quite easy for those who are " afraid"
to modify an .ini script :
if you want to launch avast for the first time in your session :
type as root : "sysctl -w kernel.shmmax=128000000 ; avastgui "
If you want to launch avast again (in the same session) just type "avastgui".
But the moderator is right , the best way is to modify the init script.