NAME

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

SYNOPSIS

cc [ flag... ] file ... -ldbm

#include <ndbm.h>
typedef struct {
     char *dptr;
     int dsize;
} datum;
int dbm_clearerr(db)
DBM *db;
void dbm_close (db)
DBM *db;
int dbm_delete(db, key)
DBM *db;
datum key;
int dbm_error(db)
DBM *db;
datum dbm_fetch(db, key)
DBM *db;
datum key;
datum dbm_firstkey(db)
DBM *db;
datum dbm_nextkey(db)
DBM *db;
DBM *dbm_open(file, flags, mode)
char *file;
int flags, mode;
int dbm_store(db, key, content, flags)
DBM *db;
datum key, content;
int flags;

DESCRIPTION

These functions maintain key/content 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(3X) 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(2V)). 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  read-
ing or writing the data base.  dbm_clearerr resets the error
condition on the named data base.

SEE ALSO

open(2), dbm(3X) in the Programmer's Reference Manual.

RETURN VALUE

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.

NOTES

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 (cp(1), cat(1), tar(1), 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/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. There are no interlocks and no reliable cache flushing; thus concurrent updating and reading is risky.