AdvancedSystemProgramming/codeFromBook/db/db.c at master · brucelau-github/AdvancedSystemProgramming

875 lines (780 loc) · 24 KB
#include "apue.h"
#include "apue_db.h"
#include <fcntl.h>		/* open & db_open flags */
#include <stdarg.h>
#include <errno.h>
#include <sys/uio.h>	/* struct iovec */
 * Internal index file constants.
 * These are used to construct records in the
 * index file and data file.
#define IDXLEN_SZ	   4	/* index record length (ASCII chars) */
#define SEP         ':'	/* separator char in index record */
#define SPACE       ' '	/* space character */
#define NEWLINE     '\n'	/* newline character */
 * The following definitions are for hash chains and free
 * list chain in the index file.
#define PTR_SZ        7	/* size of ptr field in hash chain */
#define PTR_MAX 9999999	/* max file offset = 10**PTR_SZ - 1 */
#define NHASH_DEF	 137	/* default hash table size */
#define FREE_OFF      0	/* free list offset in index file */
#define HASH_OFF PTR_SZ	/* hash table offset in index file */
typedef unsigned long	DBHASH;	/* hash values */
typedef unsigned long	COUNT;	/* unsigned counter */
 * Library's private representation of the database.
typedef struct {
  int    idxfd;  /* fd for index file */
  int    datfd;  /* fd for data file */
  char  *idxbuf; /* malloc'ed buffer for index record */
  char  *datbuf; /* malloc'ed buffer for data record*/
  char  *name;   /* name db was opened under */
  off_t  idxoff; /* offset in index file of index record */
			      /* key is at (idxoff + PTR_SZ + IDXLEN_SZ) */
  size_t idxlen; /* length of index record */
			      /* excludes IDXLEN_SZ bytes at front of record */
			      /* includes newline at end of index record */
  off_t  datoff; /* offset in data file of data record */
  size_t datlen; /* length of data record */
			      /* includes newline at end */
  off_t  ptrval; /* contents of chain ptr in index record */
  off_t  ptroff; /* chain ptr offset pointing to this idx record */
  off_t  chainoff; /* offset of hash chain for this index record */
  off_t  hashoff;  /* offset in index file of hash table */
  DBHASH nhash;    /* current hash table size */
  COUNT  cnt_delok;    /* delete OK */
  COUNT  cnt_delerr;   /* delete error */
  COUNT  cnt_fetchok;  /* fetch OK */
  COUNT  cnt_fetcherr; /* fetch error */
  COUNT  cnt_nextrec;  /* nextrec */
  COUNT  cnt_stor1;    /* store: DB_INSERT, no empty, appended */
  COUNT  cnt_stor2;    /* store: DB_INSERT, found empty, reused */
  COUNT  cnt_stor3;    /* store: DB_REPLACE, diff len, appended */
  COUNT  cnt_stor4;    /* store: DB_REPLACE, same len, overwrote */
  COUNT  cnt_storerr;  /* store error */
 * Internal functions.
static DB     *_db_alloc(int);
static void    _db_dodelete(DB *);
static int	    _db_find_and_lock(DB *, const char *, int);
static int     _db_findfree(DB *, int, int);
static void    _db_free(DB *);
static DBHASH  _db_hash(DB *, const char *);
static char   *_db_readdat(DB *);
static off_t   _db_readidx(DB *, off_t);
static off_t   _db_readptr(DB *, off_t);
static void    _db_writedat(DB *, const char *, off_t, int);
static void    _db_writeidx(DB *, const char *, off_t, int, off_t);
static void    _db_writeptr(DB *, off_t, off_t);
 * Open or create a database.  Same arguments as open(2).
db_open(const char *pathname, int oflag, ...)
	int			len, mode;
	size_t		i;
	char		asciiptr[PTR_SZ + 1],
				hash[(NHASH_DEF + 1) * PTR_SZ + 2];
					/* +2 for newline and null */
	struct stat	statbuff;
	 * Allocate a DB structure, and the buffers it needs.
