如何在UDP套接字中使用select（）来从客户端读取和写入服务器

Question

我正在尝试使用套接字创建聊天应用程序，我希望它们能够同时进行写入和读取。所以我做了一些研究，发现像select()这样的I / O多路复用将解决这个问题。但我还有一个问题。在我编译并运行它之后，两个程序中的一个没有收到消息'直到一个响应。那么，例如，服务器向客户端发送消息吧？然后客户端将在客户端响应之后才会看到该消息。您可以在下面找到此示例以及源代码。

  

客户：您好

     

服务器：你好

     

客户说“嗨”

     

对于客户等也一样

//SERVER
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <sys/select.h>//use select() for multiplexing
#include <sys/fcntl.h> // for non-blocking

#define MAX_LENGTH 1024

/* Select() params
 * int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
 * FD_SET(int fd, fd_set *set);
 * FD_CLR(int fd, fd_set *set);
 * FD_ISSET(int fd, fd_set *set);
 * FD_ZERO(fd_set *set);
*/

void error(char *message)
{
    perror(message);
    exit(1);
}

int main()
{
  fd_set original_socket;
  fd_set original_stdin;
  fd_set readfds;
  fd_set writefds;
  struct timeval tv;
  int numfd;

  int socket_fd, bytes_read;
  unsigned int address_length;
  char recieve_data[MAX_LENGTH],send_data[MAX_LENGTH];
  struct sockaddr_in server_address , client_address;

  if ((socket_fd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) 
  {
      error("socket()");
  }

  int flags = fcntl(socket_fd, F_GETFL);
  flags |= O_NONBLOCK;
  fcntl(socket_fd, F_SETFL, flags);
  //fcntl(socket_fd, F_SETFL, O_NONBLOCK); //set socket to non-blocking

  // clear the set ahead of time
  FD_ZERO(&original_socket);
  FD_ZERO(&original_stdin);
  FD_ZERO(&readfds);
  FD_ZERO(&writefds);

  // add our descriptors to the set (0 - stands for STDIN)
  FD_SET(socket_fd, &original_socket);//instead of 0 put socket_fd
  FD_SET(socket_fd, &readfds);
  FD_SET(0,&original_stdin);
  FD_SET(0, &writefds);

  // since we got s2 second, it's the "greater", so we use that for
  // the n param in select()
  numfd = socket_fd + 1;

  // wait until either socket has data ready to be recv()d (timeout 10.5 secs)
  tv.tv_sec = 10;
  tv.tv_usec = 500000;

  server_address.sin_family = AF_INET;
  server_address.sin_port = htons(5000);
  server_address.sin_addr.s_addr = INADDR_ANY;
  bzero(&(server_address.sin_zero),8);


  if (bind(socket_fd,(struct sockaddr *)&server_address, sizeof(struct sockaddr)) == -1)
  {
      error("bind()");
  }

  address_length = sizeof(struct sockaddr);

  printf("\nUDP_Server Waiting for client to respond...\n");
  fflush(stdout);
  printf("Type (q or Q) at anytime to quit\n");

  while (1)
  {
    readfds = original_socket;
    writefds = original_stdin;//problem
    int recieve = select(numfd, &readfds, &writefds,/*NULL,*/ NULL, &tv);

    if (recieve == -1) 
    {
      perror("select"); // error occurred in select()
    } 
    else if (recieve == 0) 
    {
      printf("Timeout occurred!  No data after 10.5 seconds.\n");
    } 
    else 
    {
        // one or both of the descriptors have data
        if (FD_ISSET(socket_fd, &readfds)) //if set to read
        { 
          FD_CLR(socket_fd, &readfds);
          bytes_read = recvfrom(socket_fd,recieve_data,MAX_LENGTH,0,(struct sockaddr *)&client_address, &address_length); //block call, will wait till client enters something, before proceeding
          recieve_data[bytes_read] = '\0'; //add null to the end of the buffer

          if((strcmp(recieve_data , "q") == 0) || (strcmp(recieve_data , "Q") == 0)) { //if client quit, then quit also
            printf("\nClient has exited the chat.\n");
            break;
          }

          printf("\n(%s , %d) said: %s\n",inet_ntoa(client_address.sin_addr),ntohs(client_address.sin_port),recieve_data);
        }
        else if (FD_ISSET(/*socket_fd*/0, &writefds)) //if set to write
        //else
        {
          FD_CLR(0, &writefds);
          printf("SERVER: ");
          fgets (send_data, MAX_LENGTH, stdin); //input the name with a size limit of MAX_LENGTH

          if ((strlen(send_data)>0) && (send_data[strlen (send_data) - 1] == '\n')) //if there is a trailing \n, remove it
          {
            send_data[strlen (send_data) - 1] = '\0';
          }

          if ((strcmp(send_data , "q") == 0) || (strcmp(send_data , "Q") == 0)) { //if user enters q, then quit
            sendto(socket_fd,send_data,strlen(send_data),0,(struct sockaddr *)&client_address, sizeof(struct sockaddr));
            break;
          }
          sendto(socket_fd,send_data,strlen(send_data),0,(struct sockaddr *)&client_address, sizeof(struct sockaddr));
          fflush(stdout);
        }
        else printf("\nOOPS! What happened? SERVER");
    } //end else
  }//end while

  close (socket_fd);
  return 0;
}

起初我认为这是一个尾随的换行符，但是我已经通过将'/ n'更改为'/ 0'（如果它存在）来处理这种可能性。我真的不明白为什么会发生这种情况！

//CLIENT
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h> //host struct
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <sys/select.h>//use select() for multiplexing
#include <sys/fcntl.h> // for non-blocking

