Winsock Received buffers caracters-CodePudding

I am currently trying to create an application to send messages from a server to a client after initiating the connection by sending filters from the client to the server (like a subscrition). The entire application is done but I found out that the messages I send contain special caracters and dont have the size they are supposed to have. Here is an example with the filters (which are 3 letter words) that the server receives:

Client connected!

Bytes received: 3
REceived Filters:ATL����������������������

Although it says that 3 bytes were received, it prints 25 caracters.

Here is the server side part of the code I use to receive the filters:

// Receiving and sending data on server socket

    int iSendResult;

    // buffer for received msg
    char recvbuf[3];
    int recvbuflen = 3;
    int compteurSend = 1;

    do {
        iResult = recv(ClientSocket, recvbuf, recvbuflen, 0);
        if (iResult > 0)
        {
            printf("\nBytes received: %d\n", iResult);
            printf("REceived Filters:");
            std::cout << recvbuf << "\n" << std::endl;
        }

          ...... rest of the code to send back data ......

And here is the client side part of the code I use to send te filters:

    // Sending and receiving data

        // buffer for sending filters
        const char* sendbuf = "ATL";
        int sendbuflen = strlen(sendbuf);

        // buffer for receiving
        char recvbuf[4000];
        int recvbuflen = sizeof(recvbuf);

    // Send an initial buffer
    iResult = send(ConnectSocket, sendbuf, sendbuflen, 0);
    if (iResult == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }
    std::cout << "Filters Sent: " << "'" << sendbuf << "'" << " containing " << iResult << " bytes\n\n";

While the output of the cout on the client part is correct, stating " Filters Sent: 'ATL' containing 3 bytes ", I can only understand that the issue comes from the server side and the alocation of the buffer size. Did I miss anything or did I mess up on the use of sizeof and strlen.

CodePudding user response：

std::cout << recvbuf is treating recvbuf as a null-terminated char* string, but there is no null terminator being sent by the client, and no null terminator being inserted by the server after the data received. So, operator<< ends up reading past the valid data, which is why you are seeing extra garbage being printed.

So, you can either:

update the client to send the null terminator (just make sure the server's recvbuf is large enough to receive it):

const char* sendbuf = "ATL";
int sendbuflen = strlen(sendbuf)   1; // <-- here

add a null terminator artificially on the server side:

char recvbuf[4];

do {
    iResult = recv(ClientSocket, recvbuf, sizeof(recvbuf)-1, 0);
    if (iResult > 0)
    {
        recvbuf[iResult] = '\0'; // <-- here
        printf("\nBytes received: %d\n", iResult);
        printf("Received Filters:");
        std::cout << recvbuf << "\n" << std::endl;
    }

since recv() tells you how many bytes are actually received, simply use ostream::write() instead of operator<<, eg:

std::cout.write(recvbuf, iResult);
std::cout << "\n" << std::endl;