	len = strlen(pathname);
	if ((db = _db_alloc(len)) == NULL)
		err_dump("db_open: _db_alloc error for DB");
	db->nhash   = NHASH_DEF;/* hash table size */
	db->hashoff = HASH_OFF;	/* offset in index file of hash table */
	strcpy(db->name, pathname);
	strcat(db->name, ".idx");
	if (oflag & O_CREAT) {
		va_list ap;
		va_start(ap, oflag);
		mode = va_arg(ap, int);
		va_end(ap);
		 * Open index file and data file.
		db->idxfd = open(db->name, oflag, mode);
		strcpy(db->name + len, ".dat");
		db->datfd = open(db->name, oflag, mode);
		 * Open index file and data file.
		db->idxfd = open(db->name, oflag);
		strcpy(db->name + len, ".dat");
		db->datfd = open(db->name, oflag);
	if (db->idxfd < 0 || db->datfd < 0) {
		_db_free(db);
		return(NULL);
	if ((oflag & (O_CREAT | O_TRUNC)) == (O_CREAT | O_TRUNC)) {
		 * If the database was created, we have to initialize
		 * it.  Write lock the entire file so that we can stat
		 * it, check its size, and initialize it, atomically.
		if (writew_lock(db->idxfd, 0, SEEK_SET, 0) < 0)
			err_dump("db_open: writew_lock error");
		if (fstat(db->idxfd, &statbuff) < 0)
			err_sys("db_open: fstat error");
		if (statbuff.st_size == 0) {
			 * We have to build a list of (NHASH_DEF + 1) chain
			 * ptrs with a value of 0.  The +1 is for the free
			 * list pointer that precedes the hash table.
			sprintf(asciiptr, "%*d", PTR_SZ, 0);
			hash[0] = 0;
			for (i = 0; i < NHASH_DEF + 1; i++)
				strcat(hash, asciiptr);
			strcat(hash, "\n");
			i = strlen(hash);
			if (write(db->idxfd, hash, i) != i)
				err_dump("db_open: index file init write error");
		if (un_lock(db->idxfd, 0, SEEK_SET, 0) < 0)
			err_dump("db_open: un_lock error");
	db_rewind(db);
	return(db);
 * Allocate & initialize a DB structure and its buffers.
static DB *
_db_alloc(int namelen)
	 * Use calloc, to initialize the structure to zero.
	if ((db = calloc(1, sizeof(DB))) == NULL)
		err_dump("_db_alloc: calloc error for DB");
	db->idxfd = db->datfd = -1;				/* descriptors */
	 * Allocate room for the name.
	 * +5 for ".idx" or ".dat" plus null at end.
	if ((db->name = malloc(namelen + 5)) == NULL)
		err_dump("_db_alloc: malloc error for name");
	 * Allocate an index buffer and a data buffer.
	 * +2 for newline and null at end.
	if ((db->idxbuf = malloc(IDXLEN_MAX + 2)) == NULL)
		err_dump("_db_alloc: malloc error for index buffer");
	if ((db->datbuf = malloc(DATLEN_MAX + 2)) == NULL)
		err_dump("_db_alloc: malloc error for data buffer");
	return(db);
 * Relinquish access to the database.
db_close(DBHANDLE h)
	_db_free((DB *)h);	/* closes fds, free buffers & struct */
 * Free up a DB structure, and all the malloc'ed buffers it
 * may point to.  Also close the file descriptors if still open.
static void
_db_free(DB *db)
	if (db->idxfd >= 0)
		close(db->idxfd);
	if (db->datfd >= 0)
		close(db->datfd);
	if (db->idxbuf != NULL)
		free(db->idxbuf);
	if (db->datbuf != NULL)
		free(db->datbuf);
	if (db->name != NULL)
		free(db->name);
 * Fetch a record.  Return a pointer to the null-terminated data.
