SQLite

  assert( pPage->pBt!=0 );
  assert( sqlite3PagerIswriteable(pPage->pDbPage) );
  assert( start>=pPage->hdrOffset+6+pPage->childPtrSize );
  assert( (start + size)<=pPage->pBt->usableSize );
  assert( sqlite3_mutex_held(pPage->pBt->mutex) );
  assert( size>=0 );   /* Minimum cell size is 4 */

  if( pPage->pBt->secureDelete ){
    /* Overwrite deleted information with zeros when the secure_delete
    ** option is enabled */
    memset(&data[start], 0, size);

  }

  /* Add the space back into the linked list of freeblocks.  Note that
  ** even though the freeblock list was checked by btreeInitPage(),
  ** btreeInitPage() did not detect overlapping cells or
  ** freeblocks that overlapped cells.   Nor does it detect when the
  ** cell content area exceeds the value in the page header.  If these
  ** situations arise, then subsequent insert operations might corrupt

  u16 first;

  assert( sqlite3PagerPagenumber(pPage->pDbPage)==pPage->pgno );
  assert( sqlite3PagerGetExtra(pPage->pDbPage) == (void*)pPage );
  assert( sqlite3PagerGetData(pPage->pDbPage) == data );
  assert( sqlite3PagerIswriteable(pPage->pDbPage) );
  assert( sqlite3_mutex_held(pBt->mutex) );
  if( pBt->secureDelete ){
    memset(&data[hdr], 0, pBt->usableSize - hdr);

  }
  data[hdr] = (char)flags;
  first = hdr + 8 + 4*((flags&PTF_LEAF)==0 ?1:0);
  memset(&data[hdr+1], 0, 4);
  data[hdr+7] = 0;
  put2byte(&data[hdr+5], pBt->usableSize);
  pPage->nFree = pBt->usableSize - first;
  decodeFlags(pPage, flags);

    pBt->db = db;
    sqlite3PagerSetBusyhandler(pBt->pPager, btreeInvokeBusyHandler, pBt);
    p->pBt = pBt;
  
    pBt->pCursor = 0;
    pBt->pPage1 = 0;
    pBt->readOnly = sqlite3PagerIsreadonly(pBt->pPager);
#ifdef SQLITE_SECURE_DELETE
    pBt->secureDelete = 1;
#endif
    pBt->pageSize = get2byte(&zDbHeader[16]);
    if( pBt->pageSize<512 || pBt->pageSize>SQLITE_MAX_PAGE_SIZE
         || ((pBt->pageSize-1)&pBt->pageSize)!=0 ){
      pBt->pageSize = 0;
#ifndef SQLITE_OMIT_AUTOVACUUM
      /* If the magic name ":memory:" will create an in-memory database, then
      ** leave the autoVacuum mode at 0 (do not auto-vacuum), even if

int sqlite3BtreeMaxPageCount(Btree *p, int mxPage){
  int n;
  sqlite3BtreeEnter(p);
  n = sqlite3PagerMaxPageCount(p->pBt->pPager, mxPage);
  sqlite3BtreeLeave(p);
  return n;
}

/*
** Set the secureDelete flag if newFlag is 0 or 1.  If newFlag is -1,
** then make no changes.  Always return the value of the secureDelete
** setting after the change.
*/
int sqlite3BtreeSecureDelete(Btree *p, int newFlag){
  int b;
  sqlite3BtreeEnter(p);
  if( newFlag>=0 ){
    p->pBt->secureDelete = (newFlag!=0) ? 1 : 0;
  } 
  b = p->pBt->secureDelete;
  sqlite3BtreeLeave(p);
  return b;
}
#endif /* !defined(SQLITE_OMIT_PAGER_PRAGMAS) || !defined(SQLITE_OMIT_VACUUM) */

/*
** Change the 'auto-vacuum' property of the database. If the 'autoVacuum'
** parameter is non-zero, then auto-vacuum mode is enabled. If zero, it
** is disabled. The default value for the auto-vacuum property is 
** determined by the SQLITE_DEFAULT_AUTOVACUUM macro.

  /* Increment the free page count on pPage1 */
  rc = sqlite3PagerWrite(pPage1->pDbPage);
  if( rc ) goto freepage_out;
  nFree = get4byte(&pPage1->aData[36]);
  put4byte(&pPage1->aData[36], nFree+1);

  if( pBt->secureDelete ){
    /* If the secure_delete option is enabled, then
    ** always fully overwrite deleted information with zeros.
    */
    if( (!pPage && (rc = btreeGetPage(pBt, iPage, &pPage, 0)))
     ||            (rc = sqlite3PagerWrite(pPage->pDbPage))
    ){
      goto freepage_out;
    }
    memset(pPage->aData, 0, pPage->pBt->pageSize);

  }

  /* If the database supports auto-vacuum, write an entry in the pointer-map
  ** to indicate that the page is free.
  */
  if( ISAUTOVACUUM ){
    ptrmapPut(pBt, iPage, PTRMAP_FREEPAGE, 0, &rc);
    if( rc ) goto freepage_out;

      ** to 3.6.0 or later) we should consider fixing the conditional above
      ** to read "usableSize/4-2" instead of "usableSize/4-8".
      */
      rc = sqlite3PagerWrite(pTrunk->pDbPage);
      if( rc==SQLITE_OK ){
        put4byte(&pTrunk->aData[4], nLeaf+1);
        put4byte(&pTrunk->aData[8+nLeaf*4], iPage);

        if( pPage && !pBt->secureDelete ){
          sqlite3PagerDontWrite(pPage->pDbPage);
        }

        rc = btreeSetHasContent(pBt, iPage);
      }
      TRACE(("FREE-PAGE: %d leaf on trunk page %d\n",pPage->pgno,pTrunk->pgno));
      goto freepage_out;
    }
  }

