How to allocate aligned memory only using the standard library?

I just finished a test as part of a job interview, and one question stumped me, even using Google for reference. I'd like to see what the StackOverflow crew can do with it:

The memset_16aligned function requires a 16-byte aligned pointer passed to it, or it will crash.

a) How would you allocate 1024 bytes of memory, and align it to a 16 byte boundary?
b) Free the memory after the memset_16aligned has executed.

{    
   void *mem;
   void *ptr;

   // answer a) here

   memset_16aligned(ptr, 0, 1024);

   // answer b) here    
}