How to create sockaddr from sockaddr_in / sockaddr_in6 structure - c

Question

Below is an extract of example code for IPv6 and IPv4 clinet code:
IPv6
int s;
struct sockaddr_in6 addr;
s = socket(AF_INET6, SOCK_STREAM, 0);
addr.sin6_family = AF_INET6;
addr.sin6_port = htons(5000);
inet_pton(AF_INET6, "::1", &addr.sin6_addr);
connect(s, (struct sockaddr *)&addr, sizeof(addr));
IPv4
int s;
struct sockaddr_in addr;
s = socket(AF_INET, SOCK_STREAM, 0);
addr.sin_family = AF_INET;
addr.sin_port = htons(5000);
servaddr.sin_addr.s_addr = INADDR_ANY;
connect(s, (struct sockaddr *)&addr, sizeof(addr));
My aim is to write a user defined connect API for both IPv6 and IPv4 addresses. Since both of them convert structures to struct sockaddr structure - programtically how can we convert the sockaddr_in and sockaddr_in6 to sockaddr structure (populate its member variables) and then call connect with sockaddr sturcure?

Answer 1

programtically how can we convert the sockaddr_in and sockaddr_in6 to sockaddr structure (populate its member variables) and then call connect with sockaddr structure?
A typecast is not a conversion. You are not converting anything. Just as the examples you provided show, you create a sockaddr_in or sockaddr_in6 instance as needed and then pass it as-is to connect(). You are merely typecasting the pointer that points at the struct instance. Internally, connect() will look at its input addr's sa_family field and typecast the addr back to either sockaddr_in or sockaddr_in6 accordingly to access the data fields as needed.
My aim is to write a user defined connect API for both IPv6 and IPv4 addresses.
If you want to write protocol-agnostic code, you could do something like this:
int doConnect(int family, const char *ip, u_short port)
{
struct sockaddr_storage ss = {};
socklen_t addrlen;
switch (family)
{
case AF_INET:
{
struct sockaddr_in *addr = (struct sockaddr_in *) &ss;
addr->sin_family = AF_INET;
addr->sin_port = htons(port);
inet_pton(AF_INET, ip, &addr->sin_addr);
addrlen = sizeof(struct sockaddr_in);
break;
}
case AF_INET6:
{
struct sockaddr_in6 *addr = (struct sockaddr_in6 *) &ss;
addr->sin6_family = AF_INET6;
addr->sin6_port = htons(port);
inet_pton(AF_INET6, ip, &addr->sin6_addr);
addrlen = sizeof(struct sockaddr_in6);
break;
}
default:
return -1;
}
int s = socket(family, SOCK_STREAM_IPPROTO_TCP);
if (s != -1)
{
if (connect(s, (struct sockaddr *) &ss, addrlen) < 0)
{
close(s);
s = -1;
}
}
return s;
}
int s = doConnect(AF_INET, "127.0.0.1", 5000);
int s = doConnect(AF_INET6, "::1", 5000);
However, a better solution is to use getaddrinfo() instead and let it allocate the correct sockaddr data for you based on the input values it actually parses, eg:
int doConnect(const char *ip, u_short port)
{
struct addrinfo hints = {};
hints.ai_flags = AI_NUMERICHOST;
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
struct addrinfo *addr;
char szPort[6];
sprintf(szPort, "%hu", port);
int s = -1;
int ret = getaddrinfo(ip, szPort, &hints, &addr);
if (ret == 0)
{
s = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);
if (s != -1)
{
if (connect(s, addr->ai_addr, addr->ai_addrlen) == -1)
{
close(s);
s = -1;
}
}
freeaddrinfo(addr);
}
return s;
}
int s = doConnect("127.0.0.1", 5000);
int s = doConnect("::1", 5000);
The nice thing about getaddrinfo() is that you can also use it for servers, too. Simply use the AI_PASSIVE flag in the hints.ai_flags field, and then use the resulting addrinfo item(s) to call socket() and bind().

Answer 2

programtically how can we convert the sockaddr_in and sockaddr_in6 to sockaddr
You can't, as neither sockaddr_in nor sockaddr_in6 are guaranteed to fit into a sockaddr structure.
A POSIX-compliant system will provide a sockaddr_storage type that is guaranteed to be large enough to hold any type of sockaddr structure:
The <sys/socket.h> header shall define the sockaddr_storage
structure, which shall be:
Large enough to accommodate all supported protocol-specific address structures
Aligned at an appropriate boundary so that pointers to it can be cast as pointers to protocol-specific address structures and used to
access the fields of those structures without alignment problems
The sockaddr_storage structure shall include at least the following
members:
sa_family_t ss_family
When a pointer to a sockaddr_storage structure is cast as a pointer
to a sockaddr structure, the ss_family field of the
sockaddr_storage structure shall map onto the sa_family field of
the sockaddr structure. When a pointer to a sockaddr_storage
structure is cast as a pointer to a protocol-specific address
structure, the ss_family field shall map onto a field of that
structure that is of type sa_family_t and that identifies the
protocol's address family.