char *z;                          /* Name of this chunk */
  } *aReal;                        /* list of all chunks */
  int nReal;                       /* Number of chunks */
  char *zName;                     /* Base filename of this group */
  int nName;                       /* Length of base filename */
  int flags;                       /* Flags used for original opening */
  unsigned int szChunk;            /* Chunk size used for this group */
  int bEnabled;                    /* TRUE to use Multiplex VFS for this file */

  multiplexGroup *pNext, *pPrev;   /* Doubly linked list of all group objects */
};

/*
** An instance of the following object represents each open connection
** to a file that is multiplex'ed.  This object is a 
** subclass of sqlite3_file.  The sqlite3_file object for the underlying
................................................................................
    multiplexFilename(pGroup->zName, pGroup->nName, pGroup->flags, iChunk, z);
  }
  return SQLITE_OK;
}

/* Translate an sqlite3_file* that is really a multiplexGroup* into
** the sqlite3_file* for the underlying original VFS.




*/
static sqlite3_file *multiplexSubOpen(
  multiplexGroup *pGroup,
  int iChunk,
  int *rc,


  int *pOutFlags

){
  sqlite3_file *pSubOpen = 0;
  sqlite3_vfs *pOrigVfs = gMultiplex.pOrigVfs;        /* Real VFS */

#ifdef SQLITE_ENABLE_8_3_NAMES
  /* If JOURNAL_8_3_OFFSET is set to (say) 400, then any overflow files are 
  ** part of a database journal are named db.401, db.402, and so on. A 
................................................................................
    *rc = SQLITE_FULL;
    return 0;
  }
#endif

  *rc = multiplexSubFilename(pGroup, iChunk);
  if( (*rc)==SQLITE_OK && (pSubOpen = pGroup->aReal[iChunk].p)==0 ){













    pSubOpen = sqlite3_malloc( pOrigVfs->szOsFile );
    if( pSubOpen==0 ){
      *rc = SQLITE_NOMEM;
      return 0;
    }
    pGroup->aReal[iChunk].p = pSubOpen;
    *rc = pOrigVfs->xOpen(pOrigVfs, pGroup->aReal[iChunk].z, pSubOpen,
                          pGroup->flags, pOutFlags);
    if( *rc!=SQLITE_OK ){
      sqlite3_free(pSubOpen);
      pGroup->aReal[iChunk].p = 0;
      return 0;
    }
  }
  return pSubOpen;
}




















/*
** This is the implementation of the multiplex_control() SQL function.
*/
static void multiplexControlFunc(
  sqlite3_context *context,
  int argc,
  sqlite3_value **argv
................................................................................
  }

  if( rc==SQLITE_OK ){
    /* assign pointers to extra space allocated */
    memset(pGroup, 0, sz);
    pMultiplexOpen->pGroup = pGroup;
    pGroup->bEnabled = -1;

    pGroup->szChunk = SQLITE_MULTIPLEX_CHUNK_SIZE;

    if( zName ){
      char *p = (char *)&pGroup[1];
      if( flags & SQLITE_OPEN_URI ){
        const char *zChunkSize;
        zChunkSize = sqlite3_uri_parameter(zName, "chunksize");
................................................................................
          if( n>0 ){
            pGroup->szChunk = (n+0xffff)&~0xffff;
          }else{
            /* A zero or negative chunksize disabled the multiplexor */
            pGroup->bEnabled = 0;
          }
        }

      }
      pGroup->zName = p;
      memcpy(pGroup->zName, zName, nName+1);
      pGroup->nName = nName;
    }
    if( pGroup->bEnabled ){
      /* Make sure that the chunksize is not such that the pending byte
................................................................................
      while( (sqlite3PendingByte % pGroup->szChunk)>=(pGroup->szChunk-65536) ){
        pGroup->szChunk += 65536;
      }
    }
    pGroup->flags = flags;
    rc = multiplexSubFilename(pGroup, 1);
    if( rc==SQLITE_OK ){
      pSubOpen = multiplexSubOpen(pGroup, 0, &rc, pOutFlags);
      assert( pSubOpen || rc!=SQLITE_OK );
    }
    if( rc==SQLITE_OK ){
      sqlite3_int64 sz;

