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How to get size of different kinds of types in the same function using C?
int getMoreSpace(void **pnt, int size, int add) {
    xxxxxx *tmp; /* a pointer to the same as pnt */
    if (tmp = realloc(pnt, (size+add)*sizeof(xxxxxx))) { /* get size of what pnt points to */
        *pnt=tmp;
        return 1;
    else return 0;
}

#define getMoreSpace(P, SZ, ADD) getMoreSpaceFunc(&(P), sizeof(*(P)), (SZ), (ADD))

int getMoreSpace(void **pnt, size_t elem_size, int size, int add) {
    *pnt = ...
}

int getMoreSpace(void **pnt, int size, int add, size_t eltSize)
{
  void *tmp;
  if (tmp = realloc(pnt, (size+add)*eltSize))
  {
    *pnt=tmp;
    return 1;
  }
  else
    return 0;
}
...

int *p = malloc(100 * sizeof *p);
...
if (!getMoreSpace(&p, 100, 20, sizeof *p))
{
  // panic
}