Simple server-client communication example in C

client.c

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

#define MAX_DATA_LEN 8192

int main(int argc, char *argv[]){
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);    
    struct sockaddr_in server_address;

    memset(&server_address, 0, sizeof(server_address));

    /* setup server address and port */
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(1711);
    inet_aton("127.0.0.1", &server_address.sin_addr);
    
    /* connect to the server */ 
    if(connect(sockfd, (struct sockaddr *)&server_address, sizeof(server_address)) == -1){
        perror("connection failed"); 
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    /* connected. send some data to server */
    char msg[] = "hello server";
    int bytes_sent = write(sockfd, msg, strlen(msg));
    printf("%d bytes written to server!\n", bytes_sent);

    /* waiting for response and read when available */
    char *read_buffer = malloc(MAX_DATA_LEN + 1);
    int bytes_read = read(sockfd, read_buffer, MAX_DATA_LEN);
    read_buffer[bytes_read] = '\0';
    printf("Data:\n%s\n", read_buffer);

    exit(EXIT_SUCCESS);
}

server.c

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

void usage(void){
    printf("USAGE: ./server port\n");
}

int main(int argc, char *argv[]){
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in serv_addr, client_address;
    size_t client_address_len = sizeof(client_address);
    int port_num;

    if(argc < 2){
        usage();
        exit(EXIT_FAILURE);
    }

    /* port to connect to. convert in network order */
    port_num = atoi(argv[1]);

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY; /* bind to a local address */
    serv_addr.sin_port = htons(atoi(argv[1]));

    int socket_opt = 1;
    setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &socket_opt, sizeof(socket_opt));

    if(bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) == -1){
        perror("bind failed");
    }

    if(listen(sockfd, 1024) == -1){
        perror("listen failed");
    }

    printf("Listening on port: %s\n", argv[1]);

    /* handle request from a client */
    int client_fd;
    if((client_fd = accept(sockfd, (struct sockaddr *)&client_address, (socklen_t *)&client_address_len)) == -1){
        perror("accept failed");
    }

    printf("connection from: %s:%u\n", inet_ntoa(client_address.sin_addr), client_address.sin_port);

    /* read data from client */
    int bytes_read;
    char *client_data = (char *)malloc(8192 * sizeof(char));   
    if((bytes_read = read(client_fd, client_data, 1024)) > 0){
       //*(client_data + bytes_read) = '\0'; 
       printf("data:\n%s\n", client_data);
    }

    /* write some data to client */
    char msg[] = "hello client";
    int bytes_sent = write(client_fd, msg, strlen(msg));
    printf("%d bytes sent to %s:%u\n", bytes_sent, inet_ntoa(client_address.sin_addr), client_address.sin_port);

    free(client_data);
    close(client_fd);
    close(sockfd); 
}