ptg10805159756 ADatabase Library Chapter 20
27 /*
28 * Library’s private representation of the database.
29 */
30 typedef struct {
31 int idxfd; /* fd for index file */
32 int datfd; /* fd for data file */
33 char *idxbuf; /* malloc’ed buffer for index record */
34 char *datbuf; /* malloc’ed buffer for data record*/
35 char *name; /* name db was opened under */
36 off_t idxoff; /* offset in index file of index record */
37 /* key is at (idxoff + PTR_SZ + IDXLEN_SZ) */
38 size_t idxlen; /* length of index record */
39 /* excludes IDXLEN_SZ bytes at front of record */
40 /* includes newline at end of index record */
41 off_t datoff; /* offset in data file of data record */
42 size_t datlen; /* length of data record */
43 /* includes newline at end */
44 off_t ptrval; /* contents of chain ptr in index record */
45 off_t ptroff; /* chain ptr offset pointing to this idx record */
46 off_t chainoff; /* offset of hash chain for this index record */
47 off_t hashoff; /* offset in index file of hash table */
48 DBHASH nhash; /* current hash table size */
49 COUNT cnt_delok; /* delete OK */
50 COUNT cnt_delerr; /* delete error */
51 COUNT cnt_fetchok; /* fetch OK */
52 COUNT cnt_fetcherr; /* fetch error */
53 COUNT cnt_nextrec; /* nextrec */
54 COUNT cnt_stor1; /* store: DB_INSERT, no empty, appended */
55 COUNT cnt_stor2; /* store: DB_INSERT, found empty, reused */
56 COUNT cnt_stor3; /* store: DB_REPLACE, diff len, appended */
57 COUNT cnt_stor4; /* store: DB_REPLACE, same len, overwrote */
58 COUNT cnt_storerr; /* store error */
59 } DB;[27 – 48] TheDBstructure is where we keep all the information for each open database.
TheDBHANDLEvalue that is returned bydb_openand used by all the other
functions is really just a pointer to one of these structures, but we hide that
from the callers.
Since we storepointers and lengths as ASCII in the database, we convert these
to numeric values and save them in theDBstructure. Wealso save the hash
table size even though it is fixed, just in case we decide to enhance the library
to allow callers to specify the size when the database is created (see
Exercise 20.7).
[49 – 59] The last ten fields in theDBstructurecount both successful and unsuccessful
operations. If we want to analyze the performance of our database, we can
write a function to return these statistics, but for now we only maintain the
counters.