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

...

void func(struct buffer *buf) {

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

  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }
  ...
 /* with padding, sizeof(struct buffer) may be greater than 54, causing in a small buffer overflow */
  memcpy(buf_cpy, buf, sizeof(struct buffer));
}