SDBM(3) UNIX System V(1 March 1990) SDBM(3)
NAME
sdbm, dbm_open, dbm_prep, dbm_close, dbm_fetch, dbm_store, dbm_delete,
dbm_firstkey, dbm_nextkey, dbm_hash, dbm_rdonly, dbm_error, dbm_clearerr,
dbm_dirfno, dbm_pagfno - data base subroutines
SYNOPSIS
#include <sdbm.h>
typedef struct {
char *dptr;
int dsize;
} datum;
datum nullitem = { NULL, 0 };
DBM *dbm_open(char *file, int flags, int mode)
DBM *dbm_prep(char *dirname, char *pagname, int flags, int mode)
void dbm_close(DBM *db)
datum dbm_fetch(DBM *db, key)
int dbm_store(DBM *db, datum key, datum val, int flags)
int dbm_delete(DBM *db, datum key)
datum dbm_firstkey(DBM *db)
datum dbm_nextkey(DBM *db)
long dbm_hash(char *string, int len)
int dbm_rdonly(DBM *db)
int dbm_error(DBM *db)
dbm_clearerr(DBM *db)
int dbm_dirfno(DBM *db)
int dbm_pagfno(DBM *db)
DESCRIPTION
This package allows an application to maintain a mapping of <key,value>
pairs in disk files. This is not to be considered a real database
system, but is still useful in many simple applications built around fast
retrieval of a data value from a key. This implementation uses an
external hashing scheme, called Dynamic Hashing, as described by Per-Aake
Larson in BIT 18 (1978) pp. 184-201. Retrieval of any item usually
requires a single disk access. The application interface is compatible
with the ndbm(3) library.
An sdbm database is kept in two files usually given the extensions .dir
and .pag. The .dir file contains a bitmap representing a forest of
binary hash trees, the leaves of which indicate data pages in the .pag
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file.
The application interface uses the datum structure to describe both keys
and values. A datum specifies a byte sequence of dsize size pointed to
by dptr. If you use ASCII strings as keys or values, then you must
decide whether or not to include the terminating NUL byte which sometimes
defines strings. Including it will require larger database files, but it
will be possible to get sensible output from a strings(1) command applied
to the data file.
In order to allow a process using this package to manipulate multiple
databases, the applications interface always requires a handle, a DBM *,
to identify the database to be manipulated. Such a handle can be
obtained from the only routines that do not require it, namely dbmopen()
or dbmprep(). Either of these will open or create the two necessary
files. The difference is that the latter allows explicitly naming the
bitmap and data files whereas dbmopen() will take a base file name and
call dbmprep() with the default extensions. The flags and mode
parameters are the same as for open(2).
To free the resources occupied while a database handle is active, call
dbmclose().
Given a handle, one can retrieve data associated with a key by using the
dbmfetch() routine, and associate data with a key by using the
dbmstore() routine.
The values of the flags parameter for dbmstore() can be either
DBMINSERT, which will not change an existing entry with the same key, or
DBMREPLACE, which will replace an existing entry with the same key.
Keys are unique within the database.
To delete a key and its associated value use the dbmdelete() routine.
To retrieve every key in the database, use a loop like:
for (key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))
;
The order of retrieval is unspecified.
If you determine that the performance of the database is inadequate or
you notice clustering or other effects that may be due to the hashing
algorithm used by this package, you can override it by supplying your own
dbmhash() routine. Doing so will make the database unintelligable to
any other applications that do not use your specialized hash function.
The following macros are defined in the header file:
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dbmrdonly() returns true if the database has been opened
read-only.
dbmerror() returns true if an I/O error has occurred.
dbmclearerr() allows you to clear the error flag if you think you
know what the error was and insist on ignoring it.
dbmdirfno() returns the file descriptor associated with the bitmap
file.
dbmpagfno() returns the file descriptor associated with the data
file.
SEE ALSO
open(2).
DIAGNOSTICS
Functions that return a DBM * handle will use NULL to indicate an error.
Functions that return an int will use -1 to indicate an error. The
normal return value in that case is 0. Functions that return a datum
will return nullitem to indicate an error.
As a special case of dbmstore(), if it is called with the DBMINSERT
flag and the key already exists in the database, the return value will be
1.
In general, if a function parameter is invalid, errno will be set to
EINVAL. If a write operation is requested on a read-only database, errno
will be set to ENOPERM. If a memory allocation (using malloc(3)) failed,
errno will be set to ENOMEM. For I/O operation failures errno will
contain the value set by the relevant failed system call, either read(2),
write(2), or lseek(2).
AUTHOR
Ozan S. Yigit
BUGS
The sum of key and value data sizes must not exceed PAIRMAX (1008 bytes).
The sum of the key and value data sizes where several keys hash to the
same value must fit within one bitmap page.
The .pag file will contain holes, so its apparent size is larger than its
contents. When copied through the filesystem the holes will be filled.
The contents of datum values returned are in volatile storage. If you
want to retain the values pointed to, you must copy them immediately
before another call to this package.
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The only safe way for multiple processes to (read and) update a database
at the same time, is to implement a private locking scheme outside this
package and open and close the database between lock acquisitions. It is
safe for multiple processes to concurrently access a database read-only.
APPLICATIONS PORTABILITY
For complete source code compatibility with the Berkeley Unix ndbm(3)
library, the sdbm.h header file should be installed in
/usr/include/ndbm.h.
The nullitem data item, and the dbmprep(), dbmhash(), dbmrdonly(),
dbmdirfno(), and dbmpagfno() functions are unique to this package.
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