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<H1>NDBM</H1>
Section: C Library Functions (3)<BR>Updated: May 20, 1986<BR><A HREF="#index">Index</A>
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<A NAME="lbAB"> </A>
<H2>NAME</H2>
dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr - data base subroutines
<A NAME="lbAC"> </A>
<H2>SYNOPSIS</H2>
<PRE>
<B>#include <<A HREF="file:/usr/include/ndbm.h">ndbm.h</A>>
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;
</B></PRE><A NAME="lbAD"> </A>
<H2>DESCRIPTION</H2>
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
<I><A HREF="http://localhost/cgi-bin/man/man2html?3x+dbm">dbm</A></I>(3x)
library, which managed only a single database.
<P>
<I>Key</I>s
and
<I>content</I>s
are described by the
<I>datum</I>
typedef. A
<I>datum</I>
specifies a string of
<I>dsize</I>
bytes pointed to by
<I>dptr.</I>
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.
<P>
Before a database can be accessed, it must be opened by
<I>dbm_open</I>.
This will open and/or create the files
<I>file</I><B>.dir</B>
and
<I>file</I><B>.pag</B>
depending on the flags parameter (see
<I><A HREF="http://localhost/cgi-bin/man/man2html?2+open">open</A></I>(2)).
<P>
Once open, the data stored under a key is accessed by
<I>dbm_fetch</I>
and data is placed under a key by
<I>dbm_store</I>.
The
<I>flags</I>
field can be either
<B>DBM_INSERT</B>
or
<B>DBM_REPLACE.</B>
<B>DBM_INSERT</B>
will only insert new entries into the database and will not
change an existing entry with the same key.
<B>DBM_REPLACE</B>
will replace an existing entry if it has the same key.
A key (and its associated contents) is deleted by
<I>dbm_delete</I>.
A linear pass through all keys in a database may be made,
in an (apparently) random order, by use of
<I>dbm_firstkey</I>
and
<I>dbm_nextkey</I>.
<I>Dbm_firstkey</I>
will return the first key in the database.
<I>Dbm_nextkey</I>
will return the next key in the database.
This code will traverse the data base:
<DL COMPACT>
<DT><DD>
<B>for</B>
(key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))
</DL>
<P>
<I>Dbm_error</I>
returns non-zero when an error has occurred reading or writing the database.
<I>Dbm_clearerr</I>
resets the error condition on the named database.
<A NAME="lbAE"> </A>
<H2>DIAGNOSTICS</H2>
All functions that return an
<I>int</I>
indicate errors with negative values. A zero return indicates ok.
Routines that return a
<I>datum</I>
indicate errors with a null (0)
<I>dptr.</I>
If
<I>dbm_store</I>
called with a
<I>flags</I>
value of
<B>DBM_INSERT</B>
finds an existing entry with the same key
it returns 1.
<A NAME="lbAF"> </A>
<H2>BUGS</H2>
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.
<P>
<I>Dptr</I>
pointers returned by these subroutines point into static storage
that is changed by subsequent calls.
<P>
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.
<I>Dbm_store</I>
will return an error in the event that a disk block fills with inseparable data.
<P>
<I>Dbm_delete</I>
does not physically reclaim file space,
although it does make it available for reuse.
<P>
The order of keys presented by
<I>dbm_firstkey</I>
and
<I>dbm_nextkey</I>
depends on a hashing function, not on anything interesting.
<A NAME="lbAG"> </A>
<H2>SEE ALSO</H2>
<A HREF="http://localhost/cgi-bin/man/man2html?3X+dbm">dbm</A>(3X)
<P>
<HR>
<A NAME="index"> </A><H2>Index</H2>
<DL>
<DT><A HREF="#lbAB">NAME</A><DD>
<DT><A HREF="#lbAC">SYNOPSIS</A><DD>
<DT><A HREF="#lbAD">DESCRIPTION</A><DD>
<DT><A HREF="#lbAE">DIAGNOSTICS</A><DD>
<DT><A HREF="#lbAF">BUGS</A><DD>
<DT><A HREF="#lbAG">SEE ALSO</A><DD>
</DL>
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