C++
Beispiele für Client-Server

Hallo TCP Server

Lassen Sie mich zunächst sagen, Sie sollten zuerst Beejs Guide to Network Programming besuchen und es kurz durchlesen, was die meisten dieser Dinge etwas ausführlicher erklärt. Wir erstellen hier einen einfachen TCP-Server, der zu allen eingehenden Verbindungen "Hello World" sagt und diese dann schließt. Zu beachten ist auch, dass der Server iterativ mit den Clients kommuniziert, was jeweils einen Client bedeutet. Schauen Sie sich die relevanten Manpages an, da diese wertvolle Informationen zu den einzelnen Funktionsaufrufen und Socketstrukturen enthalten können.

Wir werden den Server mit einem Port betreiben, also nehmen wir auch ein Argument für die Portnummer. Lass uns mit Code anfangen -

#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;
}

Das folgende Programm läuft als -

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

Hallo TCP-Client

Dieses Programm ist ein komplettes Hello TCP Server-Programm. Sie können eines der beiden Programme ausführen, um die Gültigkeit des anderen zu überprüfen. Der Programmablauf ist bei Hello TCP-Servern durchaus üblich. Sehen Sie sich auch diesen an.

Hier ist der Code -

#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;
}