Unexplainable usage of parentheses in the declaration - c

Question

I found this piece of C code:
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
{
struct addrinfo hints, *res, *p;
int status;
char ipstr[INET6_ADDRSTRLEN];
if (argc != 2) {
fprintf(stderr,"usage: showip hostname\n");
return 1;
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
hints.ai_socktype = SOCK_STREAM;
if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
return 2;
}
printf("IP addresses for %s:\n\n", argv[1]);
for(p = res;p != NULL; p = p->ai_next) {
void *addr;
char *ipver;
// get the pointer to the address itself,
// different fields in IPv4 and IPv6:
if (p->ai_family == AF_INET) { // IPv4
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
addr = &(ipv4->sin_addr);
ipver = "IPv4";
} else { // IPv6
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
addr = &(ipv6->sin6_addr);
ipver = "IPv6";
}
// convert the IP to a string and print it:
inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
printf(" %s: %s\n", ipver, ipstr);
}
freeaddrinfo(res); // free the linked list
return 0;
}
There are two lines that I don't know how to interpret:
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
Why does the author of this program use parentheses?
I tried following a wonderful guide about interpreting complex declarations but I still don't understand.

Answer 1

It's called casting. It converts from one type to another, but the theory behind it is a lot more complicated. Casting doesn't mean you have an object describing a Dog (for example a struct) and you can cast it to a Human. Obviously you can't.
Casting means having identical memory blocks that can map on another block. A simpler explanation is when you have a complex structure and you cast from a pointer to the structure or from the data structure to the pointer.
Also note, that a data structure doesn't necessarily mean a struct. It can also mean an array. And, an array is a concept so it can mean something like int some_data[20]; or int** some_data;.
Now, as for why the author uses the parenthesis, it's because this is the syntax. You can't use any character.
Along with further reading about casting, please look up static_cast, dynamic_cast and reinterpret_cast. These 3 functions (well, almost, but I'm trying to not complicate it even more) use native casting. static_cast is similar to (type_a*)type_b.
For some examples, look up casting from int to enum and vice-versa to get an idea.
If you want to keep it simple, don't check any C++ concepts about casting just yet until you get a hang of the concepts in C first. They are common, but C++ will fork a lot from C because of classes, polymorphism and inheritance in general.