbm.a  archives", \fIUSENET newsgroup comp.unix\fP,
  345 1987.
  346 .IP [Mar79] 4m
  347 G. N. Martin,
  348 "Spiral Storage: Incrementally  Augmentable  Hash  Addressed  Storage",
  349 \fITechnical Report #27\fP, University of Varwick, Coventry, U.K., 1979.
  350 .IP [Enb88] 4m
  351 R. J. Enbody and H. C. Du,
  352 "Dynamic Hashing  Schemes",\fIACM Computing Surveys\fP,
  353 vol. 20, no. 2, pp. 85-113, June 1988.