What do you mean by this? malloc returns memory that is contiguous in the virtual address space. It may not be contiguous in the physical address space, but that should be irrelevant for cache behavior.

Will I be thrashing the cache if I use realloc frequently?

I suppose. But if you use realloc, you should anyway ensure that you realloc geometrically growing chunks of memory (e.g., whenever you need a new buffer, you multiply its size by a constant factor like 1.2 instead of just adding an element at a time). As a result, realloc() should be infrequent enough that it normally doesn't matter.