db.c

unix环境高级编程第二版配套源代码 unix环境高级编程第二版配套源代码

int
db_delete(DBHANDLE h, const char *key)
{
	DB		*db = h;
	int		rc = 0;			/* assume record will be found */

	if (_db_find_and_lock(db, key, 1) == 0) {
		_db_dodelete(db);
		db->cnt_delok++;
	} else {
		rc = -1;			/* not found */
		db->cnt_delerr++;
	}
	if (un_lock(db->idxfd, db->chainoff, SEEK_SET, 1) < 0)
		err_dump("db_delete: un_lock error");
	return(rc);
}

/*
 * Delete the current record specified by the DB structure.
 * This function is called by db_delete and db_store, after
 * the record has been located by _db_find_and_lock.
 */
static void
_db_dodelete(DB *db)
{
	int		i;
	char	*ptr;
	off_t	freeptr, saveptr;

	/*
	 * Set data buffer and key to all blanks.
	 */
	for (ptr = db->datbuf, i = 0; i < db->datlen - 1; i++)
		*ptr++ = SPACE;
	*ptr = 0;	/* null terminate for _db_writedat */
	ptr = db->idxbuf;
	while (*ptr)
		*ptr++ = SPACE;

	/*
	 * We have to lock the free list.
	 */
	if (writew_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
		err_dump("_db_dodelete: writew_lock error");

	/*
	 * Write the data record with all blanks.
	 */
	_db_writedat(db, db->datbuf, db->datoff, SEEK_SET);

	/*
	 * Read the free list pointer.  Its value becomes the
	 * chain ptr field of the deleted index record.  This means
	 * the deleted record becomes the head of the free list.
	 */
	freeptr = _db_readptr(db, FREE_OFF);

	/*
	 * Save the contents of index record chain ptr,
	 * before it's rewritten by _db_writeidx.
	 */
	saveptr = db->ptrval;

	/*
	 * Rewrite the index record.  This also rewrites the length
	 * of the index record, the data offset, and the data length,
	 * none of which has changed, but that's OK.
	 */
	_db_writeidx(db, db->idxbuf, db->idxoff, SEEK_SET, freeptr);

	/*
	 * Write the new free list pointer.
	 */
	_db_writeptr(db, FREE_OFF, db->idxoff);

	/*
	 * Rewrite the chain ptr that pointed to this record being
	 * deleted.  Recall that _db_find_and_lock sets db->ptroff to
	 * point to this chain ptr.  We set this chain ptr to the
	 * contents of the deleted record's chain ptr, saveptr.
	 */
	_db_writeptr(db, db->ptroff, saveptr);
	if (un_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
		err_dump("_db_dodelete: un_lock error");
}

/*
 * Write a data record.  Called by _db_dodelete (to write
 * the record with blanks) and db_store.
 */
static void
_db_writedat(DB *db, const char *data, off_t offset, int whence)
{
	struct iovec	iov[2];
	static char		newline = NEWLINE;

	/*
	 * If we're appending, we have to lock before doing the lseek
	 * and write to make the two an atomic operation.  If we're
	 * overwriting an existing record, we don't have to lock.
	 */
	if (whence == SEEK_END) /* we're appending, lock entire file */
		if (writew_lock(db->datfd, 0, SEEK_SET, 0) < 0)
			err_dump("_db_writedat: writew_lock error");

	if ((db->datoff = lseek(db->datfd, offset, whence)) == -1)
		err_dump("_db_writedat: lseek error");
	db->datlen = strlen(data) + 1;	/* datlen includes newline */

	iov[0].iov_base = (char *) data;
	iov[0].iov_len  = db->datlen - 1;
	iov[1].iov_base = &newline;
	iov[1].iov_len  = 1;
	if (writev(db->datfd, &iov[0], 2) != db->datlen)
		err_dump("_db_writedat: writev error of data record");

	if (whence == SEEK_END)
		if (un_lock(db->datfd, 0, SEEK_SET, 0) < 0)
			err_dump("_db_writedat: un_lock error");
}

/*
 * Write an index record.  _db_writedat is called before
 * this function to set the datoff and datlen fields in the
 * DB structure, which we need to write the index record.
 */
static void
_db_writeidx(DB *db, const char *key,
             off_t offset, int whence, off_t ptrval)
{
	struct iovec	iov[2];
	char			asciiptrlen[PTR_SZ + IDXLEN_SZ +1];
	int				len;
	char			*fmt;

	if ((db->ptrval = ptrval) < 0 || ptrval > PTR_MAX)
		err_quit("_db_writeidx: invalid ptr: %d", ptrval);
	if (sizeof(off_t) == sizeof(long long))
		fmt = "%s%c%lld%c%d\n";
	else
		fmt = "%s%c%ld%c%d\n";
	sprintf(db->idxbuf, fmt, key, SEP, db->datoff, SEP, db->datlen);
	if ((len = strlen(db->idxbuf)) < IDXLEN_MIN || len > IDXLEN_MAX)
		err_dump("_db_writeidx: invalid length");
	sprintf(asciiptrlen, "%*ld%*d", PTR_SZ, ptrval, IDXLEN_SZ, len);

