C socket编程如何接收多条消息？

Question

我有一个服务器/客户端应用程序，我试图让服务器读取客户端发送给它的每条消息并将其发送回客户端进行打印。到目前为止，我让服务器读取第一条消息并将其发送到客户端，并且打印得很好，但是当从客户端发送第二条消息时，我尝试从 HandleTcpClient 函数打印它来测试它，它只打印 null，我不确定它是否正确接收

客户端代码:

#include <stdio.h> //include standard input/output library
#include <stdlib.h> //include standard libraries
#include <string.h> //include string headers 
#include <unistd.h> //add definitions for constansts and functions 
#include <sys/types.h> // include definitions for different data types
#include <sys/socket.h> //include socket support
#include <netinet/in.h> //define internet protocol functions
#include <arpa/inet.h> //define internet protocol functions
#include "Practical.h" //include practical header file

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

char myIP[16];
unsigned int myPort;
struct sockaddr_in server_addr,myaddr;
char username[] = "CharlieA";
  if (argc < 2 || argc > 3) // Test for correct number of arguments
    DieWithUserMessage("Parameter(s)",
        "<Server Address> [<Server Port>]");

  char *servIP = argv[1];     // First arg: server IP address (dotted quad)


  // Third arg (optional): server port (numeric).  7 is well-known echo port
  in_port_t servPort = atoi(argv[2]);  //21
printf("serv port: %d\n",servPort);


 // Create a reliable, stream socket using TCP  //23
int sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);//this block of code creates a reliable tcp stream socket and checks what the returned integer is from the socket function, the returned function will give a integer that descibes the socket. if this is 0 then kill the socket and show the user an error message.
  if (sock < 0)
    DieWithSystemMessage("socket() failed"); //26

  // Construct the server address structure //28
  struct sockaddr_in servAddr;            // Server address
  memset(&servAddr, 0, sizeof(servAddr)); // Zero out structure
  servAddr.sin_family = AF_INET;          // IPv4 address family
  // Convert address
  int rtnVal = inet_pton(AF_INET, servIP, &servAddr.sin_addr.s_addr);
  if (rtnVal == 0)
    DieWithUserMessage("inet_pton() failed", "invalid address string");
  else if (rtnVal < 0)
    DieWithSystemMessage("inet_pton() failed");
  servAddr.sin_port = htons(servPort);    // Server port
  myaddr.sin_addr.s_addr = INADDR_ANY;

  // Establish the connection to the echo server
  if (connect(sock, (struct sockaddr *) &servAddr, sizeof(servAddr)) < 0)
    DieWithSystemMessage("connect() failed"); 

 //get address of bound socket after connect function call (binds automatically with connect method)
bzero(&myaddr,sizeof(myaddr));
int len = sizeof(myaddr);
getsockname(sock,(struct sockaddr *) &myaddr, &len);
inet_ntop(AF_INET, &myaddr.sin_addr, myIP, sizeof(myIP)); //convert network address to string
myPort = ntohs(myaddr.sin_port); //convert from netshort to hostbyte order

//getlocal ip address to be sent to server
char *echoString=(char*)malloc(13*sizeof(char));
sprintf(echoString,"netsrv type0 %s %s-%u\r\n",username,myIP,myPort); //generate request string
size_t echoStringLen = strlen(echoString); // Determine input length //44
size_t iplen = strlen(myIP);


  // Send the string to the server
  ssize_t numBytes = send(sock,echoString, echoStringLen, 0);
  printf("sent: %s", echoString);

  if (numBytes < 0) //sending string to server, number of bytes of the message is equal to return value of send function, if the number of bytes is less than 0 then do not send and say to user that the send failed
    DieWithSystemMessage("send() failed");
  else if (numBytes != echoStringLen)
    DieWithUserMessage("send()", "sent unexpected number of bytes"); //51

// if the number of bytes is not equal to the input length of the string parsed as an argument then die with the message to the user saying sent unexpected number of bytes.
    //send IP to server
send(sock,myIP,iplen,0); //send client IP
  // Receive the same string back from the server  //53
  unsigned int totalBytesRcvd = 0; // Count of total bytes received

  while (totalBytesRcvd < echoStringLen) {
    char buffer[BUFSIZE]; // I/O buffer
    /* Receive up to the buffer size (minus 1 to leave space for
     a null terminator) bytes from the sender */
    numBytes = recv(sock, buffer, BUFSIZE - 1, 0);
    if (numBytes < 0)
      DieWithSystemMessage("recv() failed");
    else if (numBytes == 0)
      DieWithUserMessage("recv()", "connection closed prematurely");
    totalBytesRcvd += numBytes; // Keep tally of total bytes
    buffer[numBytes] = '\0';    // Terminate the string!
    fputs("Received: ", stdout);     // Setup to print the echoed string
    fputs(buffer, stdout);      // Print the echo buffer


  }

  fputc('\n', stdout); // Print a final linefeed //70

  close(sock);
  exit(0);
}
//closing off connections to clean up data left over.

