dbm.3x

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)dbm.3x	8.1	9/11/90
.TH dbm 3x 
.SH Name
dbminit, fetch, store, delete, firstkey, nextkey \- data base subroutines
.SH Syntax
.nf
.PP
.B typedef struct {
.B "	char *dptr;"
.B "	int dsize;"
.B } datum;
.PP
.B dbminit(file)
.B char *file;
.PP
.B datum fetch(key)
.B datum key;
.PP
.B store(key, content)
.B datum key, content;
.PP
.B delete(key)
.B datum key;
.PP
.B datum firstkey()
.PP
.B datum nextkey(key)
.B datum key;
.SH Description
.NXR "dbm keyword"
.NXR "dbminit subroutine"
.NXR "fetch subroutine"
.NXR "store subroutine"
.NXR "delete subroutine"
.NXR "firstkey subroutine"
.NXR "nextkey subroutine"
.NXR "data base subroutine"
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.
The functions are obtained with the loader option
.BR \-ldbm .
.PP
.IR Key s
and
.IR content s
are described by the
.I datum
typedef.  A
.I datum
specifies a string of
.I dsize
bytes pointed to by
.I 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.
.PP
Before a database can be accessed, it must be opened by
.PN dbminit .
At the time of this call, the files
.IB file .dir
and
.IB file .pag
must exist.
(An empty database is created by creating zero-length
`.dir' and `.pag' files.)
.PP
Once open, the data stored under a key is accessed by
.PN fetch
and data is placed under a key by
.PN store .
A key (and its associated contents) is deleted by
.PN delete .
A linear pass through all keys in a database may be made,
in an (apparently) random order, by use of
.PN firstkey
and
.PN nextkey .
The
.PN firstkey
will return the first key in the database.  With any key
.PN nextkey
will return the next key in the database.
This code will traverse the data base:
.EX 0
for (key = firstkey(); key.dptr != NULL; key = nextkey(key))
.EE
.SH Restrictions
.NXR "dbminit subroutine" "restrictions"
The 
.PN \&.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
.Pn ( cp,
.PN cat ,
.PN tp ,
.PN tar ,
.PN ar )
without filling in the holes.
.PP
The
.I dptr
pointers returned by these subroutines point into static storage
that is changed by subsequent calls.
.PP
The sum of the sizes of a key/content pair must not exceed
the internal block size (currently 1024 bytes).
Moreover all key/content pairs that hash together must fit
on a single block.  The
.PN store
will return an error in the event
that a disk block fills with inseparable data.
.PP
The
.PN delete
does not physically reclaim file space,
although it does make it available for reuse.
.SH Return Values
Routines that return a
.I datum
indicate errors with a null (0)
.I dptr.
All functions that return an
.I int
indicate errors with negative values.  A zero return indicates a successful
completion.