if( pShmNode->mutex==0 ){
        rc = SQLITE_NOMEM_BKPT;
        goto shm_open_err;
      }
    }

    if( pInode->bProcessLock==0 ){

      int openFlags = O_RDWR | O_CREAT;
      if( sqlite3_uri_boolean(pDbFd->zPath, "readonly_shm", 0) ){
        openFlags = O_RDONLY;
        pShmNode->isReadonly = 1;
      }
      pShmNode->h = robust_open(zShmFilename, openFlags, (sStat.st_mode&0777));
      if( pShmNode->h<0 ){
................................................................................

      /* If this process is running as root, make sure that the SHM file
      ** is owned by the same user that owns the original database.  Otherwise,
      ** the original owner will not be able to connect.
      */
      robustFchown(pShmNode->h, sStat.st_uid, sStat.st_gid);
  
      /* Check to see if another process is holding the dead-man switch.
      ** For a readonly_shm client, if no other process holds the DMS lock,
      ** the file cannot be opened and SQLITE_CANTOPEN_DIRTYWAL is returned.
      ** Or, for a read-write connection, if no other process holds a
      ** DMS lock the file is truncated to zero bytes in size.  */













      rc = SQLITE_OK;
      if( pShmNode->isReadonly ){
        struct flock lock;
        lock.l_whence = SEEK_SET;
        lock.l_start = UNIX_SHM_DMS;
        lock.l_len = 1;
        lock.l_type = F_WRLCK;
        if( osFcntl(pShmNode->h, F_GETLK, &lock)!=0 ) {
          rc = SQLITE_IOERR_LOCK;
        }else if( lock.l_type==F_UNLCK ){

          rc = SQLITE_CANTOPEN_DIRTYWAL;
        }

      }else if( unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1)==SQLITE_OK ){
        if( robust_ftruncate(pShmNode->h, 0) ){
          rc = unixLogError(SQLITE_IOERR_SHMOPEN, "ftruncate", zShmFilename);
        }
      }




      if( rc==SQLITE_OK ){

        rc = unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, 1);
      }
      if( rc ) goto shm_open_err;
    }
  }

  /* Make the new connection a child of the unixShmNode */

      if( pShmNode->mutex==0 ){
        rc = SQLITE_NOMEM_BKPT;
        goto shm_open_err;
      }
    }

    if( pInode->bProcessLock==0 ){
      struct flock lock;
      int openFlags = O_RDWR | O_CREAT;
      if( sqlite3_uri_boolean(pDbFd->zPath, "readonly_shm", 0) ){
        openFlags = O_RDONLY;
        pShmNode->isReadonly = 1;
      }
      pShmNode->h = robust_open(zShmFilename, openFlags, (sStat.st_mode&0777));
      if( pShmNode->h<0 ){
................................................................................

      /* If this process is running as root, make sure that the SHM file
      ** is owned by the same user that owns the original database.  Otherwise,
      ** the original owner will not be able to connect.
      */
      robustFchown(pShmNode->h, sStat.st_uid, sStat.st_gid);
  
      /* Use F_GETLK to determine the locks other processes are holding
      ** on the DMS byte. If it indicates that another process is holding
      ** a SHARED lock, then this process may also take a SHARED lock
      ** and proceed with opening the *-shm file. 
      **
      ** Or, if no other process is holding any lock, then this process
      ** is the first to open it. In this case take an EXCLUSIVE lock on the
      ** DMS byte and truncate the *-shm file to zero bytes in size. Then
      ** downgrade to a SHARED lock on the DMS byte.
      **
      ** If another process is holding an EXCLUSIVE lock on the DMS byte,
      ** return SQLITE_BUSY to the caller (it will try again). An earlier
      ** version of this code attempted the SHARED lock at this point. But
      ** this introduced a subtle race condition: if the process holding
      ** EXCLUSIVE failed just before truncating the *-shm file, then this
      ** process might open and use the *-shm file without truncating it.
      ** And if the *-shm file has been corrupted by a power failure or
      ** system crash, the database itself may also become corrupt.  */
      rc = SQLITE_OK;


      lock.l_whence = SEEK_SET;
      lock.l_start = UNIX_SHM_DMS;
      lock.l_len = 1;
      lock.l_type = F_WRLCK;
      if( osFcntl(pShmNode->h, F_GETLK, &lock)!=0 ) {
        rc = SQLITE_IOERR_LOCK;
      }else if( lock.l_type==F_UNLCK ){
        if( pShmNode->isReadonly ){
          rc = SQLITE_CANTOPEN_DIRTYWAL;

        }else{
          rc = unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1);
          if( rc==SQLITE_OK && robust_ftruncate(pShmNode->h, 0) ){
            rc = unixLogError(SQLITE_IOERR_SHMOPEN, "ftruncate", zShmFilename);
          }
        }
      }else if( lock.l_type==F_WRLCK ){
        rc = SQLITE_BUSY;
      }

      if( rc==SQLITE_OK ){
        assert( lock.l_type==F_UNLCK || lock.l_type==F_RDLCK );
        rc = unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, 1);
      }
      if( rc ) goto shm_open_err;
    }
  }

  /* Make the new connection a child of the unixShmNode */