如何获取域名和端口号？

在C语言中，获取域名和端口号通常涉及到网络编程的相关函数和数据结构，以下是一些常用的方法和步骤：

1、获取域名

：这是一个通用的网络地址解析函数，可以用于获取与主机名和服务名对应的地址信息，它能够处理IPv4、IPv6以及服务名到端口号的映射等多种情况，要获取域名www.example.com对应的IP地址，可以使用以下代码：

“`c

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <netdb.h>

#include <arpa/inet.h>

void print_address_info(const char hostname, const char service) {

struct addrinfo hints, res, p;

int status;

char ipstr[INET6_ADDRSTRLEN];

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(hostname, service, &hints, &res)) != 0) {

fprintf(stderr, "getaddrinfo: %s

", gai_strerror(status));

return;

}

printf("IP addresses for %s:

", hostname);

for(p = res; p != NULL; p = p->ai_next) {

void addr;

char ipver;

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";

}

inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);

printf(" %s: %s

", ipver, ipstr);

}

freeaddrinfo(res);

}

int main(int argc, char argv[]) {

if (argc != 3) {

fprintf(stderr, "usage: showip hostname service

");

return 1;

}

print_address_info(argv[1], argv[2]);

return 0;

}

 ：这是一个传统的网络地址解析函数，可以用于获取主机名对应的IP地址，不过需要注意的是，该函数已经被标记为过时，在新的代码中建议尽量使用getaddrinfo函数，示例代码如下：
   ```c
     #include <stdio.h>
     #include <stdlib.h>
     #include <netdb.h>
     #include <arpa/inet.h>
     void print_host_info(const char hostname) {
         struct hostent he;
         struct in_addr addr_list;
         if ((he = gethostbyname(hostname)) == NULL) {
             herror("gethostbyname");
             return;
         }
         printf("IP addresses for %s:
", hostname);
         addr_list = (struct in_addr ) he->h_addr_list;
         for(; addr_list != NULL; addr_list++) {
             printf("%s
", inet_ntoa(addr_list));
         }
     }
     int main(int argc, char argv[]) {
         if (argc != 2) {
             fprintf(stderr, "usage: showip hostname
");
             return 1;
         }
         print_host_info(argv[1]);
         return 0;
     }

2、获取端口号

从套接字地址结构体中获取：当使用套接字进行通信时，可以通过套接字地址结构体（如sockaddr_in或sockaddr_in6）来获取端口号，在服务器端接受客户端连接后，可以从客户端的套接字地址结构体中获取其端口号：

“`c

struct sockaddr_in clientAddr;

socklen_t sin_size = sizeof(clientAddr);

int new_fd = accept(sock, (struct sockaddr )&clientAddr, &sin_size);

if(new_fd<0) {

char msg[64];

bzero(msg,sizeof(msg));

sprintf(msg,"accept failed");

log::outputSysErr(msg);

} else {

// 将sockaddr强制转换为 sockaddr_in

sockaddr_in pSin = (sockaddr_in)&clientAddr;

int clientPort = ntohs(pSin->sin_port); // 将网络字节序转换为主机字节序

printf("Client port: %d

", clientPort);

}

 ：该函数用于获取与指定套接字关联的本地地址，包括端口号，在创建套接字并绑定端口后，可以使用getsockname函数获取绑定的端口号：
   ```c
     int server_fd;
     struct sockaddr_in address;
     int addrlen = sizeof(address);
     if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
         perror("socket failed");
         exit(EXIT_FAILURE);
     }
     address.sin_family = AF_INET;
     address.sin_addr.s_addr = INADDR_ANY;
     address.sin_port = htons(0); // 让系统自动分配端口
     if (bind(server_fd, (struct sockaddr )&address, sizeof(address)) < 0) {
         perror("bind failed");
         close(server_fd);
         exit(EXIT_FAILURE);
     }
     if (getsockname(server_fd, (struct sockaddr )&address, (socklen_t )&addrlen) == -1) {
         perror("getsockname failed");
         close(server_fd);
         exit(EXIT_FAILURE);
     }
     int localPort = ntohs(address.sin_port); // 将网络字节序转换为主机字节序
     printf("Local port: %d
", localPort);
     close(server_fd);

使用getpeername函数：该函数用于获取与指定套接字关联的远端地址，也包括端口号，通常在连接建立后，使用该函数获取客户端的端口号：

“`c

int server_fd, new_socket;

struct sockaddr_in address, client_addr;

int addrlen = sizeof(address);

int client_addrlen = sizeof(client_addr);

if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {

perror("socket failed");

exit(EXIT_FAILURE);

}

address.sin_family = AF_INET;

address.sin_addr.s_addr = INADDR_ANY;

address.sin_port = htons(PORT);

if (bind(server_fd, (struct sockaddr )&address, sizeof(address)) < 0) {

perror("bind failed");

close(server_fd);

exit(EXIT_FAILURE);

}

listen(server_fd, 3);

new_socket = accept(server_fd, (struct sockaddr )&client_addr, (socklen_t)&client_addrlen);

if (new_socket < 0) {

perror("accept failed");

close(server_fd);

exit(EXIT_FAILURE);

}

if (getpeername(new_socket, (struct sockaddr )&client_addr, (socklen_t )&client_addrlen) == -1) {

perror("getpeername failed");

close(new_socket);

close(server_fd);

exit(EXIT_FAILURE);

}

int clientPort = ntohs(client_addr.sin_port); // 将网络字节序转换为主机字节序

printf("Client port: %d

", clientPort);

close(new_socket);

close(server_fd);

通过这些方法，可以在C语言中有效地获取域名和端口号，从而进行网络通信和相关操作。