gcc编译出现：error: dereferencing pointer to incomplete type

#include <sys/socket.h>
#include <netdb.h>
#include <sys/types.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/in.h> 

#include <stdio.h>
#include <stdlib.h>

int main(int argc,char* argv[])
{
char host_name[128]={0};
gethostname(host_name, sizeof(host_name));
printf("host_name:%s\n",host_name);

struct addrinfo *ailist=NULL,*aip=NULL;
struct sockaddr_in *saddr;
char *addr;
int ret=getaddrinfo(host_name,"ftp",NULL,&ailist);
for(aip=ailist; aip!=NULL; aip=aip->ai_next)
{
    if(aip->ai_family==AF_INET)
    {
        saddr=(struct sockaddr_in*)aip->ai_addr;
        addr=inet_ntoa(saddr->sin_addr);
    }
    printf("addr:%s\n",addr);
}
getchar();
return 0;
}