Is using flexible array members in C bad practice?

Question

I recently read that using flexible array members in C was poor software engineering practice. However, that statement was not backed by any argument. Is this an accepted fact?

(Flexible array members are a C feature introduced in C99 whereby one can declare the last element to be an array of unspecified size. For example: )

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

Answer 1

As a side note, for C89 compatibility, such structure should be allocated like :

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

Or with macros :

#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));

Setting FLEXIBLE_SIZE to SIZE_MAX almost ensures this will fail :

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