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struct big {
  unsigned long long ull_1; /* typically 8 bytes */
  unsigned long long ull_2; /* typically 8 bytes */
  unsigned long long ull_3; /* typically 8 bytes */
  int si_4; /* typically 4 bytes */
  int si_5; /* typically 4 bytes */
};
/* ... */
size_t skip = offsetof(struct big, ull_2);
struct big *s = (struct big *)malloc(sizeof(struct big));
if (!s) {
  /* Handle malloc() error */
}

memset((char *)s + skip, 0, sizeof(struct big) - skip);
/* ... */
free(s);
s = NULL;