ndbm(3C)

ndbm: dbm_clearerr, dbm_close, dbm_delete, dbm_error, dbm_fetch, dbm_firstkey, dbm_nextkey, dbm_open, dbm_store -- database subroutines

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, int file_mode);
   

   int dbm_store(DBM *db, datum key, datum content, int store_mode);

Description

These functions maintain key pairs in a database. The functions will handle very large (a billion blocks) databases and will access a keyed item in one or two file system accesses.

keys and contents are described by the datum typedef. A datum specifies a string of dsize bytes pointed to by dptr.

   typedef struct {
           void    *dptr;
           size_t  dsize;
   } datum;

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.

Before a data base can be accessed, it must be opened by dbm_open. This will open and/or create the files file.dir and file.pag depending on the open_flags argument. 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).

A data base is closed by calling dbm_close. The argument db must be a pointer to a dbm structure previously returned from a call to dbm_open.

Once open, the data stored under a key is accessed by dbm_fetch and data is placed under a key by dbm_store.

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

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.

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 store_mode 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 dbm_clearerr function returns a 0 if the error condition is successfully cleared; a non-zero return indicates an error.

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.

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 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.

References

open(2),

Notices

The .pag file will contain holes so that its apparent size is about four times its actual content. These files cannot be copied by normal means (that is, using cp(1), cat(1), tar(1), or ar(1)) without filling in the holes.

The sum of the sizes of a key pair must not exceed the internal block size (currently 4096 bytes). Moreover all key 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.

Standards conformance

These routines conform to X/Open System Interfaces and Headers, Issue 4, Version 2.