      rc = pSubOpen->pMethods->xFileSize(pSubOpen, &sz);
      if( rc==SQLITE_OK && zName ){
        int bExists;
................................................................................
                  rc = multiplexSubFilename(pGroup, ++iChunk);
                }
              }
            }while( rc==SQLITE_OK && bExists );
          }
        }else{
          /* If the first overflow file exists and if the size of the main file
           ** is different from the chunk size, that means the chunk size is set
           ** set incorrectly.  So fix it.
           **
           ** Or, if the first overflow file does not exist and the main file is
           ** larger than the chunk size, that means the chunk size is too small.
           ** But we have no way of determining the intended chunk size, so 
           ** just disable the multiplexor all togethre.
           */
          rc = pOrigVfs->xAccess(pOrigVfs, pGroup->aReal[1].z,
              SQLITE_ACCESS_EXISTS, &bExists);

          if( rc==SQLITE_OK && bExists && sz==(sz&0xffff0000) && sz>0
              && sz!=pGroup->szChunk ){
            pGroup->szChunk = sz;
          }else if( rc==SQLITE_OK && !bExists && sz>pGroup->szChunk ){
            pGroup->bEnabled = 0;
          }
        }
      }
................................................................................
  sqlite3_int64 iOfst
){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_READ;
    }else{
      rc = pSubOpen->pMethods->xRead(pSubOpen, pBuf, iAmt, iOfst);
    }
  }else{
    while( iAmt > 0 ){
      int i = (int)(iOfst / pGroup->szChunk);
      sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, i, &rc, NULL);
      if( pSubOpen ){
        int extra = ((int)(iOfst % pGroup->szChunk) + iAmt) - pGroup->szChunk;
        if( extra<0 ) extra = 0;
        iAmt -= extra;
        rc = pSubOpen->pMethods->xRead(pSubOpen, pBuf, iAmt,
                                       iOfst % pGroup->szChunk);
        if( rc!=SQLITE_OK ) break;
................................................................................
  sqlite3_int64 iOfst
){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_WRITE;
    }else{
      rc = pSubOpen->pMethods->xWrite(pSubOpen, pBuf, iAmt, iOfst);
    }
  }else{
    while( rc==SQLITE_OK && iAmt>0 ){
      int i = (int)(iOfst / pGroup->szChunk);
      sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, i, &rc, NULL);
      if( pSubOpen ){
        int extra = ((int)(iOfst % pGroup->szChunk) + iAmt) -
                    pGroup->szChunk;
        if( extra<0 ) extra = 0;
        iAmt -= extra;
        rc = pSubOpen->pMethods->xWrite(pSubOpen, pBuf, iAmt,
                                        iOfst % pGroup->szChunk);
................................................................................
*/
static int multiplexTruncate(sqlite3_file *pConn, sqlite3_int64 size){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_TRUNCATE;
    }else{
      rc = pSubOpen->pMethods->xTruncate(pSubOpen, size);
    }
  }else if( (pGroup->flags & SQLITE_OPEN_MAIN_DB)==0 ){
    int rc2;
    int i;

    sqlite3_file *pSubOpen;
    sqlite3_vfs *pOrigVfs = gMultiplex.pOrigVfs;   /* Real VFS */
    /* delete the chunks above the truncate limit */
    for(i=(int)(size / pGroup->szChunk)+1; i<pGroup->nReal; i++){

      multiplexSubClose(pGroup, i, pOrigVfs);




    }



    pSubOpen = multiplexSubOpen(pGroup, (int)(size/pGroup->szChunk), &rc2,0);
    if( pSubOpen ){
      rc2 = pSubOpen->pMethods->xTruncate(pSubOpen, size % pGroup->szChunk);
      if( rc2!=SQLITE_OK ) rc = rc2;
    }else{
      rc = SQLITE_IOERR_TRUNCATE;
    }


