How to make recv have a little temper?

Hello everyone , I'm brother Tao .

 

today , Let's talk about TCP In network programming recv function . We know ,recv It's a blocking function , When no data is received , Will wait for the data foolishly .

that , How to let recv The function is a little grumpy , Instead of waiting foolishly ？select Functions can top . Next , Let's fight , Deepen the understanding of network programming .

                                                      Hand painted by Taoge

One .  Grumpy recv function

When recv When a function has no temper , Just wait foolishly , Till the end of time . Let's look at the server program , And let the server run first .

We see , When building a TCP After connection , The server deliberately waited 3 Seconds to send data , So can the client afford to wait ？ Are you patient ？

#include <stdio.h>
#include <winsock2.h> // winsock Interface 
#pragma comment(lib, "ws2_32.lib") // winsock Realization 
 
int main()
{
  WORD wVersionRequested;  //  Double byte ,winsock Version of the library 
  WSADATA wsaData;         // winsock Information about library version 
  
  wVersionRequested = MAKEWORD(1, 1); // 0x0101  namely :257
  
  //  load winsock Library and determine winsock edition , The system will fill in the data wsaData in 
  WSAStartup( wVersionRequested, &wsaData );
 
  // AF_INET  Said the TCP/IP Protocol family 
  // SOCK_STREAM  Said the TCP agreement 
  // 0 Is the usual default 
  unsigned int sockSrv = socket(AF_INET, SOCK_STREAM, 0);
 
  SOCKADDR_IN addrSrv;
 
  addrSrv.sin_family = AF_INET; // TCP/IP Protocol family 
  addrSrv.sin_addr.S_un.S_addr = inet_addr("0.0.0.0"); 
  addrSrv.sin_port = htons(8888); // socket Corresponding port 
 
  //  take socket Bind to something IP And port （IP Identify the host , The port identifies the communication process ）
  bind(sockSrv,(SOCKADDR*)&addrSrv, sizeof(SOCKADDR));
 
  listen(sockSrv, 5);
 
  SOCKADDR_IN addrClient;
  int len = sizeof(SOCKADDR);
  
  while(1)
  {
    unsigned int sockConn = accept(sockSrv,(SOCKADDR*)&addrClient, &len);
    printf("accept is returned\n");
 
    Sleep(3000);
    char sendBuf[100] = "hello";
    send(sockConn, sendBuf, strlen(sendBuf) + 1, 0); //  Send data to client , This is the last parameter 0
    //closesocket(sockConn);  
  }
 
  closesocket(sockSrv);
  WSACleanup();
  
  return 0;
}

next , Let's look at the client program , And let the client run . Run client program , You can find , Although the server deliberately delayed 3 second , But customers have unlimited patience , Still get data .

#include <winsock2.h>
#include <stdio.h>
#pragma comment(lib, "ws2_32.lib")
 
int main()
{
  WORD wVersionRequested;
  WSADATA wsaData;
  wVersionRequested = MAKEWORD(1, 1);
  
  WSAStartup( wVersionRequested, &wsaData );
 
  SOCKET sockClient = socket(AF_INET, SOCK_STREAM, 0);
 
  SOCKADDR_IN addrSrv;
  addrSrv.sin_addr.S_un.S_addr = inet_addr("127.0.0.1");
  addrSrv.sin_family = AF_INET;
  addrSrv.sin_port = htons(8888);
 
  int ret = connect(sockClient, (SOCKADDR*)&addrSrv, sizeof(SOCKADDR));
  printf("connect ret is %d\n", ret);
 
      
  char recvBuf[200] = "x";
  ret = recv(sockClient, recvBuf, 100, 0);
  if(0 > ret)
  {
    printf("recv error");
    return -3;
  }
 
  if(0 == ret)
  {
    printf("recv ret is %d\n", ret);
    return -4;
  }
  
  printf("%s\n", recvBuf);
 
  while(1);
 
  closesocket(sockClient);
  WSACleanup();
 
  return 0;
}

Two . Temperamental recv function

recv Function should be a little grumpy , wait for 4 Seconds later , You don't come , I won't wait . How to achieve ？ You can use select Timeout feature of function , The client program is as follows ：

#include <winsock2.h>
#include <stdio.h>
#pragma comment(lib, "ws2_32.lib")
 
int main()
{
  WORD wVersionRequested;
  WSADATA wsaData;
  wVersionRequested = MAKEWORD(1, 1);
  
  WSAStartup( wVersionRequested, &wsaData );
 
  SOCKET sockClient = socket(AF_INET, SOCK_STREAM, 0);
 
  SOCKADDR_IN addrSrv;
  addrSrv.sin_addr.S_un.S_addr = inet_addr("127.0.0.1");
  addrSrv.sin_family = AF_INET;
  addrSrv.sin_port = htons(8888);
 
  int ret = connect(sockClient, (SOCKADDR*)&addrSrv, sizeof(SOCKADDR));
  printf("connect ret is %d\n", ret);
  
  fd_set rfds;
  struct timeval timeout = {4, 0};
 
  FD_ZERO(&rfds); 
  FD_SET(sockClient, &rfds);
  ret = select(-1, &rfds, NULL, NULL, &timeout);
  printf("select ret is %d\n",ret);
  if(0 > ret)
  {
    printf("select error\n");
    return -1;
  }
   
  if(0 == ret)
  {
    printf("time out\n");
    return -2;
  }
    
  if(FD_ISSET(sockClient, &rfds))
  {
    char recvBuf[200] = "x";
    ret = recv(sockClient, recvBuf, 100, 0);
    if(0 > ret)
    {
      printf("recv error");
      return -3;
    }
 
    if(0 == ret)
    {
      printf("recv ret is %d, buf is %s\n", ret, recvBuf);
      return -4;
    }
    
    printf("%s\n", recvBuf);
    }
 
  while(1);
  closesocket(sockClient);
  WSACleanup();
 
  return 0;
}

The patience of the client is only 4 second , Client said ： The server , you TM Of , Before I establish with you TCP After relationship , If you 4 No data in seconds , I'll ignore you .

Run the program , You can see , The client can receive data , Because the server is 4 The data was transmitted in seconds , The client's patience is not reached 4 Second limit .

If you change the endurance value of the client to 2 second , What will happen then ？ After testing , Client, etc 2 Seconds later , I don't want to wait , Naturally, no data was received , Have enough temper .

In the actual work of network programming , Almost all network related operations need to set timeout , For example, connection timeout 、 Send timeout 、 Receive timeout , Everywhere .

When learning computer network and network programming , Many people have prepared a lot of eight part essays for the written interview , Like three handshakes and four waves , These preparations are necessary .

However , These are not enough . Computer network is a subject with strong practicality , I personally suggest writing more programs 、 Multi debugging 、 Grab more bags , Then compare it with the theory .