如何在連接和選擇時編程非阻塞套接字？[復制]

我正在嘗試撰寫一個使用非阻塞 TCP 套接字連接的 C 代碼以及select(). 當我閱讀關于 的手冊頁EINPROGRESS時，我感到有點困惑。

進展

套接字是非阻塞的，連接不能立即完成。可以 通過select(2)選擇poll(2)用于寫入的套接字來完成。在select(2)指示可寫性之后，用于在級別getsockopt(2)讀取選項以確定是否成功完成（為零）或未成功是此處列出的常見錯誤代碼之一，解釋失敗的原因）。SO_ERRORSOL_SOCKETconnect()SO_ERROR(SO_ERROR

有沒有我可以參考的示例代碼？盡管這是一個很老的問題，但我沒有看到有人發布完整的作業代碼。有些人建議使用兩次連接，但我不知道具體如何。

uj5u.com熱心網友回復：

當然，下面是一個小 C 程式，它使用非阻塞 TCP 連接連接到 www.google.com 的 80 埠，向它發送一個無意義的字串，然后列印出它回傳的回應：

#include <stdio.h>
#include <netdb.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
#include <sys/select.h>
#include <sys/socket.h>

static void SendNonsenseCommand(int sock)
{
   const char sendString[] = "Hello Google!  How are you!\r\n\r\n";
   if (send(sock, sendString, sizeof(sendString), 0) != sizeof(sendString)) perror("send()");
}

int main(int argc, char ** argv)
{
   // Create a TCP socket
   const int sock = socket(AF_INET, SOCK_STREAM, 0);
   if (sock < 0) {perror("socket"); return 10;}

   // Set the TCP socket to non-blocking mode
   const int flags = fcntl(sock, F_GETFL, 0);
   if (flags < 0) {perror("fcntl(F_GETFL)"); return 10;}
   if (fcntl(sock, F_SETFL, flags|O_NONBLOCK) < 0) {perror("fcntl(F_SETFL)"); return 10;}

   // Get the IP address of www.google.com
   struct hostent * he = gethostbyname("www.google.com");
   if (he == NULL) {printf("Couldn't get a hostent for www.google.com\n"); return 10;}

   // Start a non-blocking/asynchronous TCP connetion to port 80
   struct sockaddr_in saAddr;
   memset(&saAddr, 0, sizeof(saAddr));
   saAddr.sin_family = AF_INET;
   saAddr.sin_addr   = *(struct in_addr*)he->h_addr;
   saAddr.sin_port   = htons(80);

   const int connectResult = connect(sock, (const struct sockaddr *) &saAddr, sizeof(saAddr));
   int isTCPConnectInProgress = ((connectResult == -1)&&(errno == EINPROGRESS));
   if ((connectResult == 0)||(isTCPConnectInProgress))
   {
      if (isTCPConnectInProgress == 0) SendNonsenseCommand(sock);

      // TCP connection is happening in the background; our event-loop calls select() to block until it is ready
      while(1)
      {
         fd_set socketsToWatchForReadReady, socketsToWatchForWriteReady;
         FD_ZERO(&socketsToWatchForReadReady);
         FD_ZERO(&socketsToWatchForWriteReady);

         // While connecting, we'll watch the socket for ready-for-write as that will tell us when the
         // TCP connection process has completed.  After it's connected, we'll watch it for ready-for-read
         // to see what Google's web server has to say to us.
         if (isTCPConnectInProgress) FD_SET(sock, &socketsToWatchForWriteReady);
                                else FD_SET(sock, &socketsToWatchForReadReady);

         int maxFD = sock;  // if we were watching multiple sockets, we'd compute this to be the max value of all of them

         const int selectResult = select(maxFD 1, &socketsToWatchForReadReady, &socketsToWatchForWriteReady, NULL, NULL);
         if (selectResult >= 0)
         {
            if ((FD_ISSET(sock, &socketsToWatchForWriteReady))&&(isTCPConnectInProgress))
            {
               printf("Socket is ready for write!  Let's find out if the connection succeeded or not...\n");

               struct sockaddr_in junk;
               socklen_t length = sizeof(junk);
               memset(&junk, 0, sizeof(junk));
               if (getpeername(sock, (struct sockaddr *)&junk, &length) == 0)
               {
                  printf("TCP Connection succeeded, socket is ready for use!\n");
                  isTCPConnectInProgress = 0;

                  SendNonsenseCommand(sock);
               }
               else
               {
                  printf("TCP Connection failed!\n");
                  break;
               }
            }

            if (FD_ISSET(sock, &socketsToWatchForReadReady))
            {
               char buf[512];
               const int numBytesReceived = recv(sock, buf, sizeof(buf)-1, 0);
               if (numBytesReceived > 0)
               {
                  buf[numBytesReceived] = '\0';  // ensure NUL-termination before we call printf()
                  printf("recv() returned %i:  [%s]\n", numBytesReceived, buf);
               }
               else if (numBytesReceived == 0)
               {
                  printf("TCP Connection severed!\n");
                  break;
               }
               else perror("recv()");
            }
         }
         else {perror("select()"); return 10;}
      }
   }
   else perror("connect()");

   close(sock);  // just to be tidy
   return 0;
}

標籤：C 插座 选择 tcp 非阻塞

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