c - Linux, C, epoll(), read() 数据不完整？

Question

Linux, C. 以下问题仅在使用 epoll() 时发生。如果我在服务器套接字上使用 select()，则不会丢失数据。

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更新:我在 read() 中收到 errno =11(再试一次)。我需要继续还是中断 while 循环？

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我有客户端，分10次发送1280K数据(每次发送128K数据)；

我有服务器端，使用 epoll() 来观察传入的数据。

收到通知后，我使用以下代码读取数据:

nbytes = Nread(current_fd, buffer, bytes_to_be_read);

int Nread(int fd, char *buffer, size_t count)
  {
          ssize_t r;
          size_t left = count;
          printf("===>\n");
          while (left > 0){
                  r = read(fd, buffer, left);
                  printf("data: %ld\n", r);
                  if (r < 1) {
                          printf("errno: %d\n", errno);
                          break;  //I do received 2 errors of Try Again. How to try again?
                  }

                  left -= r;
                  buffer += r;
          }
          printf("=> done, %ld\n", total - left);
          return count - left;
  }

在 Nread 中，我希望读取 bytes_to_be_read (65536) 个字节并返回。

但是，当运行该工具时，我发现服务器端只收到类似这样的信息。看起来在某些情况下我没有从缓冲区读取所有数据。为什么？

1===>
data: 65536
=> done, 65536
2===>
data: 65536
=> done, 65536
3===>
data: 60734
data: 4802
=> done, 65536
4===>
data: 55934
data: -1
errno: 11
=> done, 55934  //data is not enough. the missing part should come soon. but why I am not notified?
5===>
data: 60736
data: 4800
=> done, 65536
6===>
data: 65536
=> done, 65536
7===>
data: 65536
=> done, 65536
8===>
data: 65536
=> done, 65536
9===>
data: 65536
=> done, 65536
10===>
data: 65536
=> done, 65536
11===>
data: 65536
=> done, 65536
12===>
data: 65536
=> done, 65536
13===>
data: 65536
=> done, 65536
14===>
data: 65536
=> done, 65536
15===>
data: 65536
=> done, 65536
16===>
data: 65536
=> done, 65536
17===>
data: 65536
=> done, 65536
18===>
data: 65536
=> done, 65536
19===>
data: 65536
=> done, 65536

===============

更新:

如果我使用这段代码读取所有数据，则不会有字节丢失。但性能很差:

 36 int read_all(int fd, char *buffer)
 37 {
 38         ssize_t count = 0, bytes = 0;
 39
 40         while(1){
 41                 bytes = read(fd, buffer, sizeof(buffer));
 42                 if (bytes < 1)
 43                         break;
 44                 count += bytes;
 45         }
 46         return count;
 47 }

===================================

EPOLL()

efd = epoll_create1 (0);
event.data.fd = listener_fd;
event.events = EPOLLIN | EPOLLET;
epoll_ctl(efd, EPOLL_CTL_ADD, listener_fd, &event);

while (1){
    n_fds = epoll_wait (efd, events, MAX_EPOLL_EVENTS, -1);
    for (i = 0; i < n_fds; i++){
                ...........
       if (islistener) {
             .....
            //handle new connections
       }
      else{
           bzero(buffer, recv_buf_size);
           if ((n= Nread(fd, buffer, recv_buf_size)) <= 0) {
                  //error
           }
           else
           {
                   //add bytes to total:
                    __sync_fetch_and_add(&(mythreads->total_bytes_transferred), n)    ;
           }

}

Answer 1

        break;  //I do received 2 errors of Try Again. How to try again?

回到你的 epoll 循环。

        data: 60734  //??? why I am not able to read all 65536 bytes?

因为还没有收到。

我认为您可能错过了有关如何使用 epoll 执行非阻塞 I/O 的大图。

1. epoll 告诉您何时有数据。
2. 您当时尽可能多地阅读。
3. 你回到第 1 步。