I have problem with both -n and -a option.

-a              Preallocate files on disk

    Use this if you are concerned about file fragmentation or out-of-order block storage. This option is only effective when initially creating the files and will cause startup to take longer as each entire file is written in order to reserve physical disk space. Note that all files will be created and preallocated even if the "-n" option is used to download a particular file.

I run a default 'ctorrent myfile.torrent'.  It will still preallocate files on disk resulted my usb stick run out of space and the ctorrent terminate.  Is there any way to prevent ctorrent to preallocate files on disk?

-n file_list    Specify file number(s) to download.

    Specify a priority order for downloading files in the torrent. A comma-separated list of file numbers and groups can be specified; the file numbers can be seen with the -x option. The client will preferentially request pieces of the highest-priority files, requesting a piece from a lower-priority group if a peer has no such pieces.

I run 'ctorrent -n 6 myfile.torrent'.  Again, it preallocate files on disk even though I didn't specify option -a.  I cannot specify file number to download.

Will appreciate any help.  Thanks in advance.