Casting a char array to be of type struct *

Question

In the code below could somebody perhaps explain what is happening on the line struct ether_header *eh = (struct ether_header *) sendbuf;? I understand that it

Answer 1

The line char sendbuf[BUF_SIZ] allocates a block of chars (i.e. bytes on most systems) and the cast struct ether_header *eh = (struct ether_header *) sendbuf says that you explicitly want to treat this as a struct ether_header type. There are no significant instructions from this cast, aside from (possibly) setting a CPU register.

You'll end up with two pointers to the same block of memory. Modification of one will affect the other.

That being said, it is not completely correct/safe, because the sendbuf may not be appropriately aligned to actually contain a struct ether_header.

Edit: In regard to struct aliasing rules a char* is explicitly allowed to alias any other data type, but the reverse is not necessarily true.