Sockaddr union and getaddrinfo() - c++

Question

I'm writing a UDP socket program which is provided an address from the command line.
To make sending and writing easier, I'm using getaddrinfo to convert the address to a sockaddr struct: either sockaddr_in or sockaddr_in6. Now I understand that I should use a union of sockaddrs:
typedef union address
{
struct sockaddr s;
struct sockaddr_in s4;
struct sockaddr_in6 s6;
struct sockaddr_storage ss;
} address_t;
As I understand they can't be pointers to avoid hiding strict aliasing problems. I'm having trouble seamlessly putting the information from getaddrinfo's addrinfo into this address_t:
struct addrinfo hint, *serv = NULL;
address_t addr;
hint.ai_family = AF_UNSPEC;
hint.ai_flags = 0;
hint.ai_socktype = SOCK_DGRAM;
hint.ai_protocol = IPPROTO_UDP;
ret = getaddrinfo(address_sr.c_str(), s_port.c_str(), &hint, &serv);
//address_sr and s_port are strings with the address and port
switch (serv->ai_addr) {
case AF_INET: {
addr->s4 = * (sockaddr_in*) serv->ai_addr;
//Here I want to fill the address_t struct with information
//This line causes a segfault
}
break;
case AF_INET6: {
addr->s6 = * (sockaddr_in6*) serv->ai_addr;
//Conversion here
}
break;
Also, copying the memory:
memcpy(&addr, serv->ai_addr, serv->ai_addrlen);
Causes a segfault too.
How exactly should I do this? I tried a dozen different ways and I just can't figure it out. How do I put an address from addrinfo to this union? Do I use sockaddr_storage or the sockaddr_ins?
EDIT: Editing for clarity and additional code information.

Answer 1

You need to de-reference the pointer.
addr->s4 = *(sockaddr_in*) serv->ai_addr;

Answer 2

I think you're not getting getaddrinfo right.
About the third argument, :
const struct addrinfo *hints
The hints argument points to an addrinfo structure that specifies
criteria for selecting the socket address structures returned in the
list pointed to by res. If hints is not NULL it points to an addrinfo
structure whose ai_family, ai_socktype, and ai_protocol specify
criteria that limit the set of socket addresses returned by
getaddrinfo()[...]
For example, you can ask for IPv4 address family only, and/or for datagram sockets only (which could be fine given your attempt to use UDP).
Basically, you provide an addrinfo instance, set the fields of interest, then pass a pointer to it to the function, as its third argument:
struct addrinfo hints;
memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
ret = getaddrinfo(address_sr.c_str(), s_port.c_str(), &hint, &serv);
In this example, the function could return not just one, but a whole list of address structures:
The getaddrinfo() function allocates and initializes a linked list of
addrinfo structures, one for each network address that matches node
and service, subject to any restrictions imposed by hints, and returns
a pointer to the start of the list in res. The items in the linked
list are linked by the ai_next field.
So you have to loop through the function result this way:
for (rp = serv; rp != NULL; rp = rp->ai_next)
I strongly suggest to carefully read the documentation at the link I provided. There is also a long and detailed example which could alone solve your issues.

Answer 3

sockaddr_storage is large enough to hold any sockaddr_... type, thus making address_t just as large. So, I would just memcpy() the entire serv->ai_addr in one operation and get rid of the switch, eg:
struct addrinfo hints = {};
struct addrinfo *addrs, *serv;
address_t addr;
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_DGRAM;
...
ret = getaddrinfo(address_sr.c_str(), s_port.c_str(), &hint, &addrs);
if (ret == 0)
{
for (serv = addrs; serv != NULL; serv = serv->ai_next)
{
memcpy(&addr, serv->ai_addr, serv->ai_addrlen);
...
}
freeaddrinfo(addrs);
}