invalid application of ‘sizeof’ and compilation error with struct data
<p>I created a data type which I want to send over a socket.
I'm getting a compilation error and a segmentation fault error.
The compilation error I get is <code>error: invalid application of ‘sizeof’ to incomplete type ‘struct udp_msg_t’</code> whereas the segmentation fault happens when I do the <code>memcpy</code>. What am I doing wrong?</p>

<p>Here is some of my code:</p>

<p>This is the struct I'm concerned about which I defined:</p>

<pre><code>typedef struct udp_msg {
    unsigned int udp_eid;
    u_char   udp_prefix;
    unsigned int udp_loc;
} udp_msg_t;
</code></pre>

<p>In a method I assign the memory and the values:</p>

<pre><code>void method(){
    udp_msg_t * udp_msg;

    udp_msg = (struct udp_msg_t * )calloc(1, sizeof(struct udp_msg_t));
    udp_msg-&gt;udp_eid = eid.u.prefix4.s_addr;
    udp_msg-&gt;udp_prefix = eid.prefixlen;
    udp_msg-&gt;udp_loc = loc-&gt;rloc.rloc.s_addr;

    send_rloc_udp_to_floodlight(udp_msg);
}
</code></pre>

<p>And this method actually sends the data over a socket:</p>

<pre><code>int send_rloc_udp_to_floodlight(udp_msg_t message) {
    struct sockaddr_in si_other;
    int s, i, slen = sizeof(si_other);
    char buffer[9];

    if ((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
        printf("socket");
    }

    memset((char *) &amp;si_other, 0, sizeof(si_other));

    si_other.sin_family = AF_INET;
    si_other.sin_port = htons(8888);

    if (inet_aton("127.0.0.1", &amp;si_other.sin_addr) == 0) {
        fprintf(stderr, "inet_aton() failed\n");
        exit(1);
    }


    memcpy(buffer, (char *) message.udp_eid, sizeof(unsigned int));
    memcpy(&amp;buffer[4], (char *) message.udp_prefix, sizeof(char));
    memcpy(&amp;buffer[5], (char *) message.udp_loc, sizeof(unsigned int));

    //send the message
    if (sendto(s, buffer, strlen(buffer), 0, (struct sockaddr *) &amp;si_other,
            slen) == -1) {
        printf("sendto()");
    }

    close(s);
    return 0;
}
</code></pre>