dbm(3x) dbm(3x)
Name
dbminit, fetch, store, delete, firstkey, nextkey - data base subroutines
Syntax
typedef struct {
char *dptr;
int dsize;
} datum;
dbminit(file)
char *file;
datum fetch(key)
datum key;
store(key, content)
datum key, content;
delete(key)
datum key;
datum firstkey()
datum nextkey(key)
datum key;
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. The functions are obtained with the loader option -ldbm.
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 At the time of this call, the files file.dir and file.pag must exist. (An empty
database is created by creating zero-length `.dir' and `.pag' files.)
Once open, the data stored under a key is accessed by and data is placed under a key by A key (and its associated contents) is deleted by A
linear pass through all keys in a database may be made, in an (apparently) random order, by use of and The will return the first key in the
database. With any key will return the next key in the database. This code will traverse the data base:
for (key = firstkey(); key.dptr != NULL; key = nextkey(key))
Restrictions
The four times its actual content. Older UNIX 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 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 1024 bytes). Moreover all key/content pairs
that hash together must fit on a single block. The will return an error in the event that a disk block fills with inseparable data.
The does not physically reclaim file space, although it does make it available for reuse.
Return Values
Routines that return a datum indicate errors with a null(0) dptr. All functions that return an int indicate errors with negative values.
A zero return indicates a successful completion.
dbm(3x)
