NDBM(3) — UNIX Programmer’s Manual

NAME

dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr − data base subroutines

SYNOPSIS

#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_findkey(db, key)
DBM ∗db;
datum key;

datum dbm_nextkey(db)
DBM ∗db;

int dbm_error(db)
DBM ∗db;

int dbm_clearerr(db)
DBM ∗db;

DESCRIPTION

These functions maintain key/content pairs in a data base. 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(3x) library, which managed only a single database.

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 database 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)).

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 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 dbm_delete. A linear pass through all keys in a database 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 database. Dbm_nextkey will return the next key in the database. This code will traverse the data base:

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

The other function calls do not alter the position of this search in the database. If desired dbm_findkey can be used to position the search at a particular key. This locates the given key and returns the key of the following datum. Dbm_nextkey will then return subsequent keys. The code:

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

may also be used to traverse the database. It will be slower than the previous code fragment, but has the advantage that no state pertaining to the search is held in db, thus two or more such searches may be done at the same time.

Dbm_error returns non-zero when an error has occurred reading or writing the database. Dbm_clearerr resets the error condition on the named database.

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.

NOTES

The function dbm_findkey is a RISC iX specific extension to BSD4.3.

BUGS

The data pointers returned by ndbm are char aligned, hence the data will need copying before it can be accessed as a structure.

The ‘.pag’ file will contain holes so that its apparent size is about four times its actual content. Older UNIX systems may create real file blocks for these holes when touched. These files must be copied with care − cp, tar, dump and restore will work correctly, other programs may not. cp may be used to put the holes back into a file which has been filled in.

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. 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, not on anything interesting.

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