c异步域名查询程序

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <sys/select.h>
#include <netdb.h>

int main() {
    struct hostent *host;
    struct sockaddr_in server_addr;
    int sockfd, maxfdp1;
    fd_set readset;
    struct timeval timeout;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(53);

    char domain[] = "www.example.com";
    host = gethostbyname(domain);
    if (host == NULL) {
        perror("gethostbyname");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    memcpy(&server_addr.sin_addr.s_addr, host->h_addr, host->h_length);

    FD_ZERO(&readset);
    FD_SET(sockfd, &readset);
    maxfdp1 = sockfd + 1;

    timeout.tv_sec = 5;
    timeout.tv_usec = 0;

    sendto(sockfd, domain, strlen(domain), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));

    int n = select(maxfdp1, &readset, NULL, NULL, &timeout);
    if (n < 0) {
        perror("select");
        close(sockfd);
        exit(EXIT_FAILURE);
    } else if (n == 0) {
        printf("Timeout occurred\n");
    } else {
        if (FD_ISSET(sockfd, &readset)) {
            char buffer[1024];
            struct sockaddr_in from_addr;
            socklen_t from_len = sizeof(from_addr);
            recvfrom(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr *)&from_addr, &from_len);
            printf("Received response: %s\n", buffer);
        }
    }

    close(sockfd);
    return 0;
}