	/*
	 * If we're appending, we have to lock before doing the lseek
	 * and write to make the two an atomic operation.  If we're
	 * overwriting an existing record, we don't have to lock.
	 */
	if (whence == SEEK_END)		/* we're appending */
		if (writew_lock(db->idxfd, ((db->nhash+1)*PTR_SZ)+1,
		  SEEK_SET, 0) < 0)
			err_dump("_db_writeidx: writew_lock error");

	/*
	 * Position the index file and record the offset.
	 */
	if ((db->idxoff = lseek(db->idxfd, offset, whence)) == -1)
		err_dump("_db_writeidx: lseek error");

	iov[0].iov_base = asciiptrlen;
	iov[0].iov_len  = PTR_SZ + IDXLEN_SZ;
	iov[1].iov_base = db->idxbuf;
	iov[1].iov_len  = len;
	if (writev(db->idxfd, &iov[0], 2) != PTR_SZ + IDXLEN_SZ + len)
		err_dump("_db_writeidx: writev error of index record");

	if (whence == SEEK_END)
		if (un_lock(db->idxfd, ((db->nhash+1)*PTR_SZ)+1,
		  SEEK_SET, 0) < 0)
			err_dump("_db_writeidx: un_lock error");
}

/*
 * Write a chain ptr field somewhere in the index file:
 * the free list, the hash table, or in an index record.
 */
static void
_db_writeptr(DB *db, off_t offset, off_t ptrval)
{
	char	asciiptr[PTR_SZ + 1];

	if (ptrval < 0 || ptrval > PTR_MAX)
		err_quit("_db_writeptr: invalid ptr: %d", ptrval);
	sprintf(asciiptr, "%*ld", PTR_SZ, ptrval);

	if (lseek(db->idxfd, offset, SEEK_SET) == -1)
		err_dump("_db_writeptr: lseek error to ptr field");
	if (write(db->idxfd, asciiptr, PTR_SZ) != PTR_SZ)
		err_dump("_db_writeptr: write error of ptr field");
}

/*
 * Store a record in the database.  Return 0 if OK, 1 if record
 * exists and DB_INSERT specified, -1 on error.
 */
int
db_store(DBHANDLE h, const char *key, const char *data, int flag)
{
	DB		*db = h;
	int		rc, keylen, datlen;
	off_t	ptrval;

	if (flag != DB_INSERT && flag != DB_REPLACE &&
	  flag != DB_STORE) {
		errno = EINVAL;
		return(-1);
	}
	keylen = strlen(key);
	datlen = strlen(data) + 1;		/* +1 for newline at end */
	if (datlen < DATLEN_MIN || datlen > DATLEN_MAX)
		err_dump("db_store: invalid data length");

	/*
	 * _db_find_and_lock calculates which hash table this new record
	 * goes into (db->chainoff), regardless of whether it already
	 * exists or not. The following calls to _db_writeptr change the
	 * hash table entry for this chain to point to the new record.
	 * The new record is added to the front of the hash chain.
	 */
	if (_db_find_and_lock(db, key, 1) < 0) { /* record not found */
		if (flag == DB_REPLACE) {
			rc = -1;
			db->cnt_storerr++;
			errno = ENOENT;		/* error, record does not exist */
			goto doreturn;
		}

		/*
		 * _db_find_and_lock locked the hash chain for us; read
		 * the chain ptr to the first index record on hash chain.
		 */
		ptrval = _db_readptr(db, db->chainoff);

		if (_db_findfree(db, keylen, datlen) < 0) {
			/*
			 * Can't find an empty record big enough. Append the
			 * new record to the ends of the index and data files.
			 */
			_db_writedat(db, data, 0, SEEK_END);
			_db_writeidx(db, key, 0, SEEK_END, ptrval);

			/*
			 * db->idxoff was set by _db_writeidx.  The new
			 * record goes to the front of the hash chain.
			 */
			_db_writeptr(db, db->chainoff, db->idxoff);
			db->cnt_stor1++;
		} else {
			/*
			 * Reuse an empty record. _db_findfree removed it from
			 * the free list and set both db->datoff and db->idxoff.
			 * Reused record goes to the front of the hash chain.
			 */
			_db_writedat(db, data, db->datoff, SEEK_SET);
			_db_writeidx(db, key, db->idxoff, SEEK_SET, ptrval);
			_db_writeptr(db, db->chainoff, db->idxoff);
			db->cnt_stor2++;
		}
	} else {						/* record found */
		if (flag == DB_INSERT) {
			rc = 1;		/* error, record already in db */
			db->cnt_storerr++;
			goto doreturn;
		}

