Client server examples

#include <cstring>    // sizeof()
#include <iostream>
#include <string>   

// headers for socket(), getaddrinfo() and friends
#include <arpa/inet.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/types.h>

#include <unistd.h>    // close()


int main(int argc, char *argv[])
{
    // Let's check if port number is supplied or not..
    if (argc != 2) {
        std::cerr << "Run program as 'program <port>'\n";
        return -1;
    }
    
    auto &portNum = argv[1];
    const unsigned int backLog = 8;  // number of connections allowed on the incoming queue


    addrinfo hints, *res, *p;    // we need 2 pointers, res to hold and p to iterate over
    memset(&hints, 0, sizeof(hints));

    // for more explanation, man socket
    hints.ai_family   = AF_UNSPEC;    // don't specify which IP version to use yet
    hints.ai_socktype = SOCK_STREAM;  // SOCK_STREAM refers to TCP, SOCK_DGRAM will be?
    hints.ai_flags    = AI_PASSIVE;


    // man getaddrinfo
    int gAddRes = getaddrinfo(NULL, portNum, &hints, &res);
    if (gAddRes != 0) {
        std::cerr << gai_strerror(gAddRes) << "\n";
        return -2;
    }

    std::cout << "Detecting addresses" << std::endl;

    unsigned int numOfAddr = 0;
    char ipStr[INET6_ADDRSTRLEN];    // ipv6 length makes sure both ipv4/6 addresses can be stored in this variable


    // Now since getaddrinfo() has given us a list of addresses
    // we're going to iterate over them and ask user to choose one
    // address for program to bind to
    for (p = res; p != NULL; p = p->ai_next) {
        void *addr;
        std::string ipVer;

        // if address is ipv4 address
        if (p->ai_family == AF_INET) {
            ipVer             = "IPv4";
            sockaddr_in *ipv4 = reinterpret_cast<sockaddr_in *>(p->ai_addr);
            addr              = &(ipv4->sin_addr);
            ++numOfAddr;
        }

        // if address is ipv6 address
        else {
            ipVer              = "IPv6";
            sockaddr_in6 *ipv6 = reinterpret_cast<sockaddr_in6 *>(p->ai_addr);
            addr               = &(ipv6->sin6_addr);
            ++numOfAddr;
        }

        // convert IPv4 and IPv6 addresses from binary to text form
        inet_ntop(p->ai_family, addr, ipStr, sizeof(ipStr));
        std::cout << "(" << numOfAddr << ") " << ipVer << " : " << ipStr
                  << std::endl;
    }

    // if no addresses found :(
    if (!numOfAddr) {
        std::cerr << "Found no host address to use\n";
        return -3;
    }

    // ask user to choose an address
    std::cout << "Enter the number of host address to bind with: ";
    unsigned int choice = 0;
    bool madeChoice     = false;
    do {
        std::cin >> choice;
        if (choice > (numOfAddr + 1) || choice < 1) {
            madeChoice = false;
            std::cout << "Wrong choice, try again!" << std::endl;
        } else
            madeChoice = true;
    } while (!madeChoice);



    p = res;

    // let's create a new socket, socketFD is returned as descriptor
    // man socket for more information
    // these calls usually return -1 as result of some error
    int sockFD = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
    if (sockFD == -1) {
        std::cerr << "Error while creating socket\n";
        freeaddrinfo(res);
        return -4;
    }


    // Let's bind address to our socket we've just created
    int bindR = bind(sockFD, p->ai_addr, p->ai_addrlen);
    if (bindR == -1) {
        std::cerr << "Error while binding socket\n";
        
        // if some error occurs, make sure to close socket and free resources
        close(sockFD);
        freeaddrinfo(res);
        return -5;
    }


    // finally start listening for connections on our socket
    int listenR = listen(sockFD, backLog);
    if (listenR == -1) {
        std::cerr << "Error while Listening on socket\n";

        // if some error occurs, make sure to close socket and free resources
        close(sockFD);
        freeaddrinfo(res);
        return -6;
    }

    
    // structure large enough to hold client's address
    sockaddr_storage client_addr;
    socklen_t client_addr_size = sizeof(client_addr);


    const std::string response = "Hello World";


    // a fresh infinite loop to communicate with incoming connections
    // this will take client connections one at a time
    // in further examples, we're going to use fork() call for each client connection
    while (1) {

        // accept call will give us a new socket descriptor
        int newFD
          = accept(sockFD, (sockaddr *) &client_addr, &client_addr_size);
        if (newFD == -1) {
            std::cerr << "Error while Accepting on socket\n";
            continue;
        }

        // send call sends the data you specify as second param and it's length as 3rd param, also returns how many bytes were actually sent
        auto bytes_sent = send(newFD, response.data(), response.length(), 0);
        close(newFD);
    }

    close(sockFD);
    freeaddrinfo(res);

    return 0;
}

Detecting addresses
(1) IPv4 : 0.0.0.0
(2) IPv6 : ::
Enter the number of host address to bind with: 1

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

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


int main(int argc, char *argv[])
{
    // Now we're taking an ipaddress and a port number as arguments to our program
    if (argc != 3) {
        std::cerr << "Run program as 'program <ipaddress> <port>'\n";
        return -1;
    }


    auto &ipAddress = argv[1];
    auto &portNum   = argv[2];

    addrinfo hints, *p;
    memset(&hints, 0, sizeof(hints));
    hints.ai_family   = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags    = AI_PASSIVE;

    int gAddRes = getaddrinfo(ipAddress, portNum, &hints, &p);
    if (gAddRes != 0) {
        std::cerr << gai_strerror(gAddRes) << "\n";
        return -2;
    }

    if (p == NULL) {
        std::cerr << "No addresses found\n";
        return -3;
    }

    // socket() call creates a new socket and returns it's descriptor
    int sockFD = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
    if (sockFD == -1) {
        std::cerr << "Error while creating socket\n";
        return -4;
    }

    // Note: there is no bind() call as there was in Hello TCP Server
    // why? well you could call it though it's not necessary
    // because client doesn't necessarily has to have a fixed port number
    // so next call will bind it to a random available port number

    // connect() call tries to establish a TCP connection to the specified server
    int connectR = connect(sockFD, p->ai_addr, p->ai_addrlen);
    if (connectR == -1) {
        close(sockFD);
        std::cerr << "Error while connecting socket\n";
        return -5;
    }

    std::string reply(15, ' ');

    // recv() call tries to get the response from server
    // BUT there's a catch here, the response might take multiple calls
    // to recv() before it is completely received
    // will be demonstrated in another example to keep this minimal
    auto bytes_recv = recv(sockFD, &reply.front(), reply.size(), 0);
    if (bytes_recv == -1) {
        std::cerr << "Error while receiving bytes\n";
        return -6;
    }

    std::cout << "\nClient recieved: " << reply << std::endl;
    close(sockFD);
    freeaddrinfo(p);

    return 0;
}