Thread: Question about safety/coding practices (casting between structs)

Suppose I have two structs, one of which can be considered an extension of another... is it safe to do this:

Code:

typedef struct _original
{
  int field1;
  double field2;
} original;

typedef struct _extended
{
  int field1;
  double field2;
  char * metadata;
} extended;

int main(int argc, char ** argv)
{
  original * org = malloc(sizeof(original));
  org->field1 = 10;
  org->field2 = 11.11;

  extended * ext = (extended *)org;
  ext->metadata = "metadata";
  printf("%d %lf %s\n",ext->field1, ext->field2, ext->metadata);
}

The print shows that the new struct ext has all of the right information.

I guess the other way would be to copy the information directly, but such memory copies cause unnecessary slowdown.

Is it OK for me to do the casting that way?