db_fetch(DBHANDLE h, const char *key)
	DB      *db = h;
	char	*ptr;
	if (_db_find_and_lock(db, key, 0) < 0) {
		ptr = NULL;				/* error, record not found */
		db->cnt_fetcherr++;
		ptr = _db_readdat(db);	/* return pointer to data */
		db->cnt_fetchok++;
	 * Unlock the hash chain that _db_find_and_lock locked.
	if (un_lock(db->idxfd, db->chainoff, SEEK_SET, 1) < 0)
		err_dump("db_fetch: un_lock error");
	return(ptr);
 * Find the specified record.  Called by db_delete, db_fetch,
 * and db_store.  Returns with the hash chain locked.
_db_find_and_lock(DB *db, const char *key, int writelock)
	off_t	offset, nextoffset;
	 * Calculate the hash value for this key, then calculate the
	 * byte offset of corresponding chain ptr in hash table.
	 * This is where our search starts.  First we calculate the
	 * offset in the hash table for this key.
	db->chainoff = (_db_hash(db, key) * PTR_SZ) + db->hashoff;
	db->ptroff = db->chainoff;
	 * We lock the hash chain here.  The caller must unlock it
	 * when done.  Note we lock and unlock only the first byte.
	if (writelock) {
		if (writew_lock(db->idxfd, db->chainoff, SEEK_SET, 1) < 0)
			err_dump("_db_find_and_lock: writew_lock error");
		if (readw_lock(db->idxfd, db->chainoff, SEEK_SET, 1) < 0)
			err_dump("_db_find_and_lock: readw_lock error");
	 * Get the offset in the index file of first record
	 * on the hash chain (can be 0).
	offset = _db_readptr(db, db->ptroff);
	while (offset != 0) {
		nextoffset = _db_readidx(db, offset);
		if (strcmp(db->idxbuf, key) == 0)
			break;       /* found a match */
		db->ptroff = offset; /* offset of this (unequal) record */
		offset = nextoffset; /* next one to compare */
	 * offset == 0 on error (record not found).
	return(offset == 0 ? -1 : 0);
 * Calculate the hash value for a key.
static DBHASH
_db_hash(DB *db, const char *key)
	DBHASH		hval = 0;
	for (i = 1; (c = *key++) != 0; i++)
		hval += c * i;		/* ascii char times its 1-based index */
	return(hval % db->nhash);
 * Read a chain ptr field from anywhere in the index file:
 * the free list pointer, a hash table chain ptr, or an
 * index record chain ptr.
static off_t
_db_readptr(DB *db, off_t offset)
	char	asciiptr[PTR_SZ + 1];
	if (lseek(db->idxfd, offset, SEEK_SET) == -1)
		err_dump("_db_readptr: lseek error to ptr field");
	if (read(db->idxfd, asciiptr, PTR_SZ) != PTR_SZ)
		err_dump("_db_readptr: read error of ptr field");
	asciiptr[PTR_SZ] = 0;		/* null terminate */
	return(atol(asciiptr));
 * Read the next index record.  We start at the specified offset
 * in the index file.  We read the index record into db->idxbuf
 * and replace the separators with null bytes.  If all is OK we
 * set db->datoff and db->datlen to the offset and length of the
 * corresponding data record in the data file.
static off_t
_db_readidx(DB *db, off_t offset)
	ssize_t				i;
	char			*ptr1, *ptr2;
	char			asciiptr[PTR_SZ + 1], asciilen[IDXLEN_SZ + 1];
	struct iovec	iov[2];
	 * Position index file and record the offset.  db_nextrec
	 * calls us with offset==0, meaning read from current offset.
