c++ - 如何正确轮询ncurses

Question

原始问题:我写了一些客户端代码来监视键盘的按下和服务器向其发送消息的sockfd。问题是来自服务器的第一条消息之后，epoll不再由来自服务器的消息触发。另外，每次我输入大约10次按键时，都会为sockfd触发epoll并读取一包字符(即使服务器已经发送了许多消息)。让我感到困惑的是，如果我一次只发送一个字符，epoll就能做出正确的 react 。超过一个的结果将与以前相同(epoll不 react )。

编辑:我意识到，如果将STDIN_FILENO设置为非阻塞，我将在适当的时间从服务器获取消息。但是，该程序也将进入无限循环，并始终触发STDIN_IN。我想现在的问题是如何正确地将epoll与ncurses结合使用，以使我们不会陷入无限循环。

这是我的代码:

如何使用:

clang++ client.cpp-导致-o cli

clang++ server.cpp -o ser

./ser 8080

打开另一个终端

./cli 127.0.0.1 8080

我对epoll相当陌生，所以恐怕我可能错过了一些东西。请让我知道我的代码有什么问题!

server.cpp:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <chrono>
#include <errno.h>
#include <ifaddrs.h>
#include <sys/epoll.h>

#define TO_CLI_BUF_SIZE 32
#define FROM_CLI_BUF_SIZE 8

int main(int argc, char ** argv){

  //seed rand
  srand(time(NULL));

  int sockfd; // socket
  int port; // my port to listen on
  struct sockaddr_in serveraddr; // server's address
  struct sockaddr_in clientaddr;
  socklen_t clientlen;
  int currentAddrMax = 0;
  struct hostent * hostp; //host info
  char * hostaddrp; // host adddr string
  char toClientBuf[TO_CLI_BUF_SIZE];
  char fromClientBuf[FROM_CLI_BUF_SIZE];

  if(argc != 2){
    perror("usage: file <port>");
    exit(1);
  }
  port = atoi(argv[1]);

  // create socket
  sockfd = socket(AF_INET, SOCK_DGRAM, 0);
  if(sockfd<0){
    perror("ERROR: opening socket.");
    exit(1);
  }

  bzero((char*) &serveraddr, sizeof(serveraddr));
  serveraddr.sin_family = AF_INET;
  serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
  serveraddr.sin_port = htons((unsigned short)port);

  if(bind(sockfd, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) < 0){
    perror("ERROR on bind");
    exit(1);
  }

  bzero(fromClientBuf, FROM_CLI_BUF_SIZE);
  clientlen = sizeof(clientaddr);
  int n = recvfrom(sockfd, fromClientBuf,FROM_CLI_BUF_SIZE, 0, (struct sockaddr*) &clientaddr, &(clientlen));

  while (1){ 
    bzero(toClientBuf, TO_CLI_BUF_SIZE);
    strcpy(toClientBuf, "alkjhkfqulw8fl128lh1oufo183hf1l\0"); // I want to send 32 TO_CLI_BUF_SIZE
    int amountOfBytes = TO_CLI_BUF_SIZE; // anything greater than 1 will not work
    int n = sendto(sockfd, toClientBuf, amountOfBytes, 0, (struct sockaddr *) &clientaddr, clientlen);
    if(n < 0) {
      perror("ERROR in sendto");
      exit(1);
    }

    sleep(1); // sleep 1 sec
  }

  return 0;

}

client.cpp:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 
#include <ncurses.h>
#include <sys/epoll.h>
#include <sys/fcntl.h>
#define FROM_SER_BUF_SIZE 32
#define TO_SER_BUF_SIZE 8

int main(int argc, char **argv){

  int sockfd, portno, n;
  socklen_t serverlen;
  struct sockaddr_in serveraddr;
  struct hostent *server;
  char *hostname;

  char toServerBuf[TO_SER_BUF_SIZE];
  char fromServerBuf[FROM_SER_BUF_SIZE];

  if (argc != 3) {
    perror("usage: filename <hostname> <port>\n");
    exit(0);
  }
  hostname = argv[1];
  portno = atoi(argv[2]);

  sockfd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sockfd < 0) {
    perror("ERROR: opening sockets\n");
    exit(0);
  }

  server = gethostbyname(hostname);
  if (server == NULL) {
    fprintf(stderr,"ERROR, no such host as %s\n", hostname);
    exit(0);
  }

  bzero((char *) &serveraddr, sizeof(serveraddr));
  serveraddr.sin_family = AF_INET;
  bcopy((char *)server->h_addr, 
  (char *)&serveraddr.sin_addr.s_addr, server->h_length);
  serveraddr.sin_port = htons(portno);
  serverlen = sizeof(serveraddr);
  bzero(toServerBuf, TO_SER_BUF_SIZE);
  n = sendto(sockfd, toServerBuf, TO_SER_BUF_SIZE, 0, ( struct sockaddr *) &serveraddr, serverlen);
  if (n < 0){
    perror("ERROR: sendto");
    exit(0);
  }

