UDP服务器客户端C++：sendto，recvfrom

Question

我正在尝试完成一个简单的回显服务器。目标是将消息重复返回给客户端。服务器和客户端都编译。服务器绑定到本地主机和端口 8080。客户端有地址、端口和消息。当客户端将程序执行到 sendto 部分时，它会在那里停下来等待。我的目标是将其发送到服务器，然后服务器将其发回。

问题：客户端正在发送消息，服务器可以正确接收消息，但服务器无法返回消息。请帮忙！

服务器端代码：

#include <iostream>
#include <string.h>
#include <fstream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#define PORT 8080

using namespace std;

int main() {
  int serSockDes, len, readStatus;
  struct sockaddr_in serAddr, cliAddr;
  char buff[1024] = {0};
  char msg[] = "Hello to you too!!!\n";

  //creating a new server socket
  if((serSockDes = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
    perror("socket creation error...\n");
    exit(-1);
  }

  //binding the port to ip and port
  serAddr.sin_family = AF_INET;
  serAddr.sin_port = htons(PORT);
  serAddr.sin_addr.s_addr = INADDR_ANY;

  if((bind(serSockDes, (struct sockaddr*)&serAddr, sizeof(serAddr))) < 0) {
    perror("binding error...\n");
    exit(-1);
  }

  readStatus = recvfrom(serSockDes, buff, 1024, 0, (struct sockaddr*)&cliAddr, (socklen_t*)&len);
  buff[readStatus] = '\0';
  cout<<buff;

  cout<<len;

  sendto(serSockDes, msg, strlen(msg), 0, (struct sockaddr*)&cliAddr, len);

  return 0;
}

客户端代码：

#include <iostream>
#include <fstream>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#define PORT 8080

using namespace std;

int main(int argc, char** argv) {
  int cliSockDes, readStatus, len;
  struct sockaddr_in serAddr;
  char msg[] = "Hello!!!\n";
  char buff[1024] = {0};

  //create a socket
  if((cliSockDes = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
    perror("socket creation error...\n");
    exit(-1);
  }

  //server socket address
  serAddr.sin_family = AF_INET;
  serAddr.sin_port = htons(PORT);
  serAddr.sin_addr.s_addr = INADDR_ANY;

  sendto(cliSockDes, msg, strlen(msg), 0, (struct sockaddr*)&serAddr, sizeof(serAddr));

  readStatus = recvfrom(cliSockDes, buff, 1024, 0, (struct sockaddr*)&serAddr, (socklen_t*)&len);
  buff[readStatus] = '\0';
  cout<<buff;

  return 0;
}

Answer 1

客户端正在尝试将其消息发送到INADDR_ANY，这是错误的。它需要发送到特定的IP 地址。服务器可以使用 来侦听其所有本地 IP 地址INADDR_ANY，这没问题，但是客户端发送到的 IP 地址必须是服务器侦听的 IP 地址（或者，如果客户端和服务器位于不同网段，则客户端发送到的 IP 地址必须是服务器侦听的 IP 地址）必须发送到到达服务器路由器的 IP，然后路由器必须将消息转发到服务器正在侦听的 IP）。

此外，您对两端的调用recvfrom()都sendto()缺乏足够的错误处理。特别是，addrlen参数recvfrom()指定输入时缓冲区的最大大小sockaddr，输出时返回 中存储的对等地址的实际大小sockaddr。但是您没有初始化len作为 传入的变量addrlen，所以recvfrom()可能会因您无法处理的错误而失败。

尝试更像这样的事情：

服务器：

#include <iostream>
#include <string.h>
#include <fstream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
using namespace std;

#define PORT 8080

int main() {
  int serSockDes, readStatus;
  struct sockaddr_in serAddr, cliAddr;
  socklen_t cliAddrLen;
  char buff[1024] = {0};
  char msg[] = "Hello to you too!!!\n";

  //creating a new server socket
  if ((serSockDes = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
    perror("socket creation error...\n");
    exit(-1);
  }

  //binding the port to ip and port
  serAddr.sin_family = AF_INET;
  serAddr.sin_port = htons(PORT);
  serAddr.sin_addr.s_addr = INADDR_ANY;

  if ((bind(serSockDes, (struct sockaddr*)&serAddr, sizeof(serAddr))) < 0) {
    perror("binding error...\n");
    close(serSockDes);
    exit(-1);
  }

  cliAddrLen = sizeof(cliAddr);
  readStatus = recvfrom(serSockDes, buff, 1024, 0, (struct sockaddr*)&cliAddr, &cliAddrLen);
  if (readStatus < 0) { 
    perror("reading error...\n");
    close(serSockDes);
    exit(-1);
  }

  cout.write(buff, readStatus);
  cout << endl;

  if (sendto(serSockDes, msg, strlen(msg), 0, (struct sockaddr*)&cliAddr, cliAddrLen)) < 0) { 
    perror("sending error...\n");
    close(serSockDes);
    exit(-1);
  }

  close(serSockDes);
  return 0;
}

客户：

#include <iostream>
#include <fstream>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
using namespace std;

#define PORT 8080

int main(int argc, char** argv) {
  int cliSockDes, readStatus;
  struct sockaddr_in serAddr;
  socklen_t serAddrLen;
  char msg[] = "Hello!!!\n";
  char buff[1024] = {0};

  //create a socket
  if ((cliSockDes = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
    perror("socket creation error...\n");
    exit(-1);
  }

  //server socket address
  serAddr.sin_family = AF_INET;
  serAddr.sin_port = htons(PORT);
  serAddr.sin_addr.s_addr = inet_addr("127.0.0.1");

  if (sendto(cliSockDes, msg, strlen(msg), 0, (struct sockaddr*)&serAddr, sizeof(serAddr)) < 0) {
    perror("sending error...\n");
    close(cliSockDes);
    exit(-1);
  }

  serAddrLen = sizeof(serAddr);
  readStatus = recvfrom(cliSockDes, buff, 1024, 0, (struct sockaddr*)&serAddr, &serAddrLen);
  if (readStatus < 0) {
    perror("reading error...\n");
    close(cliSockDes);
    exit(-1);
  }

  cout.write(buff, readStatus);
  cout << endl;

  close(cliSockDes);
  return 0;
}