(BSD System Compatibility)

ndbm(3bsd)

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

Synopsis

/usr/ucb/cc [flag . . . ] file . . .

#include <ndbm.h>

typedef struct {
	char *dptr;
	int dsize;
} datum;

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(char *file, int flags, int mode);

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

Description

These routines are provided for compatibility with applications originally written for BSD systems; new or ported applications should use the equivalent System V routines instead. See ndbm(3C).

These functions maintain key pairs in a data base. The functions will handle very large (a billion blocks) data base and will access a keyed item in one or two file system accesses. This package replaces the earlier dbm(3bsd) library, which managed only a single data base.

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

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 flags parameter (see open(2)).

A data base is closed by calling dbm_close.

Once open, the data stored under a key is accessed by dbm_fetch and data is placed under a key by dbm_store. The flags field can be either DBM_INSERT or DBM_REPLACE. DBM_INSERT will only insert new entries into the data base and will not change an existing entry with the same key. DBM_REPLACE will replace an existing entry if it has the same key. A key (and its associated contents) is deleted by dbm_delete. A linear pass through all keys in a data base may be made, in an (apparently) random order, by use of dbm_firstkey and dbm_nextkey. dbm_firstkey will return the first key in the data base. dbm_nextkey will return the next key in the data base. This code will traverse the data base:

   for (key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))

dbm_error returns non-zero when an error has occurred reading or writing the data base. dbm_clearerr resets the error condition on the named data base.

Return values

All functions that return an int indicate errors with negative values. A zero return indicates no error. Routines that return a datum indicate errors with a NULL (0) dptr. If dbm_store is called with a flags value of DBM_INSERT and finds an existing entry with the same key, it returns 1.

References

``BSD system libraries and header files'' in Programming with system calls and libraries

cc(1bsd), dbm(3bsd) ld(1bsd), ndbm(3C), open(2)

Notices

The .pag file will contain holes so that its apparent size is about four times 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 (that is, using cp(1), cat(1), tar(1), or ar(1)) without filling in the holes.

dptr pointers returned by these subroutines point into static storage that is changed by subsequent calls.

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.

dbm_delete does not physically reclaim file space, although it does make it available for reuse.

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.