对等方重置连接：套接字写入错误C ++-Java

Question

I am trying to write a server-client application for file transfer. The client is written in Java and the server is written in C++.

Unfortunately I have the following error:

java.net.SocketException: Connection reset by peer: socket write error"

Here is my code for client:

 import java.io.*;
 import java.net.Socket;


 public class Proba_binar 
 {
     public static void main(String[] args) 
     {
         byte[] buffer = null;  
         byte[] auxByte = new byte[1000];  
         String fileName = "1.jpg";
         File a_file = new File(fileName);  
         try  
         {
             // Create a socket
             Socket socket = new Socket("192.168.14.146", 8888);

             BufferedWriter out = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));

             BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));


             // Read file
             FileInputStream fis = new FileInputStream(fileName);    
             int length = (int)a_file.length();  
             buffer = new byte[length];  
             fis.read(buffer);  
             fis.close();  

             // Send file length
             System.out.println("length = " + Integer.toString(length));
             out.write(Integer.toString(length) + "\n");
             out.flush();

             // Send file
             int imageSize = buffer.length;
             char[] auxChar = new char[1000];
             int nr_transf = imageSize / 1000;
             int rest_byte = imageSize % 1000;
             System.out.println("nr_transf = " + nr_transf);

             for(int j = 0; j < nr_transf; j++)
             {
                 // send series of 1000 bytes
                 for(int i = 0; i < 1000; i++)
                 {
                     auxChar[i] = (char)buffer[j*1000+i];
                     auxByte[i] = buffer[j*1000+i];
                 }
                 out.write(auxChar);
                 out.flush();
             }

             // send last bytes
             for(int i = 0; i < rest_byte; i++)
             {
                 auxChar[i] = (char)buffer[1000*nr_transf+i];
                 auxByte[i] = buffer[1000*nr_transf+i];
             }
             out.write(auxChar, 0, rest_byte);
             out.flush();

             out.close();
            in.close();
            socket.close();
            System.out.println("Transfer finished!");
         }  
         catch(IOException e)  
         {  
             e.printStackTrace();  
         } 
     }
 }

And the code for server:

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

 {

     WSADATA wsa;

     SOCKET s , new_socket;

     struct sockaddr_in server , client;

     int c, bytecount, nr_transf, rest_byte, i;

     int recv_size, file_size;

     char message[1000];

    char buffer[1000];
    int buffer_len = 1000;
    FILE *f = fopen("out.jpg", "wb");

     printf("\nInitialising Winsock...");
     if (WSAStartup(MAKEWORD(2, 2),&wsa) != 0)
     {
         printf("Failed. Error Code : %d",WSAGetLastError());
         return 1;
    }

     printf("Initialised.\n");

     //Create a socket
     if((s = socket(AF_INET, SOCK_STREAM, 0 )) == INVALID_SOCKET)
     {
         printf("Could not create socket : %d" , WSAGetLastError());
        getch();
        return 0;
     }

     printf("Socket created.\n");

     //Prepare the sockaddr_in structure
     server.sin_family = AF_INET;
     server.sin_addr.s_addr = INADDR_ANY;
     server.sin_port = htons( 8888 );

     //Bind
     if(bind(s, (struct sockaddr*)&server, sizeof(server)) == SOCKET_ERROR)
     {
         printf("Bind failed with error code : %d" , WSAGetLastError());
        getch();
        return 0;
     }

     puts("Bind done");

     //Listen to incoming connections
     listen(s, 3);

     //Accept and incoming connection
     puts("Waiting for incoming connections...");

     c = sizeof(struct sockaddr_in);
     new_socket = accept(s, (struct sockaddr*)&client, &c);
     if (new_socket == INVALID_SOCKET)
     {
         printf("accept failed with error code : %d", WSAGetLastError());
        getch();
        return 0;
     }

     puts("Connection accepted");

     //Receive FILE DIMENSION from client

     if((recv_size = recv(new_socket, message, 1000, 0)) == SOCKET_ERROR)
     {
         puts("recv failed");
        getch();
     }     
        message[recv_size] = '\0';
    file_size = atoi(message);

    printf("\nfile_size = %d", file_size);

    nr_transf = file_size / 1000;
    rest_byte = file_size % 1000;

    //Receive FILE from client

    for(i = 0; i < nr_transf; i++)

    {

        // receive 1000 bytes

        if((bytecount = recv(new_socket, buffer, buffer_len, 0)) == SOCKET_ERROR)   

        {

            printf("Receive failed auxChar");

            getch();

            return 0;

        }

        fwrite(buffer, 1, buffer_len, f);

    }

    // receive last bytes

    if((bytecount = recv(new_socket, buffer, rest_byte, 0)) == SOCKET_ERROR)    

    {

        printf("Receive failed rest_byte");

        getch();

        return 0;

    }

    fwrite(buffer, 1, rest_byte, f);

    fclose(f);

    printf("Receive finished!");





     closesocket(s);

     WSACleanup();



    getch();

     return 0;

 }

I made equivalent server in Java and works perfectly. I do not know what the problem is in c + + version. Thanks in advance!

Answer 1

recv() does not guarantee that it will read all bytes requested:

...calling recv will return as much data as is currently available—up to the size of the buffer specified...

This means that the for / recv loop is incorrectly structured and it is probably the case that the server believes it has read all the data but it has not. This results in the server closing the socket before the client has sent all the data, resulting in the error reported by the Java client.

Example (untested) restructuring of the recv loop:

int totalBytesRead = 0;
int bytesRead;
while (totalBytesRead < file_size)
{
    if((bytesRead = recv(new_socket, buffer, buffer_len, 0)) == SOCKET_ERROR)
    {
        /* Handle failure. */
        break;
    }

    if (bytesRead != fwrite(buffer, 1, bytesRead, f))
    {
        /* Handle failure. */
        break;
    }

    totalBytesRead += file_size;
}

Remember that connected sockets are stream based, using mutilple send() s does not imply that the matching recv() s will read the data in the same "chunks" as they were sent. In addition to the data content retrieval error already mentioned, this equally applies to the reading of the file size. There is no guarantee that entire file size will be read.

I would recommend designing a simple protocol to ensure the different types of data are correctly read. For example, client sends:

<file_size>\\n<file_content>

the server reads all data up to the newline character and converts it into the file size. The server then knows for certain it has the correct file size. The server then reads file_size bytes from the socket. Finally, server responds to indicate success or failure:

success\\n

the client reads up to the newline character to obtain the response.