NAME

dbm: dbminit, dbmclose, fetch, store, delete, firstkey, nextkey - data base subroutines

SYNOPSIS

cc [ flag... ] file ...  -ldbm #include <dbm.h>
typedef struct {
char *dptr;
int dsize;
} datum;
dbminit(file)
char *file;
dbmclose
datum fetch(key)
datum key;
store(key, content)
datum key, content;
delete(key)
datum key;
datum firstkey
datum nextkey(key)
datum key;

DESCRIPTION

Note: the dbm library has been superceded by ndbm(3), and is now implemented using ndbm. 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 -libdbm. 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 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. A database may be closed by calling dbmclose. You must close a database before opening a new one. Once open, the data stored under a key is accessed by fetch and data is placed under a key by store. 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))

SEE ALSO

ndbm(3).

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.

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 1024 bytes).  Moreover
all key/content pairs that hash together must fit on a  sin-
gle  block.   store will return an error in the event that 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  first-
key  and  nextkey depends on a hashing function, not on any-
thing interesting.  There are no interlocks and no  reliable
cache  flushing;  thus  concurrent  updating  and reading is
risky.