  }
  multiplexLeave();
  return rc;
}

/* Pass xSync requests through to the original VFS without change
*/
................................................................................
static int multiplexFileSize(sqlite3_file *pConn, sqlite3_int64 *pSize){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  int i;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_FSTAT;
    }else{
      rc = pSubOpen->pMethods->xFileSize(pSubOpen, pSize);
    }
  }else{
    sqlite3_vfs *pOrigVfs = gMultiplex.pOrigVfs;
    *pSize = 0;
    for(i=0; rc==SQLITE_OK; i++){
      sqlite3_file *pSubOpen = 0;
      int exists = 0;
      rc = multiplexSubFilename(pGroup, i);
      if( rc!=SQLITE_OK ) break;
      if( pGroup->nReal>i && pGroup->aReal[i].p!=0 ){
        exists = 1;
      }else if( (pGroup->flags & SQLITE_OPEN_DELETEONCLOSE)==0 ){
        const char *zReal = pGroup->aReal[i].z;
        rc = pOrigVfs->xAccess(pOrigVfs, zReal, SQLITE_ACCESS_EXISTS, &exists);
      }
      if( exists==0 ){
        /* stop at first "gap" or IO error. */
        break;
      }
      if( rc==SQLITE_OK ){
        pSubOpen = multiplexSubOpen(pGroup, i, &rc, NULL);
      }
      assert( pSubOpen || rc!=SQLITE_OK );
      if( rc==SQLITE_OK ){
        sqlite3_int64 sz = 0;
        rc = pSubOpen->pMethods->xFileSize(pSubOpen, &sz);
        if( rc==SQLITE_OK && sz>pGroup->szChunk ){
          rc = SQLITE_IOERR_FSTAT;
        }

        *pSize = i*(sqlite3_int64)pGroup->szChunk + sz;
      }
    }
  }
  multiplexLeave();
  return rc;
}

/* Pass xLock requests through to the original VFS unchanged.
*/
static int multiplexLock(sqlite3_file *pConn, int lock){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    return pSubOpen->pMethods->xLock(pSubOpen, lock);
  }
  return SQLITE_BUSY;
}

/* Pass xUnlock requests through to the original VFS unchanged.
*/
static int multiplexUnlock(sqlite3_file *pConn, int lock){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    return pSubOpen->pMethods->xUnlock(pSubOpen, lock);
  }
  return SQLITE_IOERR_UNLOCK;
}

/* Pass xCheckReservedLock requests through to the original VFS unchanged.
*/
static int multiplexCheckReservedLock(sqlite3_file *pConn, int *pResOut){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    return pSubOpen->pMethods->xCheckReservedLock(pSubOpen, pResOut);
  }
  return SQLITE_IOERR_CHECKRESERVEDLOCK;
}

/* Pass xFileControl requests through to the original VFS unchanged,
................................................................................
      break;
    case SQLITE_FCNTL_SIZE_HINT:
    case SQLITE_FCNTL_CHUNK_SIZE:
      /* no-op these */
      rc = SQLITE_OK;
      break;
    default:
      pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL);
      if( pSubOpen ){
        rc = pSubOpen->pMethods->xFileControl(pSubOpen, op, pArg);
        if( op==SQLITE_FCNTL_VFSNAME && rc==SQLITE_OK ){
         *(char**)pArg = sqlite3_mprintf("multiplex/%z", *(char**)pArg);
        }
      }
      break;
................................................................................
}

/* Pass xSectorSize requests through to the original VFS unchanged.
*/
static int multiplexSectorSize(sqlite3_file *pConn){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen && pSubOpen->pMethods->xSectorSize ){
    return pSubOpen->pMethods->xSectorSize(pSubOpen);
  }
  return DEFAULT_SECTOR_SIZE;
}

/* Pass xDeviceCharacteristics requests through to the original VFS unchanged.
*/
static int multiplexDeviceCharacteristics(sqlite3_file *pConn){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    return pSubOpen->pMethods->xDeviceCharacteristics(pSubOpen);
  }
  return 0;
}

/* Pass xShmMap requests through to the original VFS unchanged.
................................................................................
  int iRegion,                    /* Region to retrieve */
  int szRegion,                   /* Size of regions */
  int bExtend,                    /* True to extend file if necessary */
  void volatile **pp              /* OUT: Mapped memory */
){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    return pSubOpen->pMethods->xShmMap(pSubOpen, iRegion, szRegion, bExtend,pp);
  }
  return SQLITE_IOERR;
}