		/*
		 * We are replacing an existing record.  We know the new
		 * key equals the existing key, but we need to check if
		 * the data records are the same size.
		 */
		if (datlen != db->datlen) {
			_db_dodelete(db);	/* delete the existing record */

			/*
			 * Reread the chain ptr in the hash table
			 * (it may change with the deletion).
			 */
			ptrval = _db_readptr(db, db->chainoff);

			/*
			 * Append new index and data records to end of files.
			 */
			_db_writedat(db, data, 0, SEEK_END);
			_db_writeidx(db, key, 0, SEEK_END, ptrval);

			/*
			 * New record goes to the front of the hash chain.
			 */
			_db_writeptr(db, db->chainoff, db->idxoff);
			db->cnt_stor3++;
		} else {
			/*
			 * Same size data, just replace data record.
			 */
			_db_writedat(db, data, db->datoff, SEEK_SET);
			db->cnt_stor4++;
		}
	}
	rc = 0;		/* OK */

doreturn:	/* unlock hash chain locked by _db_find_and_lock */
	if (un_lock(db->idxfd, db->chainoff, SEEK_SET, 1) < 0)
		err_dump("db_store: un_lock error");
	return(rc);
}

/*
 * Try to find a free index record and accompanying data record
 * of the correct sizes.  We're only called by db_store.
 */
static int
_db_findfree(DB *db, int keylen, int datlen)
{
	int		rc;
	off_t	offset, nextoffset, saveoffset;

	/*
	 * Lock the free list.
	 */
	if (writew_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
		err_dump("_db_findfree: writew_lock error");

	/*
	 * Read the free list pointer.
	 */
	saveoffset = FREE_OFF;
	offset = _db_readptr(db, saveoffset);

	while (offset != 0) {
		nextoffset = _db_readidx(db, offset);
		if (strlen(db->idxbuf) == keylen && db->datlen == datlen)
			break;		/* found a match */
		saveoffset = offset;
		offset = nextoffset;
	}

	if (offset == 0) {
		rc = -1;	/* no match found */
	} else {
		/*
		 * Found a free record with matching sizes.
		 * The index record was read in by _db_readidx above,
		 * which sets db->ptrval.  Also, saveoffset points to
		 * the chain ptr that pointed to this empty record on
		 * the free list.  We set this chain ptr to db->ptrval,
		 * which removes the empty record from the free list.
		 */
		_db_writeptr(db, saveoffset, db->ptrval);
		rc = 0;

		/*
		 * Notice also that _db_readidx set both db->idxoff
		 * and db->datoff.  This is used by the caller, db_store,
		 * to write the new index record and data record.
		 */
	}

	/*
	 * Unlock the free list.
	 */
	if (un_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
		err_dump("_db_findfree: un_lock error");
	return(rc);
}

/*
 * Rewind the index file for db_nextrec.
 * Automatically called by db_open.
 * Must be called before first db_nextrec.
 */
void
db_rewind(DBHANDLE h)
{
	DB		*db = h;
	off_t	offset;

	offset = (db->nhash + 1) * PTR_SZ;	/* +1 for free list ptr */

	/*
	 * We're just setting the file offset for this process
	 * to the start of the index records; no need to lock.
	 * +1 below for newline at end of hash table.
	 */
	if ((db->idxoff = lseek(db->idxfd, offset+1, SEEK_SET)) == -1)
		err_dump("db_rewind: lseek error");
}

/*
 * Return the next sequential record.
 * We just step our way through the index file, ignoring deleted
 * records.  db_rewind must be called before this function is
 * called the first time.
 */
char *
db_nextrec(DBHANDLE h, char *key)
{
	DB		*db = h;
	char	c;
	char	*ptr;

	/*
	 * We read lock the free list so that we don't read
	 * a record in the middle of its being deleted.
	 */
	if (readw_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
		err_dump("db_nextrec: readw_lock error");

	do {
		/*
		 * Read next sequential index record.
		 */
		if (_db_readidx(db, 0) < 0) {
			ptr = NULL;		/* end of index file, EOF */
			goto doreturn;
		}

		/*
		 * Check if key is all blank (empty record).
		 */
		ptr = db->idxbuf;
		while ((c = *ptr++) != 0  &&  c == SPACE)
			;	/* skip until null byte or nonblank */
	} while (c == 0);	/* loop until a nonblank key is found */

	if (key != NULL)
		strcpy(key, db->idxbuf);	/* return key */
	ptr = _db_readdat(db);	/* return pointer to data buffer */
	db->cnt_nextrec++;

doreturn:
	if (un_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
		err_dump("db_nextrec: un_lock error");
	return(ptr);
}