UNIX for Advanced & Expert Users

I am having a doubt with type casting in C. Very often in network programming, we have something like this:

char *data = ...;
struct iphdr *ip = (struct iphdr *) data;

where iphdr is define in netinet/ip.h.

Let us say that the size of the content of held by 'data' is much more than sizeof(struct iphdr). But why are we still able to cast ? Does it mean that while casting to struct iphdr, the compiler attempts to take the first sizeof(struct iphdr) bytes from data and assigns it to 'ip' ? What if the data variable has lesser bytes than sizeof(struct iphdr)?

It is up to the programmer to set up a correct cast. If the object has fewer bytes than what it is cast to - then you can read/write past the end of the object, a data overrun. Which results, usually, in odd behavior, data corruption or sometimes a segfault or a sigbus error...