/* Pass xShmLock requests through to the original VFS unchanged.
................................................................................
  sqlite3_file *pConn,       /* Database file holding the shared memory */
  int ofst,                  /* First lock to acquire or release */
  int n,                     /* Number of locks to acquire or release */
  int flags                  /* What to do with the lock */
){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    return pSubOpen->pMethods->xShmLock(pSubOpen, ofst, n, flags);
  }
  return SQLITE_BUSY;
}

/* Pass xShmBarrier requests through to the original VFS unchanged.
*/
static void multiplexShmBarrier(sqlite3_file *pConn){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    pSubOpen->pMethods->xShmBarrier(pSubOpen);
  }
}

/* Pass xShmUnmap requests through to the original VFS unchanged.
*/
static int multiplexShmUnmap(sqlite3_file *pConn, int deleteFlag){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL);
  if( pSubOpen ){
    return pSubOpen->pMethods->xShmUnmap(pSubOpen, deleteFlag);
  }
  return SQLITE_OK;
}

/************************** Public Interfaces *****************************/

    char *z;                          /* Name of this chunk */
  } *aReal;                        /* list of all chunks */
  int nReal;                       /* Number of chunks */
  char *zName;                     /* Base filename of this group */
  int nName;                       /* Length of base filename */
  int flags;                       /* Flags used for original opening */
  unsigned int szChunk;            /* Chunk size used for this group */
  unsigned char bEnabled;          /* TRUE to use Multiplex VFS for this file */
  unsigned char bTruncate;         /* TRUE to enable truncation of databases */
  multiplexGroup *pNext, *pPrev;   /* Doubly linked list of all group objects */
};

/*
** An instance of the following object represents each open connection
** to a file that is multiplex'ed.  This object is a 
** subclass of sqlite3_file.  The sqlite3_file object for the underlying
................................................................................
    multiplexFilename(pGroup->zName, pGroup->nName, pGroup->flags, iChunk, z);
  }
  return SQLITE_OK;
}

/* Translate an sqlite3_file* that is really a multiplexGroup* into
** the sqlite3_file* for the underlying original VFS.
**
** For chunk 0, the pGroup->flags determines whether or not a new file
** is created if it does not already exist.  For chunks 1 and higher, the
** file is created only if createFlag is 1.
*/
static sqlite3_file *multiplexSubOpen(
  multiplexGroup *pGroup,    /* The multiplexor group */


  int iChunk,                /* Which chunk to open.  0==original file */
  int *rc,                   /* Result code in and out */
  int *pOutFlags,            /* Output flags */
  int createFlag             /* True to create if iChunk>0 */
){
  sqlite3_file *pSubOpen = 0;
  sqlite3_vfs *pOrigVfs = gMultiplex.pOrigVfs;        /* Real VFS */

#ifdef SQLITE_ENABLE_8_3_NAMES
  /* If JOURNAL_8_3_OFFSET is set to (say) 400, then any overflow files are 
  ** part of a database journal are named db.401, db.402, and so on. A 
................................................................................
    *rc = SQLITE_FULL;
    return 0;
  }
#endif

  *rc = multiplexSubFilename(pGroup, iChunk);
  if( (*rc)==SQLITE_OK && (pSubOpen = pGroup->aReal[iChunk].p)==0 ){
    int flags, rc2, bExists;
    if( iChunk==0 ) createFlag = (pGroup->flags & SQLITE_OPEN_CREATE)!=0;
    flags = pGroup->flags;
    if( createFlag ){
      flags |= SQLITE_OPEN_CREATE;
    }else if( pGroup->aReal[iChunk].z==0 ){
      return 0;
    }else{
      *rc = pOrigVfs->xAccess(pOrigVfs, pGroup->aReal[iChunk].z,
                              SQLITE_ACCESS_EXISTS, &bExists);
      if( *rc || !bExists ) return 0;
      flags &= ~SQLITE_OPEN_CREATE;
    }
    pSubOpen = sqlite3_malloc( pOrigVfs->szOsFile );
    if( pSubOpen==0 ){
      *rc = SQLITE_NOMEM;
      return 0;
    }
    pGroup->aReal[iChunk].p = pSubOpen;
    rc2 = pOrigVfs->xOpen(pOrigVfs, pGroup->aReal[iChunk].z, pSubOpen,
                          flags, pOutFlags);
    if( rc2!=SQLITE_OK ){
      sqlite3_free(pSubOpen);
      pGroup->aReal[iChunk].p = 0;
      return 0;
    }
  }
  return pSubOpen;
}

