Advanced Programming in the UNIX® Environment

ptg10805159

766 ADatabase Library Chapter 20

317 /*

318 * Position index file and record the offset. db_nextrec

319 * calls us with offset==0, meaning read from current offset.

320 * We still need to call lseek to record the current offset.

321 */

322 if ((db->idxoff = lseek(db->idxfd, offset,

323 offset == 0? SEEK_CUR : SEEK_SET)) == -1)

324 err_dump("_db_readidx: lseek error");

325 /*

326 * Read the ascii chain ptr and the ascii length at

327 * the front of the index record. This tells us the

328 * remaining size of the index record.

329 */

330 iov[0].iov_base=asciiptr;

331 iov[0].iov_len =PTR_SZ;

332 iov[1].iov_base=asciilen;

333 iov[1].iov_len =IDXLEN_SZ;

334 if ((i = readv(db->idxfd, &iov[0], 2)) != PTR_SZ + IDXLEN_SZ) {

335 if (i == 0 && offset == 0)

336 return(-1); /* EOF for db_nextrec */

337 err_dump("_db_readidx: readv error of index record");

338 }

339 /*

340 * This is our return value; always >= 0.

341 */

342 asciiptr[PTR_SZ]=0;/*null terminate */

343 db->ptrval=atol(asciiptr); /* offset of next key in chain */

344 asciilen[IDXLEN_SZ]=0;/*null terminate */

345 if ((db->idxlen = atoi(asciilen)) < IDXLEN_MIN ||

346 db->idxlen>IDXLEN_MAX)

347 err_dump("_db_readidx: invalid length");

[317 – 324] We start by seeking to the index file offset provided by the caller.Werecord

the offset in theDBstructure, so even if the caller wants to read the recordat

the current file offset (by settingoffsetto 0), we still need to calllseekto

determine the current offset. Since an index recordwill never be stored at

offset 0 in the index file, we can safely overload the value of 0 to mean ‘‘read

from the current offset.’’

[325 – 338] We callreadvto read the two fixed-length fields at the beginning of the

index record: the chain pointer to the next index recordand the size of the

variable-length index recordthat follows.

[339 – 347] We convert the offset of the next record to an integer and store it in the

ptrvalfield (this will be used as the return value for this function). Then

we convert the length of the index recordinto an integer and save it in the

idxlenfield.