      ** later on.  
      **
      ** Unless SQLite is compiled in secure-delete mode. In this case,
      ** the dropCell() routine will overwrite the entire cell with zeroes.
      ** In this case, temporarily copy the cell into the aOvflSpace[]
      ** buffer. It will be copied out again as soon as the aSpace[] buffer
      ** is allocated.  */
      if( pBt->secureDelete ){
        memcpy(&aOvflSpace[apDiv[i]-pParent->aData], apDiv[i], szNew[i]);
        apDiv[i] = &aOvflSpace[apDiv[i]-pParent->aData];

      }
      dropCell(pParent, i+nxDiv-pParent->nOverflow, szNew[i], &rc);
    }
  }

  /* Make nMaxCells a multiple of 4 in order to preserve 8-byte
  ** alignment */
  nMaxCells = (nMaxCells + 3)&~3;

int sqlite3BtreeClose(Btree*);
int sqlite3BtreeSetCacheSize(Btree*,int);
int sqlite3BtreeSetSafetyLevel(Btree*,int,int);
int sqlite3BtreeSyncDisabled(Btree*);
int sqlite3BtreeSetPageSize(Btree *p, int nPagesize, int nReserve, int eFix);
int sqlite3BtreeGetPageSize(Btree*);
int sqlite3BtreeMaxPageCount(Btree*,int);
int sqlite3BtreeSecureDelete(Btree*,int);
int sqlite3BtreeGetReserve(Btree*);
int sqlite3BtreeSetAutoVacuum(Btree *, int);
int sqlite3BtreeGetAutoVacuum(Btree *);
int sqlite3BtreeBeginTrans(Btree*,int);
int sqlite3BtreeCommitPhaseOne(Btree*, const char *zMaster);
int sqlite3BtreeCommitPhaseTwo(Btree*);
int sqlite3BtreeCommit(Btree*);

struct BtShared {
  Pager *pPager;        /* The page cache */
  sqlite3 *db;          /* Database connection currently using this Btree */
  BtCursor *pCursor;    /* A list of all open cursors */
  MemPage *pPage1;      /* First page of the database */
  u8 readOnly;          /* True if the underlying file is readonly */
  u8 pageSizeFixed;     /* True if the page size can no longer be changed */
  u8 secureDelete;      /* True if secure_delete is enabled */
#ifndef SQLITE_OMIT_AUTOVACUUM
  u8 autoVacuum;        /* True if auto-vacuum is enabled */
  u8 incrVacuum;        /* True if incr-vacuum is enabled */
#endif
  u16 pageSize;         /* Total number of bytes on a page */
  u16 usableSize;       /* Number of usable bytes on each page */
  u16 maxLocal;         /* Maximum local payload in non-LEAFDATA tables */

    /* If there are dirty pages in the page cache with page numbers greater
    ** than Pager.dbSize, this means sqlite3PagerTruncateImage() was called to
    ** make the file smaller (presumably by auto-vacuum code). Do not write
    ** any such pages to the file.
    **
    ** Also, do not write out any page that has the PGHDR_DONT_WRITE flag
    ** set (set by sqlite3PagerDontWrite()).


    */
    if( pgno<=pPager->dbSize && 0==(pList->flags&PGHDR_DONT_WRITE) ){
      i64 offset = (pgno-1)*(i64)pPager->pageSize;   /* Offset to write */
      char *pData;                                   /* Data to write */    

      /* Encode the database */
      CODEC2(pPager, pList->pData, pgno, 6, return SQLITE_NOMEM, pData);

*/
#ifndef NDEBUG
int sqlite3PagerIswriteable(DbPage *pPg){
  return pPg->flags&PGHDR_DIRTY;
}
#endif


/*
** A call to this routine tells the pager that it is not necessary to
** write the information on page pPg back to the disk, even though
** that page might be marked as dirty.  This happens, for example, when
** the page has been added as a leaf of the freelist and so its
** content no longer matters.
**

    IOTRACE(("CLEAN %p %d\n", pPager, pPg->pgno))
    pPg->flags |= PGHDR_DONT_WRITE;
#ifdef SQLITE_CHECK_PAGES
    pPg->pageHash = pager_pagehash(pPg);
#endif
  }
}


/*
** This routine is called to increment the value of the database file 
** change-counter, stored as a 4-byte big-endian integer starting at 
** byte offset 24 of the pager file.
**
** If the isDirectMode flag is zero, then this is done by calling 

    }
    if( ALWAYS(pBt) ){
      newMax = sqlite3BtreeMaxPageCount(pBt, newMax);
    }
    returnSingleInt(pParse, "max_page_count", newMax);
  }else

  /*
  **  PRAGMA [database.]secure_delete
  **  PRAGMA [database.]secure_delete=ON/OFF
  **
  ** The first form reports the current setting for the
  ** secure_delete flag.  The second form changes the secure_delete
  ** flag setting and reports thenew value.
  */
  if( sqlite3StrICmp(zLeft,"secure_delete")==0 ){
    Btree *pBt = pDb->pBt;
    int b = -1;
    assert( pBt!=0 );
    if( zRight ){
      b = getBoolean(zRight);
    }
    b = sqlite3BtreeSecureDelete(pBt, b);
    returnSingleInt(pParse, "secure_delete", b);
  }else

  /*
  **  PRAGMA [database.]page_count
  **
  ** Return the number of pages in the specified database.
  */
  if( sqlite3StrICmp(zLeft,"page_count")==0 ){
    int iReg;