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struct buffer {
    size_t size;
    char buffer[50];
};

...

void func(struct buffer *buf) {

  /* assuming sizeof(size_t) is 4, this equals 54 */
  struct buffer *buf_cpy = malloc((sizeof(size_t)+(sizeof(char)*50))); /* assuming sizeof(size_t) is 4, this equals 54 */

  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }

  ...

  memcpy(buf_cpy, buf, sizeof(struct buffer)); /* with padding, sizeof(struct buffer) may equal 56, causing in a small buffer overflow */
  memcpy(buf_cpy, buf, sizeof(struct buffer));
}