Hi, I installed cygwin recently and wanted to do some network programming with it. Since I have never used unix sockets before, I'm reading Beej's Guide To Network Programming. I took this from section 5.1 and tried to compile it. I get a few errors that I can't figure out :-/.

** showip.c -- show IP addresses for a host given on the command line

#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);
            ipver = "IPv4";
        } else { // IPv6
            struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
            addr = &(ipv6->sin6_addr);
            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;

Here are my errors:
stuff.c: In function `main':
stuff.c:14: error: storage size of 'hints' isn't known
stuff.c:16: error: `INET6_ADDRSTRLEN' undeclared (first use in this function)
stuff.c:16: error: (Each undeclared identifier is reported only once
stuff.c:16: error: for each function it appears in.)
stuff.c:34: error: dereferencing pointer to incomplete type
stuff.c:40: error: dereferencing pointer to incomplete type
stuff.c:41: error: dereferencing pointer to incomplete type
stuff.c:45: error: dereferencing pointer to incomplete type
stuff.c:46: error: dereferencing pointer to incomplete type
stuff.c:51: error: dereferencing pointer to incomplete type

I added this to my code and got rid of the `INET6_ADDRSTRLEN' undeclared error:


I figured it out after about a bit of searching. For anyone having this same problem: apparently, Cygwin can't use IPv6 by default. I went to http://win6.jp/Cygwin/ and downloaded the files I needed there. I unzipped it into my cygwin directory, and had to replace cygwin1.dll in the bin directory with new-cygwin1.dll.

I tried to put together the same program in C on our development server but ran into the same issue. I don't believe it's platform-related as much as it is leaving out a header include.

In reference to these two lines of Beej's showip.c example:

addr = &(ipv4->sin_addr);
addr = &(ipv6->sin6_addr);

If you're getting this error:

error: dereferencing pointer to incomplete type

Chances are you left out:

#include <netinet/in.h>


Matt Borja
Borja Webs, LLC

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