DBM(3X)

NAME

dbminit, fetch, store, delete, firstkey, nextkey − database subroutines

USAGE

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 database. The functions will handle very large (a billion blocks) databases and will find a keyed item in one or two file system accesses. Use the loader option −ldbm to obtain these functions.

The datum typedef decsribes the keys and contents. 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 you can access a database, you must open it with dbminit. 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, fetch accesses data stored under a key; store places data under a key. Delete removes a key (and its associated contents). A linear pass through all keys in a database may be made, in an (apparently) random order, by use of firstkey and nextkey. Firstkey will return the first key in the database. With any key, nextkey will return the next key in the database. This code will traverse the database:

for (key = firstkey(); key.dptr != NULL; key = nextkey(key))

NOTES

The “.pag” file will contain holes; its apparent size is about four times larger than its content. These files cannot be copied by normal means (cp, cat, tp, tar, ar) without filling in the holes.

Dptr pointers returned by these subroutines point into static storage that subsequent calls change.

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. Store will return an error if a disk block fills with inseparable data.

Delete does not physically reclaim file space, although it does make it available for reuse.

The order of keys that firstkey and nextkey present depends on a hashing function.

DIAGNOSTICS