Flexible array members in C - bad?

struct header {
    size_t len;
    unsigned char data[];
};

struct header {
  size_t len;
  unsigned char data[1];
};

ptr = malloc(sizeof(struct header) + (n-1));

struct header
{
 size_t len;
 unsigned char data[];
};

unsigned char data[0];

foo = malloc(sizeof(header) + N * sizeof(data[0]));
  foo->len = N;

struct header
    {
     size_t len;
     unsigned char *data;
    };

struct header *my_header
  = malloc(offsetof(struct header, data) + n * sizeof my_header->data);

#define FLEXIBLE_SIZE SIZE_MAX /* or whatever maximum length for an array */
#define SIZEOF_FLEXIBLE(type, member, length)
  ( offsetof(type, member) + (length) * sizeof ((type *)0)->member[0] )

struct header {
  size_t len;
  unsigned char data[FLEXIBLE_SIZE];
};

...

size_t n = 123;
struct header *my_header = malloc(SIZEOF_FLEXIBLE(struct header, data, n));

struct header *my_header = malloc(sizeof *my_header);

struct header {
    size_t len;
    unsigned char *data;
};

   struct header *p;
   p = malloc(sizeof(*p) + len + 1 );
   p->data = (unsigned char*) (p + 1 );  // memory after p is mine!