problem with snort and the file "features.log"

hi to all,

i got a problem with my snort.... i' m trying to calculate the features
of a traffic data using the preprocessor of snort and tcpreplay...
unfortunately when i use these commands:
tcpreplay -i lo -p 4 /../../outside.tcpdump
and with snort :
../snort -i lo -c ../../snort.conf

when i go to see the file /var/log/snort/features.log is empty ...
the file has been genereted each time and subscribe but is alway
empty.
the same thing is for the file alert.log