man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

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 databases (up to a billion blocks) and will access a keyed item in one or two file system accesses. This package replaces the earlier dbm 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 meaning 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 initialized 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 initialized 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 writing. 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 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 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_nextkey().

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 checking.

#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
Committed
MT-Level
Unsafe
Standard

See Also

ar(1), cat(1), cp(1), tar(1), open(2), 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 architecture-specific (for example, they depend on the architecture's byte order.) These files are not guaranteed to be portable across architectures.