ndbm(3)ndbm(3)Name
dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey,
dbm_nextkey, dbm_setpblksiz, dbm_error, dbm_clearerr - database subrou‐
tines
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 func‐
tions 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 library, which managed only a single database.
The keys and contents are described by the typedef. A specifies a
string of bytes pointed to by 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 function.
This will open and/or create the files file.dir and file.pag depending
on the flags parameter (see
Once open, the data stored under a key is accessed by the function and
data is placed under a key by the 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 func‐
tion. A linear pass through all keys in a database may be made, in an
(apparently) random order, by use of the and the functions. The func‐
tion will return the first key in the database. The function will
return the next key in the database. This code will traverse the data‐
base:
(key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))
The 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 passed to the function is that
returned from the function.
The function returns non-zero when an error has occurred reading or
writing the database. The 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 ( without filling in the holes.
The 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 function will return an
error in the event that a disk block fills with inseparable data.
The function does not physically reclaim file space, although it does
make it available for reuse.
The order of keys presented by the and functions depends on a hashing
function.
Diagnostics
All functions that return an indicate errors with negative values. A
zero return indicates ok. Routines that return a indicate errors with
a null (0) If called with a flags value of DBM_INSERT finds an existing
entry with the same key it returns 1.
See Alsondbm(3)
