client.c

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>

int main(int argc, char *argv[])
{
    int sockfd = 0;
    struct sockaddr_in serv_addr;
    int done;
    char *line = NULL;
    size_t len = 0;
    int nread;

    if (argc != 2)
    {
        printf("\n Usage: %s <ip of server> \n", argv[0]);
        return 1;
    }

    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("\n Error : Could not create socket \n");
        return 1;
    }
    memset(&serv_addr, 0, sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(5001);

    if (inet_pton(AF_INET, argv[1], &serv_addr.sin_addr) <= 0)
    {
        printf("\n inet_pton error occured\n");
        return 1;
    }

    if (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
    {
        printf("\n Error : Connect Failed \n");
        return 1;
    }

    printf("Connected to server... Send message to server, or type 'quit' to exit\n");

    done = 0;
    while (!done)
    {
        nread = getline(&line, &len, stdin);
        nread--;
        line[nread] = '\0';
        if (strcmp(line, "ENDE") == 0)
        {
            done = 1;
            printf("Closing connection with the server..\n");
        }

        int i, key = 3;
        char ch;
        
        for (i = 0; line[i] != '\0'; ++i)
        {
            ch = line[i];
            if (ch >= 'a' && ch <= 'z')
            {
                ch = ch + key;
                if (ch > 'z')
                {
                    ch = ch - 'z' + 'a' - 1;
                }
                line[i] = ch;
            }
            else if (ch >= 'A' && ch <= 'Z')
            {
                ch = ch + key;
                if (ch > 'Z')
                {
                    ch = ch - 'Z' + 'A' - 1;
                }
                line[i] = ch;
            }
        }
        write(sockfd, line, strlen(line));
    }
    close(sockfd);
    return 0;
}