external script collecting invironment variables

Hi all, I'm run a "home made" script whenever I receive an accounting- start, stop or alive. In the script I get the value of all the attributes set in the environment variables (example : val = getenv("USER_NAME") ). I want to count how many attributes contain no value (or doesn't not exist) so I have a check (example : if(val == NULL) ) and I have a counter variable. I check with NULL because I thought when an attribute has no value, the value would be NULL butt apparently it is not always the case. I guess it is different when the attribute is sent without a value and when it is not sent at all. I would like to know what value contains in these two cases. Anyone? Thanks! -- Vicky