NDBM(3C)NDBM(3C)NAME
ndbm, dbm_clearerr, dbm_close, dbm_delete, dbm_error, dbm_fetch,
dbm_firstkey, dbm_nextkey, dbm_open, dbm_store - database functions
SYNOPSIS
#include <ndbm.h>
int dbm_clearerr(DBM *db);
void dbm_close(DBM *db);
int dbm_delete(DBM *db, datum key);
int dbm_error(DBM *db);
datum dbm_fetch(DBM *db, datum key);
datum dbm_firstkey(DBM *db);
datum dbm_nextkey(DBM *db);
DBM *dbm_open(const char *file, int open_flags, mode_t file_mode);
int dbm_store(DBM *db, datum key, datum content, int store_mode);
DESCRIPTION
These functions create, access and modify a database. They maintain
key/content pairs in a database. The functions will handle large data‐
bases (up to a billion blocks) and will access a keyed item in one or
two file system accesses. This package replaces the earlier dbm(3UCB)
library, which managed only a single database.
keys and contents are described by the datum typedef. A datum consists
of at least two members, dptr and dsize. The dptr member points to an
object that is dsize bytes in length. Arbitrary binary data, as well
as ASCII character strings, may be stored in the object pointed to by
dptr.
The database is stored in two files. One file is a directory containing
a bit map of keys and has .dir as its suffix. The second file contains
all data and has .pag as its suffix.
The dbm_open() function opens a database. The file argument to the
function is the pathname of the database. The function opens two files
named file.dir and file.pag. The open_flags argument has the same mean‐
ing as the flags argument of open(2) except that a database opened for
write-only access opens the files for read and write access. The
file_mode argument has the same meaning as the third argument of
open(2).
The dbm_close() function closes a database. The argument db must be a
pointer to a dbm structure that has been returned from a call to
dbm_open().
The dbm_fetch() function reads a record from a database. The argument
db is a pointer to a database structure that has been returned from a
call to dbm_open(). The argument key is a datum that has been initial‐
ized by the application program to the value of the key that matches
the key of the record the program is fetching.
The dbm_store() function writes a record to a database. The argument
db is a pointer to a database structure that has been returned from a
call to dbm_open(). The argument key is a datum that has been initial‐
ized by the application program to the value of the key that identifies
(for subsequent reading, writing or deleting) the record the program is
writing. The argument content is a datum that has been initialized by
the application program to the value of the record the program is writ‐
ing. The argument store_mode controls whether dbm_store() replaces any
pre-existing record that has the same key that is specified by the key
argument. The application program must set store_mode to either
DBM_INSERT or DBM_REPLACE. If the database contains a record that
matches the key argument and store_mode is DBM_REPLACE, the existing
record is replaced with the new record. If the database contains a
record that matches the key argument and store_mode is DBM_INSERT, the
existing record is not replaced with the new record. If the database
does not contain a record that matches the key argument and store_mode
is either DBM_INSERT or DBM_REPLACE, the new record is inserted in the
database.
The dbm_delete() function deletes a record and its key from the data‐
base. The argument db is a pointer to a database structure that has
been returned from a call to dbm_open(). The argument key is a datum
that has been initialized by the application program to the value of
the key that identifies the record the program is deleting.
The dbm_firstkey() function returns the first key in the database. The
argument db is a pointer to a database structure that has been returned
from a call to dbm_open().
The dbm_nextkey() function returns the next key in the database. The
argument db is a pointer to a database structure that has been returned
from a call to dbm_open(). The dbm_firstkey() function must be called
before calling dbm_nextkey(). Subsequent calls to dbm_nextkey() return
the next key until all of the keys in the database have been returned.
The dbm_error() function returns the error condition of the database.
The argument db is a pointer to a database structure that has been
returned from a call to dbm_open().
The dbm_clearerr() function clears the error condition of the database.
The argument db is a pointer to a database structure that has been
returned from a call to dbm_open().
These database functions support key/content pairs of at least 1024
bytes.
RETURN VALUES
The dbm_store() and dbm_delete() functions return 0 when they succeed
and a negative value when they fail.
The dbm_store() function returns 1 if it is called with a flags value
of DBM_INSERT and the function finds an existing record with the same
key.
The dbm_error() function returns 0 if the error condition is not set
and returns a non-zero value if the error condition is set.
The return value of dbm_clearerr() is unspecified .
The dbm_firstkey() and dbm_nextkey() functions return a key datum. When
the end of the database is reached, the dptr member of the key is a
null pointer. If an error is detected, the dptr member of the key is a
null pointer and the error condition of the database is set.
The dbm_fetch() function returns a content datum. If no record in the
database matches the key or if an error condition has been detected in
the database, the dptr member of the content is a null pointer.
The dbm_open() function returns a pointer to a database structure. If
an error is detected during the operation, dbm_open() returns a (DBM
*)0.
ERRORS
No errors are defined.
USAGE
The following code can be used to traverse the database:
for(key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))
The dbm_ functions provided in this library should not be confused in
any way with those of a general-purpose database management system.
These functions do not provide for multiple search keys per entry, they
do not protect against multi-user access (in other words they do not
lock records or files), and they do not provide the many other useful
database functions that are found in more robust database management
systems. Creating and updating databases by use of these functions is
relatively slow because of data copies that occur upon hash collisions.
These functions are useful for applications requiring fast lookup of
relatively static information that is to be indexed by a single key.
The dptr pointers returned by these functions may point into static
storage that may be changed by subsequent calls.
The dbm_delete() function does not physically reclaim file space,
although it does make it available for reuse.
After calling dbm_store() or dbm_delete() during a pass through the
keys by dbm_firstkey() and dbm_nextkey(), the application should reset
the database by calling dbm_firstkey() before again calling dbm_nex‐
tkey().
EXAMPLES
Example 1 Using the Database Functions
The following example stores and retrieves a phone number, using the
name as the key. Note that this example does not include error check‐
ing.
#include <ndbm.h>
#include <stdio.h>
#include <fcntl.h>
#define NAME "Bill"
#define PHONE_NO "123-4567"
#define DB_NAME "phones"
main()
{
DBM *db;
datum name = {NAME, sizeof (NAME)};
datum put_phone_no = {PHONE_NO, sizeof (PHONE_NO)};
datum get_phone_no;
/* Open the database and store the record */
db = dbm_open(DB_NAME, O_RDWR | O_CREAT, 0660);
(void) dbm_store(db, name, put_phone_no, DBM_INSERT);
/* Retrieve the record */
get_phone_no = dbm_fetch(db, name);
(void) printf("Name: %s, Phone Number: %s\n", name.dptr,
get_phone_no.dptr);
/* Close the database */
dbm_close(db);
return (0);
}
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Standard │
├────────────────────┼─────────────────┤
│MT-Level │ Unsafe │
└────────────────────┴─────────────────┘
SEE ALSOar(1), cat(1), cp(1), tar(1), open(2), dbm(3UCB), netconfig(4),
attributes(5), standards(5)NOTES
The .pag file will contain holes so that its apparent size may be
larger than 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(1), cat(1), tar(1), ar(1))
without filling in the holes.
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. dbm_store() will return an
error in the event that a disk block fills with inseparable data.
The order of keys presented by dbm_firstkey() and dbm_nextkey() depends
on a hashing function.
There are no interlocks and no reliable cache flushing; thus concurrent
updating and reading is risky.
The database files (file.dir and file.pag) are binary and are architec‐
ture-specific (for example, they depend on the architecture's byte
order.) These files are not guaranteed to be portable across architec‐
tures.
Sep 17, 2001 NDBM(3C)