第二条消息是发送客户端IP注释行

服务器:

#include <stdio.h> //include standard input/output library
#include <stdlib.h> //include standard libraries
#include <string.h> //include string headers 
#include <sys/types.h> //add definitions for constansts and functions 
#include <sys/socket.h> // include definitions for different data types
#include <netinet/in.h>  //define internet protocol functions
#include <arpa/inet.h> //define internet protocol functions
#include "Practical.h" //include pactical

static const int MAXPENDING = 5; // Maximum outstanding connection requests
static const int servPort = 48031;
int main(int argc) {//run on command line = "echoSvr <port>";argc = 2 command and parameter-  argv[0] = echoSvr and argv[1] = <port>

  // Create socket for incoming connections
  int servSock; // Socket descriptor for server
  if ((servSock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
    DieWithSystemMessage("socket() failed");
// this block of code is creating a socket stream to accept the incoming connections from clients 

  // Construct local address structure
  struct sockaddr_in servAddr;                  // Local address; internet socket address structure
  memset(&servAddr, 0, sizeof(servAddr));       // Zero out structure
  servAddr.sin_family = AF_INET;                // IPv4 address family
  servAddr.sin_addr.s_addr = htonl(INADDR_ANY); // Any incoming interface; host to network long[integer]
  servAddr.sin_port = htons(servPort);          // Local port; host to network short[integer]

  // Bind to the local address
  if (bind(servSock, (struct sockaddr*) &servAddr, sizeof(servAddr)) < 0)//cast servaddr as generic socket address structure
    DieWithSystemMessage("bind() failed");

  // Mark the socket so it will listen for incoming connections
  if (listen(servSock, MAXPENDING) < 0)
    DieWithSystemMessage("listen() failed");

setvbuf (stdout, NULL, _IONBF, 0);
printf("Listening on port: %d \n" , servPort);
printf("awaiting connection from client.... \n");
// this block of code binds the socket to the address of the server and tells the binded to socket to begin listening in for connections coming from client machines 
  for (;;) { // Run forever
    struct sockaddr_in clntAddr; // Client address
    // Set length of client address structure (in-out parameter)
    socklen_t clntAddrLen = sizeof(clntAddr);

    // Wait for a client to connect
    int clntSock = accept(servSock, (struct sockaddr *) &clntAddr, &clntAddrLen);
    if (clntSock < 0)
      DieWithSystemMessage("accept() failed");
//this block of code waits for a client to connect to the socket and then accepts the connection from the client and prints the clients details out to screen
    // clntSock is connected to a client!

    char clntName[INET_ADDRSTRLEN]; // String to contain client address
    if (inet_ntop(AF_INET, &clntAddr.sin_addr.s_addr, clntName,
        sizeof(clntName)) != NULL)
      printf("Handling client %s/%d\n", clntName, ntohs(clntAddr.sin_port));
    else
      puts("Unable to get client address");

    HandleTCPClient(clntSock);
  }
}

HandleTCPClient函数:

void HandleTCPClient(int clntSocket) {
  char buffer[BUFSIZE]; // Buffer for echo string
  char *clientIP;

  unsigned int clientPort;
  // Receive message from client
  ssize_t numBytesRcvd = recv(clntSocket, buffer, BUFSIZE, 0);
  if (numBytesRcvd < 0)
    DieWithSystemMessage("recv() failed");

//get ip and port of clntSocket to apply to greeting string

  // Send greeting string and receive again until end of stream
  while (numBytesRcvd > 0) { // 0 indicates end of stream
    // Echo message back to client
    ssize_t numBytesSent = send(clntSocket, buffer, numBytesRcvd, 0);
    if (numBytesSent < 0)
      DieWithSystemMessage("send() failed");
    else if (numBytesSent != numBytesRcvd)
      DieWithUserMessage("send()", "sent unexpected number of bytes");

    // See if there is more data to receive
    numBytesRcvd = recv(clntSocket, buffer, BUFSIZE, 0);
    if (numBytesRcvd < 0)
      DieWithSystemMessage("recv() failed");

    //recv client ip and assign to variable to hold
    recv(clntSocket,clientIP,100,0);
    printf("clientIP : %s" ,clientIP);
  }

close(clntSocket); // Close client socket
}

我正在尝试使用 printf 函数打印 clientIP，这就是我得到 null 的地方，它似乎没有收到它，这里有很多代码，我将其全部发布以防万一需要

Answer 1

这是很多代码，所以我没有看其中的大部分。然而，我确实注意到:

char *echoString=(char*)malloc(13*sizeof(char));
sprintf(echoString,"netsrv type0 %s %s-%u\r\n",username,myIP,myPort);

你没有在任何地方解释 13 来自哪里，但它显然不足以容纳 sprintf 将生成的格式化字符串。它足以容纳一个 12 个字符的字符串(加上 NUL 终止符)，这可以让您达到 netsrv type0。这需要修复，因为 sprintf 会破坏不属于它的随机内存，但这可能不是您眼前的问题。 (使用 snprintf。更简单的是，如果您使用的是 Linux/OSX/FreeBSD，则使用 asprintf。)

我认为您的问题可能是这样的:

numBytesRcvd = recv(clntSocket, buffer, BUFSIZE, 0);
//recv client ip and assign to variable to hold
recv(clntSocket,clientIP,100,0);
printf("clientIP : %s" ,clientIP);

您永远不会查看第一个 recv() 调用收到的数据。也许您有这样的误解:每次 send 都会以某种方式标记正在发送的数据，以便 recv 只会准确读取一次 send 发送的数据。事实并非如此。 TCP是一种流式传输协议(protocol)；数据只是一系列无区别的字节，每个 recv() 接收任何可用的数据，但受到调用中的大小限制。因此，两个 send() 调用的结果很可能会出现在第一个 recv() 调用中。

我认为这可能在您用来编写该代码的任何文本/教程/指南中进行了解释，但如果没有，我强烈建议找到 W. R. Stevens' Unix Network Programming 的副本.

简而言之:如果您想发送“消息”，您需要弄清楚如何以一种方式来分隔它们，以便接收者可以知道一条消息在哪里结束以及下一条消息在哪里开始。许多旧的互联网协议(protocol)使用的一个非常简单的策略是用换行符序列结束每条消息，并确保任何消息中都没有换行符。