Windows（客户端）和Linux之间的TCP通信

Question

I'm working on a robot that is controlled by an xbox controller connected to a windows computer and commands are sent to a pcduino through a tcp connection. I have it working by sending a string of 1's and 0's to tell the pcduino which motors to turn on. I'm trying to optimize it by just sending an int and using bit masks to make the decisions on the pcduino but I can't get the pcduino to receive the int correctly. I tested the windows function sending the command with sokit and its sending the correct values but the pcduino is receiving the same number even when the commands are changing.

This is what its doing:

Windows -> PCDuino

command = 1 -> sendBuff = 73932

cmdstring = 1 -> n = 1

---

command = 1025 -> sendBuff = 73932

cmdstring = 1025 -> n = 4

---

My windows functions are:

bool Client::Send(char * smsg)
{
    int iResult = send(ConnectSocket, smsg, strlen(smsg), 0);

    if (iResult == SOCKET_ERROR)
    {
        std::cout << "Sending Message has failed: " << WSAGetLastError() << "\n";
        Stop();
        return false;
    }
    return true;
}

---

    bool sendCommand()
{
    cmdbuffer << command;
    cmdstring = cmdbuffer.str();

    if (!client->Send((char *)cmdstring.c_str()))
    {
        std::cout << "Disconnected from Server. Press Enter to Exit";
        std::cin.ignore();
        std::cin.get();
        return false;
    }
    return true;
}

---

PCDuino Program

/******************************************************************************/

/*Connection Globals*/
int listenfd = 0, connfd = 0;
int n;
struct sockaddr_in serv_addr;
char sendBuff[1025];
time_t ticks;

/******************************************************************************/


void setup()
{

    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    memset(&serv_addr, '0', sizeof(serv_addr));
    memset(sendBuff, '0', sizeof(sendBuff));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(5000);

    bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr));

    listen(listenfd, 10);

    connfd = accept(listenfd, (struct sockaddr*)NULL, NULL);

/******************************************************************************/


    digitalWrite(MineMode, HIGH);
}

void loop()
{
    recBuff = 0;
    deviceFlag = 0;

    //Read Socket

/******************************************************************************/

    n = read(connfd, sendBuff, strlen(sendBuff));
    recBuff = atoi(sendBuff);

/******************************************************************************/
    }

---

I have a printf after the read call and that's where I am getting the 73932 number. I think I have everything you guys need but if there's anything else I need to add let me know. I'm stumped...I don't know if its just a casting problem or what.

Answer 1

bool Client::Send(char * smsg)

you not change data here, so use const char * at least

send(ConnectSocket, smsg, strlen(smsg), 0)

If you send variable length message via TCP , you should tell somehow another side of connection how many bytes you send, for example use first 4 bytes as length

n = read(connfd, sendBuff, strlen(sendBuff));

you not receive any bytes here yet, so strlen(sendBuff) is useless and dangerous, you should make sure that you receive data with ending 0 byte, before call strlen.

Plus you should never assume that if you send N bytes by one chunk, you recieve N bytes on another side by one chunk, so you client should be finite state machine:

1. Wait 4 bytes with length of packet (call read in loop until it return 4 bytes to you in sum). Convert 4 bytes to length of message (N).

    size_t read_bytes = 0; while (read_bytes < 4) { ssize_t n = read(); if (n < 0) handle_error(); else if (n == 0) handle_n_equal_to_0_case(); else read_bytes += n; } 

2. Wait N bytes (call read in loop similar to (1)). Not forget to use rest of bytes that you receive on step 1.