When I create a table from within Informix the *.dat and *.idx files in the unix operating system are created with permissions of 660.
If I create files directly from unix the permissions are 777 as the umask is set to 000.
Where does informix control the setting of permisions and how can I alter this?
I don't want to run chmod over the files, I would like the Informix default to 666.
Thanks

Another user can delete , re-write or cat /dev/null to that file but in the point of database , another user can't read or write or do anything with data in that file before dba user (informix) doesn't grant yet

You don't want to change the permissions. It's much saves if only the user informix controls it.
In the SE manual, they say that file-permissions must be 660.
Why do you want to change them?