accessing initial common structure members

[Chuck Kozak]

I have seen code where the first N members of different structures
are accessed by code that treats the structures as identical.  I
looked through K&R II to check whether this practice is legal and
cannot find any mention of it.  This practice works with GNU C on a
SUN-4 and I cannot think of a reason why it would not work on other
architectures, but I would like to know whether it is a safe or bad
practice.
 
An example of what I mean is given below.
 
struct common {
	struct common *next;
	struct common *prev;
	int type;
};
 
struct type_1 {
	struct common *next;
	struct common *prev;
	int type;
	... type_1 specific members ...
};
 
struct type_2 {
	struct common *next;
	struct common *prev;
	int type;
	... type_2 specific members ...
};
 
void common_function(struct common *pnt)
{
	... access the next, prev, and type members ...
}
 
... other code ...
   struct type_1 type_1_data;
 
   common_function((struct common *) &type_1_data);
...
 
thanks for your help,
 
Chuck Kozak
kozak at grad2.cis.upenn.edu