SQLite

    /* EVIDENCE-OF: R-22710-53328 The third and fourth bytes of each
    ** freeblock form a big-endian integer which is the size of the freeblock
    ** in bytes, including the 4-byte header. */
    size = get2byte(&aData[pc+2]);
    if( (x = size - nByte)>=0 ){
      testcase( x==4 );
      testcase( x==3 );
      if( pc < pPg->cellOffset+2*pPg->nCell || size+pc > usableSize ){
        *pRc = SQLITE_CORRUPT_PGNO(pPg->pgno);
        return 0;
      }else if( x<4 ){
        /* EVIDENCE-OF: R-11498-58022 In a well-formed b-tree page, the total
        ** number of bytes in fragments may not exceed 60. */
        if( aData[hdr+7]>57 ) return 0;

*/
static int freeSpace(MemPage *pPage, u16 iStart, u16 iSize){
  u16 iPtr;                             /* Address of ptr to next freeblock */
  u16 iFreeBlk;                         /* Address of the next freeblock */
  u8 hdr;                               /* Page header size.  0 or 100 */
  u8 nFrag = 0;                         /* Reduction in fragmentation */
  u16 iOrigSize = iSize;                /* Original value of iSize */
  u32 iLast = pPage->pBt->usableSize-4; /* Largest possible freeblock offset */
  u32 iEnd = iStart + iSize;            /* First byte past the iStart buffer */
  unsigned char *data = pPage->aData;   /* Page content */

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

  /* Overwrite deleted information with zeros when the secure_delete
  ** option is enabled */
  if( pPage->pBt->btsFlags & BTS_FAST_SECURE ){
    memset(&data[iStart], 0, iSize);
  }

  /* The list of freeblocks must be in ascending order.  Find the 
  ** spot on the list where iStart should be inserted.
  */
  hdr = pPage->hdrOffset;
  iPtr = hdr + 1;
  if( data[iPtr+1]==0 && data[iPtr]==0 ){
    iFreeBlk = 0;  /* Shortcut for the case when the freelist is empty */
  }else{
    while( (iFreeBlk = get2byte(&data[iPtr]))<iStart ){
      if( iFreeBlk<iPtr+4 ){
        if( iFreeBlk==0 ) break;
        return SQLITE_CORRUPT_PGNO(pPage->pgno);
      }
      iPtr = iFreeBlk;
    }

    if( iFreeBlk>iLast ) return SQLITE_CORRUPT_PGNO(pPage->pgno);

    assert( iFreeBlk>iPtr || iFreeBlk==0 );
  
    /* At this point:
    **    iFreeBlk:   First freeblock after iStart, or zero if none
    **    iPtr:       The address of a pointer to iFreeBlk
    **
    ** Check to see if iFreeBlk should be coalesced onto the end of iStart.

        iSize = iEnd - iPtr;
        iStart = iPtr;
      }
    }
    if( nFrag>data[hdr+7] ) return SQLITE_CORRUPT_PGNO(pPage->pgno);
    data[hdr+7] -= nFrag;
  }
  if( iStart==get2byte(&data[hdr+5]) ){

    /* The new freeblock is at the beginning of the cell content area,
    ** so just extend the cell content area rather than create another
    ** freelist entry */
    if( iPtr!=hdr+1 ) return SQLITE_CORRUPT_PGNO(pPage->pgno);
    put2byte(&data[hdr+1], iFreeBlk);
    put2byte(&data[hdr+5], iEnd);
  }else{
    /* Insert the new freeblock into the freelist */
    put2byte(&data[iPtr], iStart);






    put2byte(&data[iStart], iFreeBlk);
    put2byte(&data[iStart+2], iSize);
  }
  pPage->nFree += iOrigSize;
  return SQLITE_OK;
}

/*
** Decode the flags byte (the first byte of the header) for a page
** and initialize fields of the MemPage structure accordingly.

  assert( sqlite3_mutex_held(pPage->pBt->mutex) );
  data = pPage->aData;
  ptr = &pPage->aCellIdx[2*idx];
  pc = get2byte(ptr);
  hdr = pPage->hdrOffset;
  testcase( pc==get2byte(&data[hdr+5]) );
  testcase( pc+sz==pPage->pBt->usableSize );
  if( pc < (u32)get2byte(&data[hdr+5]) || pc+sz > pPage->pBt->usableSize ){
    *pRC = SQLITE_CORRUPT_BKPT;
    return;
  }
  rc = freeSpace(pPage, pc, sz);
  if( rc ){
    *pRC = rc;
    return;