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

/* ... */

void func(struct buffer *buf) {

  /* assuming sizeof(size_t) is 4, sizeof(size_t)+sizeof(char)*50 equals 54 */
  struct buffer *buf_cpy = (struct buffer *)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
   * some data to be written outside the bounds of the memory allocated
   */
  memcpy(buf_cpy, buf, sizeof(struct buffer));

  /* ... */

  free(buf_cpy);
}

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

/* ... */

void func(struct buffer *buf) {

  struct buffer *buf_cpy = (struct buffer *)malloc(sizeof(struct buffer));
  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }

  /* ... */

  memcpy(buf_cpy, buf, sizeof(struct buffer));

  /* ... */

  free(buf_cpy);
}