Sample getaddrinfo Code Compiles in Linux but not FreeBSD.

Martin McCormick martin at
Wed Jan 18 13:31:22 UTC 2012

	Here is a sample program kindly provided in the
                      Beej's Guide to Network Programming

Using Internet Sockets

   Brian "Beej Jorgensen" Hall

	The code is said to be in the public domain so it is
posted here as it compiles and runs perfectly under Linux but
fails in two places with the following errors: I named it nsl.c.

nsl.c: In function 'main':
nsl.c:38: error: dereferencing pointer to incomplete type
nsl.c:42: error: dereferencing pointer to incomplete type

	You will see that in both places, the code was
performing the same operation of assigning a value to a pointer
so I am suspecting a prototyping issue but am not sure and hope
someone can help me cut through the forest a little more quickly.
He did provide suggestions for users of Sunos who have reported
errors, but for FreeBSD, the errors did not change. Here is the
sample code with the two error-generating lines marked.

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>

int main(int argc, char *argv[])
    struct addrinfo hints, *res, *p;
    int status;
    char ipstr[INET6_ADDRSTRLEN];

    if (argc != 2) {
        fprintf(stderr,"usage: showip hostname\n");
        return 1;

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
    hints.ai_socktype = SOCK_STREAM;

    if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
        return 2;

    printf("IP addresses for %s:\n\n", argv[1]);

    for(p = res;p != NULL; p = p->ai_next) {
        void *addr;
        char *ipver;

        // get the pointer to the address itself,
        // different fields in IPv4 and IPv6:
        if (p->ai_family == AF_INET) { // IPv4
            struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
            addr = &(ipv4->sin_addr);/*error*/
            ipver = "IPv4";
        } else { // IPv6
            struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
            addr = &(ipv6->sin6_addr);/*error*/
            ipver = "IPv6";

        // convert the IP to a string and print it:
        inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
        printf("  %s: %s\n", ipver, ipstr);

    freeaddrinfo(res); // free the linked list

    return 0;

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