Implementation of an Stack without Linked Lists in C
makestack should return a struct stringstack * instead of void *.
malloc and realloc may fail. Check their return values.
push doesn't copy the string (it only makes a redundant copy of the pointer). It is OK as long as you can guarantee that the string parameter has a lifetime long enough. Consider a scenario:

void foo(struct stringstack * stack)
{
    char bar[] = "aaa";
    push(stack, bar);
}


Upon the return, bar disappears, and the stack entry points to the pretty random place in the application stack. Do not expect to have "aaa" there.

To actually copy the string, do

 *(ptr-&gt;stack + ptr-&gt;count) = strdup(stringtext);


which may introduce other problems in interacting with client (who owns the string after peek?). Unfortunately, C doesn't offer a clean solution.
You may want to shrink the stack when ptr-&gt;count hits the low watermark of ptr-&gt;slots.