/*
** Return the size, in bytes, of chunk number iChunk.  If that chunk
** does not exist, then return 0.  This function does not distingish between
** non-existant files and zero-length files.
*/
static sqlite3_int64 multiplexSubSize(
  multiplexGroup *pGroup,    /* The multiplexor group */
  int iChunk,                /* Which chunk to open.  0==original file */
  int *rc                    /* Result code in and out */
){
  sqlite3_file *pSub;
  sqlite3_int64 sz = 0;

  pSub = multiplexSubOpen(pGroup, iChunk, rc, NULL, 0);
  if( pSub==0 ) return 0;
  *rc = pSub->pMethods->xFileSize(pSub, &sz);
  return sz;
}    

/*
** This is the implementation of the multiplex_control() SQL function.
*/
static void multiplexControlFunc(
  sqlite3_context *context,
  int argc,
  sqlite3_value **argv
................................................................................
  }

  if( rc==SQLITE_OK ){
    /* assign pointers to extra space allocated */
    memset(pGroup, 0, sz);
    pMultiplexOpen->pGroup = pGroup;
    pGroup->bEnabled = -1;
    pGroup->bTruncate = (flags & SQLITE_OPEN_MAIN_DB)==0;
    pGroup->szChunk = SQLITE_MULTIPLEX_CHUNK_SIZE;

    if( zName ){
      char *p = (char *)&pGroup[1];
      if( flags & SQLITE_OPEN_URI ){
        const char *zChunkSize;
        zChunkSize = sqlite3_uri_parameter(zName, "chunksize");
................................................................................
          if( n>0 ){
            pGroup->szChunk = (n+0xffff)&~0xffff;
          }else{
            /* A zero or negative chunksize disabled the multiplexor */
            pGroup->bEnabled = 0;
          }
        }
        if( sqlite3_uri_parameter(zName, "truncate") ) pGroup->bTruncate = 1;
      }
      pGroup->zName = p;
      memcpy(pGroup->zName, zName, nName+1);
      pGroup->nName = nName;
    }
    if( pGroup->bEnabled ){
      /* Make sure that the chunksize is not such that the pending byte
................................................................................
      while( (sqlite3PendingByte % pGroup->szChunk)>=(pGroup->szChunk-65536) ){
        pGroup->szChunk += 65536;
      }
    }
    pGroup->flags = flags;
    rc = multiplexSubFilename(pGroup, 1);
    if( rc==SQLITE_OK ){
      pSubOpen = multiplexSubOpen(pGroup, 0, &rc, pOutFlags, 0);
      if( pSubOpen==0 && rc==SQLITE_OK ) rc = SQLITE_CANTOPEN;
    }
    if( rc==SQLITE_OK ){
      sqlite3_int64 sz;

