ndbm(NS)                       6 January 1993                       ndbm(NS)


 Name

    ndbm, dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete,
    dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr - database subroutines

 Syntax


    #include<ndbm.h>

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

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

    void dbm_close (db)
    DBM*db;

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

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

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

    datum dbm_firstkey(db)
    DBM*db;

    datum dbm_nextkey(db)
    DBM*db;

    int dbm_error(db)
    DBM *db;

    int dbm_clearerr(db)
    DBM *db;


 Description

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

    keys and contents are described by the datum typedef.  A datum specifies
    a string of dsize bytes to which dptr points.  Arbitrary binary data, as
    well as normal ASCII strings, are allowed.  The database 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 dbmopen.  This
    will open and/or create the files file.dir and file.pag depending on the
    flags parameter (see open(S)).

    A database is closed by calling dbmclose.

    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 reading or writ-
    ing the database.  dbmclearerr() resets the error condition on the named
    database.

 See also

    ar(CP), cat(C), cp(C), dbm(NS) open(S) and tar(C).

 Diagnostics

    All functions that return an int indicate errors with negative values.  A
    zero return indicates no error.  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.

 Bugs

    The .pag file will contain holes so that its apparent size is about four
    times its actual content.  Older versions of the UNIX operating system
    may create real file blocks for these holes when touched.  These files
    cannot be copied by normal means (cp(C), cat(C), tar(C), ar(C)) 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 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.

    There are no interlocks and no reliable cache flushing; thus concurrent
    updating and reading is risky.