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copied!<p>If you want your server to listen on IPV4 and IPV6 addresses you <strike>need to</strike> could set up two socket to listen on.</p> <p><code>getaddrinfo()</code> might return information for more then one internet address for given host and/or service.</p> <p>The member <code>ai_family</code> of the <code>hints</code> structure passed specifies which address family the caller is interest in. If <code>AF_UNSPEC</code> is specified IPV4 and IPV6 addresses might be returned.</p> <p>To find out if there are such addresses available you might like to mod you code like so:</p> <pre><code>int open_listenfd(int port, int * pfdSocketIpV4, int * pfdSocketIpV6) { ... struct addrinfo hints = {0}; hints.ai_family = AF_UNSPEC; hints.ai_socktype = SOCK_STREAM; hints.ai_flags = AI_PASSIVE|AI_ADDRCONFIG; hints.ai_next = NULL; struct addrinfo * res = NULL; int err=getaddrinfo(hostname, portname, &hints, &res); free(pName); if (err) { return -1; } struct addrinfo * pAddrInfoIpV4 = NULL; struct addrinfo * pAddrInfoIpV6 = NULL; { struct addrinfo * pAddrInfo = res; /* Loop over all address infos found until a IPV4 and a IPV6 address is found. */ while (pAddrInfo) { if (!pAddrInfoIpV4 && AF_INET == pAddrInfo->ai_family) { pAddrInfoIpV4 = pAddrInfo; /* Take first IPV4 address available */ } else if (!pAddrInfoIpV6 && AF_INET6 == pAddrInfo->ai_family) { pAddrInfoIpV6 = pAddrInfo; /* Take first IPV6 address available */ } else { break; /* Already got an IPV4 and IPV6 address, so skip the rest */ } pAddrInfo= pAddrInfo->ai_next; /* Get next address info, if any */ } } if (pAddrInfoIpV4) { ... /* create, bind and make IPV4 socket listen */ int fdSocketIpV4 = socket(pAddrInfoIpV4->ai_family,... *pfdSocketIpV4 = fdSocketIpV4; } if (pAddrInfoIpV6) { /* create, bind and make IPV6 socket listen */ int fdSocketIpV6 = socket(pAddrInfoIpV6->ai_family,... *pfdSocketIpV6 = fdSocketIpV6; } freeaddrinfo(res); ... </code></pre> <p>Then call it like so:</p> <pre><code>... int fdSocketIpV4 = -1; int fdSocketIpV6 = -1; if (0 > open_listenfd(port, &fdSocketIpV4, &fdSocketIpV6)) { printf("Error executing 'open_listenfd()'\n"); } else { ... /* go for accepting connectings on 'fdSocketIpV4' and 'fdSocketIpV6' */ </code></pre> <hr> <p><strong>Update:</strong></p> <p>As commented by <em>Per Johansson</em> an alternative approach would be to set up a dual stack socket, supporting both, Ipv4 and Ipv6, as mentioned by this answer: <a href="https://stackoverflow.com/q/1618240/694576">How to support both IPv4 and IPv6 connections</a></p>

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