域名转换成IP地址

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

#define MAXLINE 1024

int main(int argc, char **argv) {
    struct addrinfo *p, *listp, hints;
    char buf[MAXLINE];
    int rc, flags;

    if(argc!=2) {
        fprintf(stderr, "usage: %s <domain name>\n", argv[0]);
        exit(0);
    }

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family=AF_INET;
    hints.ai_socktype=SOCK_STREAM;

    if((rc=getaddrinfo(argv[1], "ntp", &hints, &listp)) != 0) {
        fprintf(stderr, "getaddrinfo error: %s\n", gai_strerror(rc));
        exit(1);
    }

    flags=NI_NUMERICHOST;
    for(p=listp;p;p=p->ai_next) {
        if((rc=getnameinfo(p->ai_addr, p->ai_addrlen, buf, MAXLINE, NULL, 0, flags)) != 0) {
            fprintf(stderr, "getnameinfo error: %s\n", gai_strerror(rc));
            exit(1);
        }
        printf("%s\n", buf);
    }

    freeaddrinfo(listp);

    exit(0);
}