$home/manuals/unix_v8/3/dbm

DBM(3X)                                                                DBM(3X)

NAME
       dbminit, fetch, store, delete, firstkey, nextkey - database subroutines

SYNOPSIS
       typedef struct {
            char *dptr;
            int dsize;
       } datum;

       dbminit(file)
       char *file;

       datum fetch(key)
       datum key;

       store(key, value)
       datum key, value;

       delete(key)
       datum key;

       datum firstkey()

       datum nextkey(key)
       datum key;

DESCRIPTION
       These functions maintain key/value pairs in a data base.  The functions
       will handle very large databases in one or two file system accesses per
       key.  The functions are loaded with ld(1) option -ldbm.

       Keys  and values are described by the datum typedef.  A datum specifies
       a string of dsize bytes pointed to by dptr.  Arbitrary binary data,  as
       well  as normal ASCII strings, are allowed.  The data base is stored in
       two files.  One file is a directory containing a bit map and has ‘.dir'
       as its suffix.  The second file contains all data and has ‘.pag' as its
       suffix.

       Before a database can be accessed, it must be opened  by  dbminit.   At
       the time of this call, the files file.dir and file.pag must exist.  (An
       empty  database  is  created  by creating zero-length ‘.dir' and ‘.pag'
       files.)

       Once open, the data stored under a key is accessed by fetch and data is
       placed under a key by store.   A  key  and  its  associated  value  are
       deleted by delete.  A linear pass through all keys in a database may be
       made,  in  random order, by use of firstkey and nextkey.  Firstkey will
       return the first key in the database.  With any key nextkey will return
       the next key in the database.  This code will traverse the data base:

              for(key = firstkey(); key.dptr != NULL; key = nextkey(key))

DIAGNOSTICS
       All functions that return an int indicate errors with negative  values.
       A zero return indicates success.  Routines that return a datum indicate
       errors with a null (0) dptr.

SEE ALSO
       cbt(3)

BUGS
       The  ‘.pag'  file will contain holes so that its apparent size is about
       four times its actual content.  These files cannot be copied by  normal
       means (cp, cat, tp, tar, ar) without filling in the holes.
       Dptr  pointers  returned by these subroutines point into static storage
       that is changed by subsequent calls.
       The sum of the sizes of a key/value pair must not exceed a fixed inter‐
       nal block size.  Moreover all key/value pairs that hash  together  must
       fit  on a single block.  Store will return an error in the event that a
       disk block fills with inseparable data.
       Delete does not physically reclaim file space, although it does make it
       available for reuse.

                                                                       DBM(3X)