#define MAX_LENGTH 1024
#define HOST "localhost" //127.0.0.1

void error(char *message)
{
    perror(message);
    exit(1);
}

int main()
{
  fd_set original_socket;
  fd_set original_stdin;
  fd_set readfds;
  fd_set writefds;
  struct timeval tv;
  int numfd;

  int socket_fd,bytes_recieved;
  unsigned int address_length;
  struct sockaddr_in server_address;
  struct hostent *host;
  char send_data[MAX_LENGTH],recieve_data[MAX_LENGTH];

  host = (struct hostent *) gethostbyname((char *)HOST);//127.0.0.1

  if ((socket_fd = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
  {
    error("socket()");
  }

  int flags = fcntl(socket_fd, F_GETFL);
  flags |= O_NONBLOCK;
  fcntl(socket_fd, F_SETFL, flags);
  //fcntl(socket_fd, F_SETFL, O_NONBLOCK); //set socket to non-blocking

  // clear the set ahead of time
  FD_ZERO(&original_socket);
  FD_ZERO(&original_stdin);
  FD_ZERO(&readfds);
  FD_ZERO(&writefds);

  // add our descriptors to the set (0 - stands for STDIN)
  FD_SET(socket_fd, &original_socket);//instead of 0 put socket_fd
  FD_SET(socket_fd, &readfds);
  FD_SET(0,&original_stdin);
  FD_SET(0, &writefds);

  // since we got s2 second, it's the "greater", so we use that for
  // the n param in select()
  numfd = socket_fd + 1;

  // wait until either socket has data ready to be recv()d (timeout 10.5 secs)
  tv.tv_sec = 10;
  tv.tv_usec = 500000;

  server_address.sin_family = AF_INET;
  server_address.sin_port = htons(5000);
  server_address.sin_addr = *((struct in_addr *)host->h_addr);
  bzero(&(server_address.sin_zero),8);

  address_length = sizeof(struct sockaddr);
  printf("Type (q or Q) at anytime to quit\n");

  while (1)
  {
    readfds = original_socket;
    writefds = original_stdin;//problem

    int recieve = select(numfd, &readfds, /*NULL*/&writefds, NULL, &tv);
    if (recieve == -1) 
    {
      perror("select"); // error occurred in select()
    } 
    else if (recieve == 0) 
    {
      printf("Timeout occurred!  No data after 10.5 seconds.\n");
    } 
    else 
    {
      // one or both of the descriptors have data
      if (FD_ISSET(socket_fd, &readfds)) //if set to read
      {
        FD_CLR(socket_fd, &readfds);//clear the set
        bytes_recieved = recvfrom(socket_fd,recieve_data, sizeof(recieve_data),0,(struct sockaddr *)&server_address,&address_length);
        recieve_data[bytes_recieved]= '\0';

        if((strcmp(recieve_data , "q") == 0) || (strcmp(recieve_data , "Q") == 0)) //if client quit, then quit also
        { 
          printf("\nServer has exited the chat.\n");
          break;
        }

        printf("\n(%s , %d) said: %s\n",inet_ntoa(server_address.sin_addr),ntohs(server_address.sin_port),recieve_data);
        //inet_ntoa returns an ip address ipv4 style, ex: 127.0.0.1, and ntohs returns the port in the converted byte ordering
      }

      else if (FD_ISSET(0/*socket_fd*/, &writefds)) //if set to write
      //else
      {
        FD_CLR(0, &writefds);
        printf("ClIENT: ");
        fgets (send_data, MAX_LENGTH, stdin);

        if ((strlen(send_data)>0) && (send_data[strlen (send_data) - 1] == '\n')) //remove trailing newline, if exists
        { 
           send_data[strlen (send_data) - 1] = '\0';
        }

        if ((strcmp(send_data , "q") == 0) || strcmp(send_data , "Q") == 0) //if user quits, then send an invisible message to server to quit also
        { 
          sendto(socket_fd, send_data, strlen(send_data), 0, (struct sockaddr *)&server_address, sizeof(struct sockaddr));
          break;
        }

        else 
        {
          sendto(socket_fd, send_data, strlen(send_data), 0, (struct sockaddr *)&server_address, sizeof(struct sockaddr));
        }

      }
       else printf("\nOOPS! What happened? CLIENT");
    } //end else
  } // end while

  close (socket_fd);
}

Answer 1

你的错误是select()等待套接字可读（这是正确的）并且STDIN是可写的（这是错误的，因为终端在大多数时间是可写的）。您必须等待套接字或STDIN可读，i。即两个文件描述符都在readfds中设置，NULL可以传递而不是&writefds。