  if(connect(sockfd, (struct sockaddr *)&serveraddr, serverlen) < 0) { 
    printf("\n Error : Connect Failed \n"); 
    exit(0); 
  } 
  fcntl(sockfd, F_SETFL, O_NONBLOCK); 
  nodelay(stdscr, true);
  fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK); 
  initscr();
  noecho();
  int keyboardtick = 0;
  int servertick = 0;
  int ep = epoll_create1(0);
  struct epoll_event e1,e2, e[2]; // e1 for serverfd, e2 for stdin
  memset(&e1, 0, sizeof(struct epoll_event));
  e1.events = EPOLLIN; 
  e1.data.fd = sockfd;
  epoll_ctl(ep, EPOLL_CTL_ADD, sockfd, &e1); 
  memset(&e2, 0, sizeof(struct epoll_event));
  e2.events = EPOLLIN; 
  e2.data.fd = STDIN_FILENO;
  epoll_ctl(ep, EPOLL_CTL_ADD, STDIN_FILENO, &e2);

  mvprintw(0,0,"ticks from server: %d",servertick);
  mvprintw(2,0,"ticks from keyboard: %d",keyboardtick);
  while (1){ 
    int n = epoll_wait(ep, e, 2, -1);
    for(int i = 0; i < n; i++){
      if (e[i].data.fd == sockfd) { // from server
        //
        bzero(fromServerBuf, FROM_SER_BUF_SIZE);
        n = recvfrom(sockfd, fromServerBuf, FROM_SER_BUF_SIZE, 0,( struct sockaddr *) &serveraddr, &serverlen);
        if(n < 0) {
          perror("ERROR in recv");
          exit(1);
        }
        servertick+=n;
        mvprintw(0,0,"ticks from server: %d",servertick);
      }else if(e[i].data.fd == STDIN_FILENO){
        char c = getch();
        keyboardtick++;
        mvprintw(2,0,"ticks from keyboard: %d",keyboardtick);
      }
    }
    refresh();
  }


  endwin();

  return 0;
}

Answer 1

e1.events = EPOLLIN|EPOLLET;

您已选择通过此设置使用边沿触发的epoll事件。

epoll手册页详细讨论了其含义以及如何正确使用边缘触发的epoll事件。您应该查看该描述。

让我们继续:

    int n = epoll_wait(ep, e, 2, -1);

// ...
        n = recvfrom(sockfd, fromServerBuf, FROM_SER_BUF_SIZE, 0,( struct sockaddr *) &serveraddr, &serverlen);

如果此套接字触发了epoll接收了更多的FROM_SER_BUF_SIZE字节，则此操作仅读取第一个FROM_SER_BUF_SIZE字节-换句话说，发送方足够快地发送足够的数据，这将读取第一个FROM_SER_BUF_SIZE字节就好了。当它返回到epoll_wait()时，它将再次等待。即使套接字上还有更多数据要读取。这就是边缘触发的epoll事件的工作方式。您正在以下描述边缘触发的epoll事件:

The problem is after the first message from the server, epoll is no longer triggered by messages from the server. Also, every time I enter key-presses around 10 times, epoll is triggered for sockfd and one pack of characters are read (even though the server has already sent alot of messages).

正确的。您的epoll是边缘触发的。所显示的逻辑仅使用第一个数据报，即使套接字有更多未读数据报，也将再次调用epoll。

What adds to my confusion is the if I only send one character at a time, epoll is able to react properly. Anything more than one, will have the same result as before (epoll doesn't react).

再次纠正。如果仅发送一条消息，则由显示的逻辑对其进行处理，然后epoll再次等待。

边缘触发的epoll将等待，直到该事件再次在套接字上发生。句号故事结局。如果该事件已经在套接字上发生，则边缘触发的epoll将等待而不是立即返回。 TLDR:如果套接字中有未读的数据，则不会立即返回epoll_wait的POLLIN。仅在收到更多数据后返回。如果曾经。这就是边缘触发的epoll的工作方式。

如果您打算使用沿边沿触发的epoll逻辑，则手册页说明您必须使用非阻塞套接字，并且仅在套接字完全为空或已满(换句话说，反复循环读取或写入内容)后再调用epoll_wait直到你不能)。

或者，不要使用边缘触发的epoll事件。但是，即使在非边缘触发的epoll的情况下，您也应使用非阻塞套接字。您将不得不深入阅读手册页，但是您会发现epoll包含poll，poll包含select，您将在其中找到以下gem:

   Under Linux, select() may report a socket file descriptor as "ready for
   reading", while nevertheless a subsequent read blocks.  This could  for
   example  happen  when  data  has arrived but upon examination has wrong
   checksum and is discarded.  There may be other circumstances in which a
   file  descriptor is spuriously reported as ready.  Thus it may be safer
   to use O_NONBLOCK on sockets that should not block.

因此，实际上，对于最大的可移植性，事实证明，使用边缘触发的epoll还是水平触发的epoll并不重要。为了避免追赶鬼，最佳实践似乎是(根据可用的文档)，始终将非阻塞式套接字与select/poll/epoll一起使用，并继续向前插入铲子，直到用完铲子的雪为止然后召集一次选择/投票/投票。