NDBM(3-BSD)         RISC/os Reference Manual          NDBM(3-BSD)



NAME
     dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete,
          dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr -
          data base subroutines

SYNOPSIS
     #include <ndbm.h>

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

     DBM *dbmopen(file, flags, mode)
         char *file;
         int flags, mode;

     void dbmclose(db)
         DBM *db;

     datum dbmfetch(db, key)
         DBM *db;
         datum key;

     int dbmstore(db, key, content, flags)
         DBM *db;
         datum key, content;
         int flags;

     int dbmdelete(db, key)
         DBM *db;
         datum key;

     datum dbmfirstkey(db)
         DBM *db;

     datum dbmnextkey(db)
         DBM *db;

     int dbmerror(db)
         DBM *db;

     int dbmclearerr(db)
         DBM *db;

DESCRIPTION
     These functions maintain key/content pairs in a data base.
     The functions will handle very large (a billion blocks)
     databases and will access a keyed item in one or two file
     system accesses.  This package replaces the earlier dbm(3X)
     library, which managed only a single database.




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     keys and contents 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 suf-
     fix.  The second file contains all data and has `.pag' as
     its suffix.

     Before a database can be accessed, it must be opened by
     dbmopen.  This will open and/or create the files file.dir
     and file.pag depending on the flags parameter (see open(2)).

     Once open, the data stored under a key is accessed by
     dbmfetch and data is placed under a key by dbmstore.  The
     flags field can be either DBMINSERT or DBMREPLACE.
     DBMINSERT will only insert new entries into the database
     and will not change an existing entry with the same key.
     DBMREPLACE will replace an existing entry if it has the
     same key.  A key (and its associated contents) is deleted by
     dbmdelete.  A linear pass through all keys in a database
     may be made, in an (apparently) random order, by use of
     dbmfirstkey and dbmnextkey.  dbmfirstkey will return the
     first key in the database.  dbmnextkey will return the next
     key in the database.  This code will traverse the data base:

          for (key = dbm_firstkey(db); key.dptr != NULL; key =
          dbm_nextkey(db))

     dbmerror returns non-zero when an error has occurred read-
     ing or writing the database.  dbmclearerr resets the error
     condition on the named database.

DIAGNOSTICS
     All functions that return an int indicate errors with nega-
     tive values.  A zero return indicates ok.  Routines that
     return a datum indicate errors with a null (0) dptr. If
     dbmstore called with a flags value of DBMINSERT finds an
     existing entry with the same key it returns 1.

ERRORS
     The `.pag' file will contain holes so that its apparent size
     is about four times its actual content.  Older UNIX systems
     may create real file blocks for these holes when touched.
     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/content pair must not exceed
     the internal block size (currently 4096 bytes).  Moreover
     all key/content pairs that hash together must fit on a



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NDBM(3-BSD)         RISC/os Reference Manual          NDBM(3-BSD)



     single block.  dbmstore will return an error in the event
     that a disk block fills with inseparable data.

     dbmdelete does not physically reclaim file space, although
     it does make it available for reuse.

     The order of keys presented by dbmfirstkey and dbmnextkey
     depends on a hashing function, not on anything interesting.

SEE ALSO
     dbm(3X)












































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