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

/* ... */

void func(const struct buffer *buf) {

  /* Assumes sizeof( struct buffer) =
     sizeof( size_t) + 50 * sizeof( char) = 54 */
  struct buffer *buf_cpy = (struct buffer *)malloc(54);
  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }
  /*
   * withWith 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);
}

enum {buffer_size = 50};

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

/* ... */

void func(const 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);
}