	 * We still need to call lseek to record the current offset.
	if ((db->idxoff = lseek(db->idxfd, offset,
	  offset == 0 ? SEEK_CUR : SEEK_SET)) == -1)
		err_dump("_db_readidx: lseek error");
	 * Read the ascii chain ptr and the ascii length at
	 * the front of the index record.  This tells us the
	 * remaining size of the index record.
	iov[0].iov_base = asciiptr;
	iov[0].iov_len  = PTR_SZ;
	iov[1].iov_base = asciilen;
	iov[1].iov_len  = IDXLEN_SZ;
	if ((i = readv(db->idxfd, &iov[0], 2)) != PTR_SZ + IDXLEN_SZ) {
		if (i == 0 && offset == 0)
			return(-1);		/* EOF for db_nextrec */
		err_dump("_db_readidx: readv error of index record");
	 * This is our return value; always >= 0.
	asciiptr[PTR_SZ] = 0;        /* null terminate */
	db->ptrval = atol(asciiptr); /* offset of next key in chain */
	asciilen[IDXLEN_SZ] = 0;     /* null terminate */
	if ((db->idxlen = atoi(asciilen)) < IDXLEN_MIN ||
	  db->idxlen > IDXLEN_MAX)
		err_dump("_db_readidx: invalid length");
	 * Now read the actual index record.  We read it into the key
	 * buffer that we malloced when we opened the database.
	if ((i = read(db->idxfd, db->idxbuf, db->idxlen)) != db->idxlen)
		err_dump("_db_readidx: read error of index record");
	if (db->idxbuf[db->idxlen-1] != NEWLINE)	/* sanity check */
		err_dump("_db_readidx: missing newline");
	db->idxbuf[db->idxlen-1] = 0;	 /* replace newline with null */
	 * Find the separators in the index record.
	if ((ptr1 = strchr(db->idxbuf, SEP)) == NULL)
		err_dump("_db_readidx: missing first separator");
	*ptr1++ = 0;				/* replace SEP with null */
	if ((ptr2 = strchr(ptr1, SEP)) == NULL)
		err_dump("_db_readidx: missing second separator");
	*ptr2++ = 0;				/* replace SEP with null */
	if (strchr(ptr2, SEP) != NULL)
		err_dump("_db_readidx: too many separators");
	 * Get the starting offset and length of the data record.
	if ((db->datoff = atol(ptr1)) < 0)
		err_dump("_db_readidx: starting offset < 0");
	if ((db->datlen = atol(ptr2)) <= 0 || db->datlen > DATLEN_MAX)
		err_dump("_db_readidx: invalid length");
	return(db->ptrval);		/* return offset of next key in chain */
 * Read the current data record into the data buffer.
 * Return a pointer to the null-terminated data buffer.
static char *
_db_readdat(DB *db)
	if (lseek(db->datfd, db->datoff, SEEK_SET) == -1)
		err_dump("_db_readdat: lseek error");
	if (read(db->datfd, db->datbuf, db->datlen) != db->datlen)
		err_dump("_db_readdat: read error");
	if (db->datbuf[db->datlen-1] != NEWLINE)	/* sanity check */
		err_dump("_db_readdat: missing newline");
	db->datbuf[db->datlen-1] = 0; /* replace newline with null */
	return(db->datbuf);		/* return pointer to data record */
 * Delete the specified record.
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++;
		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)
	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;
	if ((db->ptrval = ptrval) < 0 || ptrval > PTR_MAX)
		err_quit("_db_writeidx: invalid ptr: %d", ptrval);
	sprintf(db->idxbuf, "%s%c%lld%c%ld\n", key, SEP,
	  (long long)db->datoff, SEP, (long)db->datlen);
	len = strlen(db->idxbuf);
	if (len < IDXLEN_MIN || len > IDXLEN_MAX)
		err_dump("_db_writeidx: invalid length");
	sprintf(asciiptrlen, "%*lld%*d", PTR_SZ, (long long)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, "%*lld", PTR_SZ, (long long)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.
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++;
			 * 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++;
			 * 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.
_db_findfree(DB *db, int keylen, int datlen)
	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 */
		 * 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);
		 * 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.
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.
db_nextrec(DBHANDLE h, char *key)
	DB		*db = h;
	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");
		 * 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++;
	if (un_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
		err_dump("db_nextrec: un_lock error");
	return(ptr);
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