ndbm(3)

Name

dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey, dbm_nextkey, dbm_setpblksiz, dbm_error, dbm_clearerr − database subroutines

Syntax

#include <ndbm.h>

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

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

void dbm_close(db)
DBM *db;

datum dbm_fetch(db, key)
DBM *db;
datum key;

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

int dbm_delete(db, key)
DBM *db;
datum key;

datum dbm_firstkey(db)
DBM *db;

datum dbm_nextkey(db)
DBM *db;

int dbm_setpblksiz(db, size)
DBM *db;
int size;

int dbm_error(db)
DBM *db;

int dbm_clearerr(db)
DBM *db;

Arguments

The arguments are described in the Description section.

Description

These functions maintain key/content 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. This package replaces the earlier dbm() library, which managed only a single database.

The 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 database 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 database can be accessed, it must be opened by the dbm_open function. This will open and/or create the files file.dir and file.pag depending on the flags parameter (see open().)).

Once open, the data stored under a key is accessed by the dbm_fetch function and data is placed under a key by the dbm_store function. The flags field can be either DBM_INSERT or DBM_REPLACE. DBM_INSERT will only insert new entries into the database 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 the dbm_delete function. A linear pass through all keys in a database may be made, in an (apparently) random order, by use of the dbm_firstkey and the dbm_nextkey functions. The dbm_firstkey function will return the first key in the database. The dbm_nextkey function will return the next key in the database. This code will traverse the database:

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

The dbm_setpblksiz function returns an EINVAL error if the value passed is not between 1024 and 32767, inclusive. The function sets the block size of the page file to something higher than the current value, which is 1024. Digital recommends a value of 4096. The db passed to the function is that returned from the db_open function.

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

Restrictions

The .pag file will contain holes so that its apparent size is about four times its actual content. Older systems may create real file blocks for these holes when touched. These files cannot be copied by normal means ( cp, cat, tp, tar, ar) without filling in the holes.

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

The sum of the sizes of a key/content pair must not exceed the internal block size (currently 4096 bytes). Moreover all key/content pairs that hash together must fit on a single block. The dbm_store function will return an error in the event that a disk block fills with inseparable data.

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

The order of keys presented by the dbm_firstkey and dbm_nextkey functions depends on a hashing function.

Diagnostics

All functions that return an int indicate errors with negative values. A zero return indicates ok. Routines that return a datum indicate errors with a null (0) dptr. If dbm_store called with a flags value of DBM_INSERT finds an existing entry with the same key it returns 1.

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