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    <p>Your <code>print_socket_info</code> function is wrong. If takes a <code>struct sockaddr_in *</code> (IPv4 socket address structure) but it's meant to support both IPv4 and IPv6.</p> <p>You must declare <code>print socket_info</code> to take a generic <code>struct sockaddr *</code> (socket address of any type). For good measure: rename the <code>sin</code> argument to <code>sa</code> to indicate that it's of the generic type, not <code>struct sockaddr_in</code> type. Then, inside the function, you check <code>sin-&gt;sin_family</code> to find out what the actual family is and continue by casting <code>sin</code> to either a <code>struct sockaddr_in *</code> or <code>struct sockaddr_in6</code> as appropriate.</p> <p>What's happening in your existing function is that you're just treating it as a <code>struct sockaddr_in *</code> throughout the whole function with the following results (at least on Linux):</p> <ul> <li>Checking <code>sin_family</code> is OK because the family is guaranteed to be at the same offset into the structure for all types of sockaddrs, whether it is <code>struct sockaddr_in</code> or <code>struct sockaddr_in6</code> or even <code>struct sockaddr_un</code> for UNIX domain sockets or the sockaddr structures for all the other obscure address families too.</li> <li>You are lucky to get away with it when checking <code>sin_port</code> because <code>sin_port</code> in <code>struct sockaddr_in</code> happens to lie at the same offset in the structure as <code>sin6_port</code> in <code>struct sockaddr_in6</code>.</li> <li>If doesn't work for <code>sin_addr</code> because for IPv4, <code>sin_addr</code> comes right after <code>sin_port</code> in the structure, but for IPv6, some other field is found at that location (namely, <code>sin6_flowinfo</code>). <code>sin6_addr</code> is somewhere else.</li> </ul> <p>Another thing that's wrong with <code>print_socket_info</code> is that your string buffer only has enough space for an IPv4 address string because it is declared with length <code>INET_ADDRSTRLEN</code> which is too short for IPv6 (for IPv6 you need <code>INET6_ADDRSTRLEN</code>. And the function does not need to take a parameter <code>protocol</code>. This information is already embedded in the sockaddr.</p> <pre><code>int print_socket_info(int sock_fd, struct sockaddr *sa){ char dbg[INET6_ADDRSTRLEN]; /* the larger of the two sizes */ char *famstr; unsigned short port; switch (sa-&gt;sa_family): case AF_INET4: inet_ntop(AF_INET4, &amp;(((struct sockaddr_in *)sa)-&gt;sin_addr), dbg, sizeof(dbg)); port = ((struct sockaddr_in *)sa)-&gt;sin_port; break; case AF_INET6: inet_ntop(AF_INET6, &amp;(((struct sockaddr_in6 *)sa)-&gt;sin6_addr), dbg, sizeof(dbg)); port = ((struct sockaddr_in6 *)sa)-&gt;sin6_port; break; default: strcpy(dst, "UNKNOWN"); port = 0; } printf("============ SOCKET INFORMATION =============\n"); printf("!** socket: %d\n", sock_fd); printf("!** info-&gt;ai_addr: sockaddr_in(\n"); famstr = fam2str(sa-&gt;sa_family); printf("!** sa_family: %s\n", famstr); printf("!** sin[6]_port: %d\n", ntohs(port)); printf("!** sin[6]_addr: in_addr( s_addr : '%s' )\n", dbg); printf("!**)\n"); printf("=============================================\n"); return 1; } </code></pre>
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    1. COAh, okay, thank you. I seem to not have a problem using IPv4, so I'll use that for now (I just need the git-r-dun answer for now), but I'll correct that later. Thank you for all your help.
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