Don't interview for positions in languages you've forgotten, then, or brush up and re-learn stuff before the interview. It's like riding a bike...it comes back to you.
Not being able to remember pointer syntax is a serious problem if you're being hired to program in C. I can understand why they'd want to weed out anyone who can't get that, particularly given how many candidates they have.
PS: Ok, if it was really just a C job of course I would focus on that.
Edit: Dammit it is like riding a bike. I have been having C flashbacks to horrible old code. O Malloc how do I hate the let me count the ways.
Around 5 years ago I was spent a lot of time working with really old Object Pascal and some C code that was upgraded to OS 8/9 and then left to rot. Let me just say that low level networking code on a OS that does not have threads is a PITA. They had something like interrupts but you could not allocate memory during them. You had to first allocate using Handel's and then do something before working with actual pointers as part of your run loop and then use that memory during the interrupts. Meh, I think it might have been the most fun coding project I have ever been on, but I don't want to do that again.