how do you cast sockaddr structure to a sockaddr_in - C++ networking sockets ubuntu UDP

Question

I am trying to get the client address, but i am unsure how do i cast the sockaddr structure to sockaddr_in?

struct sockaddr_in cliAddr, servAddr;

    n = recvfrom(sd, msg, MAX_MSG, 0,(struct sockaddr *) cliAddr,sizeof(cliAddr));

 //i tried this but it does not work
    struct sockaddr cliSockAddr = (struct sockaddr *) cliAddr; 
    char *ip = inet_ntoa(cliSockAddr.sin_addr);

Thanks in advance! :)

i've found questions that brought me to this step: Getting IPV4 address from a sockaddr structure

Sorry to avoid confusion, this is my real implementation where "ci" is an object to store pointers such as sockaddr_in.

    /* receive message */
    n = recvfrom(*(ci->getSd()), msg, MAX_MSG, 0,(struct sockaddr *) ci->getCliAddr(),ci->getCliLen());

    char *ip = inet_ntoa(ci->getCliAddr().sin_addr);

i will get the following errors:

udpserv.cpp:166: error: request for member ‘sin_addr’ in ‘ci->clientInfo::getCliAddr()’, which is of non-class type ‘sockaddr_in*’

Your code won't even compile. You are assigning a pointer to a structure, declare `cliSockAddr` as a pointer. — Some programmer dude, Jul 27 '12 at 08:20
What you are really asking is the reverse of what it says in your title. — user207421, Jul 27 '12 at 09:39

score 27 · Accepted Answer · answered Sep 02 '13 at 19:23

I would point out that if this is actually C++ the idiomatic way to do this would be:

sockaddr *sa = ...; // struct not needed in C++
char ip[INET6_ADDRSTRLEN] = {0};

switch (sa->sa_family) {
  case AF_INET: {
    // use of reinterpret_cast preferred to C style cast
    sockaddr_in *sin = reinterpret_cast<sockaddr_in*>(sa);
    inet_ntop(AF_INET, &sin->sin_addr, ip, INET6_ADDRSTRLEN);
    break;
  }
  case AF_INET6: {
    sockaddr_in6 *sin = reinterpret_cast<sockaddr_in6*>(sa);
    // inet_ntoa should be considered deprecated
    inet_ntop(AF_INET6, &sin->sin6_addr, ip, INET6_ADDRSTRLEN);
    break;
  }
  default:
    abort();
}

This sample code handles IPv4 and IPv6 addresses and also would be considered more C++ idiomatic than either of the suggested implementations.

I really like that you've used reinterpret_cast. It makes everything so much cleaner, and less pointers need to be used. On a side note, I didn't set sa as a pointer to begin with. I simply passed the address to it in via reference in like this (mine is a school project and only using IPv4 -- inet_ntoa(reinterpret_cast(&clientIpAddress)->sin_addr) — adprocas, Nov 27 '16 at 03:54

score 17 · Answer 2 · answered Jul 27 '12 at 08:23

17

It is actually very simple!

struct sockaddr *sa = ...;

if (sa->sa_family == AF_INET)
{
    struct sockaddr_in *sin = (struct sockaddr_in *) sa;
    ip = inet_ntoa(sin->sin_addr);
}

answered Jul 27 '12 at 08:23

Some programmer dude

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3

And while you're doing that, make your program IPv6 capable by checking for AF_INET6 too. ;) – onitake Jul 27 '12 at 11:10
3

I know this is an old thread but for sake of correctness, @bmatheny's answer below is the correct one since this question is tagged "C++". `reinterpret_cast(sa)` is the right way to convert the two types and though the old C conversion still works, it is too obscure and won't let clear the intentions for any reader. – j4x Jul 02 '18 at 19:29

score 3 · Answer 3 · answered Jul 27 '12 at 08:34

3

I think this will compile just fine for you and do what you want.

struct sockaddr_in cliAddr={}, servAddr={};

socklen_t cliAddrLength = sizeof(cliAddr);

n = recvfrom(sd, msg, MAX_MSG, 0,(struct sockaddr *)&cliAddr, &cliAddrLength);

answered Jul 27 '12 at 08:34

selbie

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how do you cast sockaddr structure to a sockaddr_in - C++ networking sockets ubuntu UDP

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