dbm.3x

unix v7是最后一个广泛发布的研究型UNIX版本

.TH DBM 3X 
.SH NAME
dbminit, fetch, store, delete, firstkey, nextkey \- data base subroutines
.SH SYNOPSIS
.nf
.PP
.B "typedef struct { char *dptr; int dsize; } 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
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 filesystem 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
.I 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
.I fetch
and data is placed under a key
by
.IR store .
A key (and its associated contents)
is deleted by
.IR delete .
A linear pass through all keys in a database
may be made,
in an (apparently) random order,
by use of
.I firstkey
and
.IR nextkey .
.I Firstkey
will return the first key
in the database.
With any key
.I nextkey
will return the next key in the database.
This code will traverse the data base:
.PP
	for(key=firstkey(); key.dptr!=NULL; key=nextkey(key))
.SH DIAGNOSTICS
All functions that return an
.I int
indicate errors with negative values.
A zero return indicates ok.
Routines that return a
.I datum
indicate errors with a null (0)
.I dptr.
.SH BUGS
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.
.PP
.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 512 bytes).
Moreover all key/content pairs that hash
together must fit on a single block.
.I Store
will return an error in the event that
a disk block fills with inseparable data.
.PP
.I Delete
does not physically reclaim file space,
although it does make it available for reuse.
.PP
The order of keys presented by
.I firstkey
and
.I nextkey
depends on a hashing function, not on anything
interesting.