      rc = pSubOpen->pMethods->xFileSize(pSubOpen, &sz);
      if( rc==SQLITE_OK && zName ){
        int bExists;
................................................................................
                  rc = multiplexSubFilename(pGroup, ++iChunk);
                }
              }
            }while( rc==SQLITE_OK && bExists );
          }
        }else{
          /* If the first overflow file exists and if the size of the main file
          ** is different from the chunk size, that means the chunk size is set
          ** set incorrectly.  So fix it.
          **
          ** Or, if the first overflow file does not exist and the main file is
          ** larger than the chunk size, that means the chunk size is too small.
          ** But we have no way of determining the intended chunk size, so 
          ** just disable the multiplexor all togethre.
          */
          rc = pOrigVfs->xAccess(pOrigVfs, pGroup->aReal[1].z,
              SQLITE_ACCESS_EXISTS, &bExists);
          bExists = multiplexSubSize(pGroup, 1, &rc)>0;
          if( rc==SQLITE_OK && bExists  && sz==(sz&0xffff0000) && sz>0
              && sz!=pGroup->szChunk ){
            pGroup->szChunk = sz;
          }else if( rc==SQLITE_OK && !bExists && sz>pGroup->szChunk ){
            pGroup->bEnabled = 0;
          }
        }
      }
................................................................................
  sqlite3_int64 iOfst
){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL, 0);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_READ;
    }else{
      rc = pSubOpen->pMethods->xRead(pSubOpen, pBuf, iAmt, iOfst);
    }
  }else{
    while( iAmt > 0 ){
      int i = (int)(iOfst / pGroup->szChunk);
      sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, i, &rc, NULL, 1);
      if( pSubOpen ){
        int extra = ((int)(iOfst % pGroup->szChunk) + iAmt) - pGroup->szChunk;
        if( extra<0 ) extra = 0;
        iAmt -= extra;
        rc = pSubOpen->pMethods->xRead(pSubOpen, pBuf, iAmt,
                                       iOfst % pGroup->szChunk);
        if( rc!=SQLITE_OK ) break;
................................................................................
  sqlite3_int64 iOfst
){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL, 0);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_WRITE;
    }else{
      rc = pSubOpen->pMethods->xWrite(pSubOpen, pBuf, iAmt, iOfst);
    }
  }else{
    while( rc==SQLITE_OK && iAmt>0 ){
      int i = (int)(iOfst / pGroup->szChunk);
      sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, i, &rc, NULL, 1);
      if( pSubOpen ){
        int extra = ((int)(iOfst % pGroup->szChunk) + iAmt) -
                    pGroup->szChunk;
        if( extra<0 ) extra = 0;
        iAmt -= extra;
        rc = pSubOpen->pMethods->xWrite(pSubOpen, pBuf, iAmt,
                                        iOfst % pGroup->szChunk);
................................................................................
*/
static int multiplexTruncate(sqlite3_file *pConn, sqlite3_int64 size){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL, 0);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_TRUNCATE;
    }else{
      rc = pSubOpen->pMethods->xTruncate(pSubOpen, size);
    }
  }else{

    int i;
    int iBaseGroup = (int)(size / pGroup->szChunk);
    sqlite3_file *pSubOpen;
    sqlite3_vfs *pOrigVfs = gMultiplex.pOrigVfs;   /* Real VFS */
    /* delete the chunks above the truncate limit */
    for(i = pGroup->nReal-1; i>iBaseGroup && rc==SQLITE_OK; i--){
      if( pGroup->bTruncate ){
        multiplexSubClose(pGroup, i, pOrigVfs);
      }else{
        pSubOpen = multiplexSubOpen(pGroup, i, &rc, 0, 0);
        if( pSubOpen ){
          rc = pSubOpen->pMethods->xTruncate(pSubOpen, 0);
        }
      }
    }
    if( rc==SQLITE_OK ){
      pSubOpen = multiplexSubOpen(pGroup, iBaseGroup, &rc, 0, 0);
      if( pSubOpen ){
        rc = pSubOpen->pMethods->xTruncate(pSubOpen, size % pGroup->szChunk);



      }
    }
    if( rc ) rc = SQLITE_IOERR_TRUNCATE;
  }
  multiplexLeave();
  return rc;
}

/* Pass xSync requests through to the original VFS without change
*/
................................................................................
static int multiplexFileSize(sqlite3_file *pConn, sqlite3_int64 *pSize){
  multiplexConn *p = (multiplexConn*)pConn;
  multiplexGroup *pGroup = p->pGroup;
  int rc = SQLITE_OK;
  int i;
  multiplexEnter();
  if( !pGroup->bEnabled ){
    sqlite3_file *pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL, 0);
    if( pSubOpen==0 ){
      rc = SQLITE_IOERR_FSTAT;
    }else{
      rc = pSubOpen->pMethods->xFileSize(pSubOpen, pSize);
    }
  }else{

