Advanced Programming in the UNIX® Environment

ptg10805159

Section 20.8 Source Code 765

287 /*

288 * Read a chain ptr field from anywhere in the index file:

289 * the free list pointer, a hash table chain ptr, or an

290 * index record chain ptr.

291 */

292 static off_t

293 _db_readptr(DB *db, off_t offset)

294 {

295 char asciiptr[PTR_SZ + 1];

296 if (lseek(db->idxfd, offset, SEEK_SET) == -1)

297 err_dump("_db_readptr: lseek error to ptr field");

298 if (read(db->idxfd, asciiptr, PTR_SZ) != PTR_SZ)

299 err_dump("_db_readptr: read error of ptr field");

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

301 return(atol(asciiptr));

302 }

303 /*

304 * Read the next index record. We start at the specified offset

305 * in the index file. We read the index record into db->idxbuf

306 * and replace the separators with null bytes. If all is OK we

307 * set db->datoff and db->datlen to the offset and length of the

308 * corresponding data record in the data file.

309 */

310 static off_t

311 _db_readidx(DB *db, off_t offset)

312 {

313 ssize_t i;

314 char *ptr1, *ptr2;

315 char asciiptr[PTR_SZ + 1], asciilen[IDXLEN_SZ + 1];

316 struct iovec iov[2];

[287 – 302] _db_readptrreads any one of three different chain pointers: (a) the pointer

at the beginning of the index file that points to the first index record on the

free list, (b) the pointers in the hash table that point to the first index record

on each hash chain, and (c) the pointers that arestored at the beginning of

each index record(whether the index record is part of a hash chain or on the

free list). We convert the pointer from ASCII to a long integer before

returning it. No locking is done by this function; that is up to the caller.

[303 – 316] The_db_readidxfunction is used to read the record at the specified offset

from the index file. On success, the function will return the offset of the next

record in the list. In this case, the function will populate several fields in the

DBstructure:idxoffcontains the offset of the current record in the index

file,ptrvalcontains the offset of the next index entry in the list,idxlen

contains the length of the current index record,idxbufcontains the actual

index record,datoffcontains the offset of the record in the data file, and

datlencontains the length of the data record.