NAME

dbminit, fetch, store, delete, firstkey, nextkey, dbmclose − data base subroutines

SYNOPSIS

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;

dbmclose()

DESCRIPTION

These functions maintain key/content pairs in a data base.  The functions will handle very large (a billion blocks (block = 1024 bytes)) databases and will locate a keyed item in one or two file system accesses.  This package is superseded by the newer ndbm(3X) library, which manages multiple databases. The functions can be accessed by giving the −ldbm option to ld(1) or cc(1).

Key and content parameters are described by the datum type.  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 of keys 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 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, the data stored under a key is accessed by fetch and data is placed under a key by store. Storing data on an existing key will replace the existing data. A key (and its associated contents) is deleted by delete. 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 data base:

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

A database may be closed by calling dbmclose.  The user must close a database before opening a new one. 

DIAGNOSTICS

All functions that return an int indicate errors with negative values and success with zero.  Routines that return a datum indicate errors with a null dptr. 

WARNINGS

The .pag file will contain holes so that its apparent size is about four times its actual content.  Some older UNIX systems create real file blocks for these holes when touched.  These files cannot be copied by normal means (such as cp(1), cat(1), tar(1), or ar(1)) without expansion.

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.  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 presented by firstkey and nextkey depends on a hashing function, not on anything interesting. 

AUTHOR

Dbm(3X) was developed by the University of California, Berkeley.

SEE ALSO

ndbm(3X). 

Hewlett-Packard Company  —  May 11, 2021