    *pSize = 0;
    for(i=0; rc==SQLITE_OK; i++){



















      sqlite3_int64 sz = multiplexSubSize(pGroup, i, &rc);




      if( sz==0 ) break;
      *pSize = i*(sqlite3_int64)pGroup->szChunk + sz;

    }
  }
  multiplexLeave();
  return rc;
}

/* Pass xLock requests through to the original VFS unchanged.
*/
static int multiplexLock(sqlite3_file *pConn, int lock){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    return pSubOpen->pMethods->xLock(pSubOpen, lock);
  }
  return SQLITE_BUSY;
}

/* Pass xUnlock requests through to the original VFS unchanged.
*/
static int multiplexUnlock(sqlite3_file *pConn, int lock){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    return pSubOpen->pMethods->xUnlock(pSubOpen, lock);
  }
  return SQLITE_IOERR_UNLOCK;
}

/* Pass xCheckReservedLock requests through to the original VFS unchanged.
*/
static int multiplexCheckReservedLock(sqlite3_file *pConn, int *pResOut){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    return pSubOpen->pMethods->xCheckReservedLock(pSubOpen, pResOut);
  }
  return SQLITE_IOERR_CHECKRESERVEDLOCK;
}

/* Pass xFileControl requests through to the original VFS unchanged,
................................................................................
      break;
    case SQLITE_FCNTL_SIZE_HINT:
    case SQLITE_FCNTL_CHUNK_SIZE:
      /* no-op these */
      rc = SQLITE_OK;
      break;
    default:
      pSubOpen = multiplexSubOpen(pGroup, 0, &rc, NULL, 0);
      if( pSubOpen ){
        rc = pSubOpen->pMethods->xFileControl(pSubOpen, op, pArg);
        if( op==SQLITE_FCNTL_VFSNAME && rc==SQLITE_OK ){
         *(char**)pArg = sqlite3_mprintf("multiplex/%z", *(char**)pArg);
        }
      }
      break;
................................................................................
}

/* Pass xSectorSize requests through to the original VFS unchanged.
*/
static int multiplexSectorSize(sqlite3_file *pConn){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen && pSubOpen->pMethods->xSectorSize ){
    return pSubOpen->pMethods->xSectorSize(pSubOpen);
  }
  return DEFAULT_SECTOR_SIZE;
}

/* Pass xDeviceCharacteristics requests through to the original VFS unchanged.
*/
static int multiplexDeviceCharacteristics(sqlite3_file *pConn){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    return pSubOpen->pMethods->xDeviceCharacteristics(pSubOpen);
  }
  return 0;
}

/* Pass xShmMap requests through to the original VFS unchanged.
................................................................................
  int iRegion,                    /* Region to retrieve */
  int szRegion,                   /* Size of regions */
  int bExtend,                    /* True to extend file if necessary */
  void volatile **pp              /* OUT: Mapped memory */
){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    return pSubOpen->pMethods->xShmMap(pSubOpen, iRegion, szRegion, bExtend,pp);
  }
  return SQLITE_IOERR;
}

/* Pass xShmLock requests through to the original VFS unchanged.
................................................................................
  sqlite3_file *pConn,       /* Database file holding the shared memory */
  int ofst,                  /* First lock to acquire or release */
  int n,                     /* Number of locks to acquire or release */
  int flags                  /* What to do with the lock */
){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    return pSubOpen->pMethods->xShmLock(pSubOpen, ofst, n, flags);
  }
  return SQLITE_BUSY;
}

/* Pass xShmBarrier requests through to the original VFS unchanged.
*/
static void multiplexShmBarrier(sqlite3_file *pConn){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    pSubOpen->pMethods->xShmBarrier(pSubOpen);
  }
}

/* Pass xShmUnmap requests through to the original VFS unchanged.
*/
static int multiplexShmUnmap(sqlite3_file *pConn, int deleteFlag){
  multiplexConn *p = (multiplexConn*)pConn;
  int rc;
  sqlite3_file *pSubOpen = multiplexSubOpen(p->pGroup, 0, &rc, NULL, 0);
  if( pSubOpen ){
    return pSubOpen->pMethods->xShmUnmap(pSubOpen, deleteFlag);
  }
  return SQLITE_OK;
}

/************************** Public Interfaces *****************************/