client

#include <iostream>
#include <string>
#include <cstring>

#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <signal.h>

using namespace std;

int main()
{
    signal(SIGPIPE, SIG_IGN);

    const int PORT = 54000;

    int sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock < 0)
    {
        cerr << "socket error\n";
        return 1;
    }

    sockaddr_in hint{};
    hint.sin_family = AF_INET;
    hint.sin_port = htons(PORT);
    inet_pton(AF_INET, "127.0.0.1", &hint.sin_addr);

    if (connect(sock, (sockaddr*)&hint, sizeof(hint)) < 0)
    {
        cerr << "connect error\n";
        close(sock);
        return 1;
    }

    char buf[256];
    string input;

    while (true)
    {
        cout << "Enter integer: ";
        getline(cin, input);

        if (input.empty())
            continue;

        send(sock, input.c_str(), input.size(), MSG_NOSIGNAL);

        memset(buf, 0, sizeof(buf));

        ssize_t bytesReceived = recv(sock, buf, sizeof(buf) - 1, 0);
        if (bytesReceived <= 0)
            break;

        cout << "Server replied: " << buf << endl;
    }

    close(sock);

    return 0;
}

server

#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <signal.h>

using namespace std;

int main()
{
    signal(SIGPIPE, SIG_IGN);

    const int PORT = 54000;

    int serverSocket = socket(AF_INET, SOCK_STREAM, 0);
    if (serverSocket < 0)
    {
        cerr << "socket error\n";
        return 1;
    }

    int opt = 1;
    setsockopt(serverSocket, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

    sockaddr_in hint{};
    hint.sin_family = AF_INET;
    hint.sin_port = htons(PORT);
    hint.sin_addr.s_addr = INADDR_ANY;

    if (::bind(serverSocket, (sockaddr*)&hint, (socklen_t)sizeof(hint)) < 0)
    {
        cerr << "bind error\n";
        close(serverSocket);
        return 1;
    }

    if (listen(serverSocket, SOMAXCONN) < 0)
    {
        cerr << "listen error\n";
        close(serverSocket);
        return 1;
    }

    cerr << "Server started. Waiting for client...\n";

    sockaddr_in client{};
    socklen_t clientSize = sizeof(client);

    int clientSocket = accept(serverSocket, (sockaddr*)&client, &clientSize);
    if (clientSocket < 0)
    {
        cerr << "accept error\n";
        close(serverSocket);
        return 1;
    }

    char buf[256];

    while (true)
    {
        memset(buf, 0, sizeof(buf));

        ssize_t bytesReceived = recv(clientSocket, buf, sizeof(buf) - 1, 0);
        if (bytesReceived <= 0)
            break;

        long value = strtol(buf, nullptr, 10);
        long result = value + 1;

        string out = to_string(result);

        send(clientSocket, out.c_str(), out.size(), 0);
    }

    close(clientSocket);
    close(serverSocket);

    return 0;
}