a couple of newbie question

I know this question is probably fairly elementary, but I could not
find the answer in the docs or mailing list archives.  How does one handle
NULL values in the embedded SQL interface?  For example, if I have a
statement:

exec sql insert into test_table values(:foo, :x, :y)

and foo represents a CHAR attribute (represented by a string in C) that I
want to set to NULL, how do I go about doing this?  Similarly, how do
I tell that a value put into a variable by a fetch call is NULL or
not?  

On a different note, is there any list of error codes for the actual
postgreSQL errors, or is the only thing a user can get back is the
text message in sqlca.sqlerrm.sqlerrmc (e.g. no associated error #)?

Any help would be greatly appreciated.

thanks,
Matt