echo server using select with minor modification from http://www.geeksforgeeks.org/socket-programming-in-cc-handling-multiple-clients-on-server-without-multi-threading/

select_echo_example.c

//Example code: A simple server side code, which echos back the received message.
//Handle multiple socket connections with select and fd_set on Linux 
#include <stdio.h> 
#include <string.h>   //strlen 
#include <stdlib.h> 
#include <errno.h> 
#include <unistd.h>   //close 
#include <arpa/inet.h>    //close 
#include <sys/types.h> 
#include <sys/socket.h> 
#include <netinet/in.h> 
#include <sys/time.h> //FD_SET, FD_ISSET, FD_ZERO macros 
    
#define TRUE   1 
#define FALSE  0 
    
int main(int argc , char *argv[])  
{  
    int opt = TRUE;
    int master_socket , addrlen , new_socket , client_socket[30] , 
          max_clients = 30 , activity, i , valread , sd;
    int max_sd;
    struct sockaddr_in address;
        
    char buffer[1025];  //data buffer of 1K 
    
    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s [port]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    //set of socket descriptors 
    fd_set readfds;
        
    //a message 
    char *message = "ECHO Daemon v1.0 \r\n";
    
    //initialise all client_socket[] to 0 so not checked 
    for (i = 0; i < max_clients; i++)  
    {  
        client_socket[i] = 0;
    }  
        
    //create a master socket 
    if( (master_socket = socket(AF_INET , SOCK_STREAM , 0)) == 0)  
    {  
        perror("socket failed");
        exit(EXIT_FAILURE);
    }  
    
    //set master socket to allow multiple connections , 
    //this is just a good habit, it will work without this 
    if( setsockopt(master_socket, SOL_SOCKET, SO_REUSEADDR, (char *)&opt, 
          sizeof(opt)) < 0 )  
    {  
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }  
    
    //type of socket created 
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(atoi(argv[1]));
        
    //bind the socket to localhost port 8888 
    if (bind(master_socket, (struct sockaddr *)&address, sizeof(address))<0)  
    {  
        perror("bind failed");
        exit(EXIT_FAILURE);
    }  
        
    //try to specify maximum of 3 pending connections for the master socket 
    if (listen(master_socket, 3) < 0)  
    {  
        perror("listen");
        exit(EXIT_FAILURE);
    }  
        
    //accept the incoming connection 
    addrlen = sizeof(address);
    puts("Waiting for connections ...");
        
    while(TRUE)  
    {  
        //clear the socket set 
        FD_ZERO(&readfds);
    
        //add master socket to set 
        FD_SET(master_socket, &readfds);
        max_sd = master_socket;
            
        //add child sockets to set 
        for ( i = 0 ; i < max_clients ; i++)  
        {  
            //socket descriptor 
            sd = client_socket[i];
                
            //if valid socket descriptor then add to read list 
            if(sd > 0)  
                FD_SET( sd , &readfds);
                
            //highest file descriptor number, need it for the select function 
            if(sd > max_sd)  
                max_sd = sd;
        }  
    
        //wait for an activity on one of the sockets , timeout is NULL , 
        //so wait indefinitely 
        activity = select( max_sd + 1 , &readfds , NULL , NULL , NULL);
      
        if ((activity < 0) && (errno!=EINTR))  
        {  
            printf("select error");
        }  
            
        //If something happened on the master socket , 
        //then its an incoming connection 
        if (FD_ISSET(master_socket, &readfds))  
        {  
            if ((new_socket = accept(master_socket, 
                    (struct sockaddr *)&address, (socklen_t*)&addrlen))<0)  
            {  
                perror("accept");
                exit(EXIT_FAILURE);
            }  
            
            //inform user of socket number - used in send and receive commands 
            printf("New connection , socket fd is %d , ip : %s , port: %d\n" ,
                new_socket , inet_ntoa(address.sin_addr) , ntohs(address.sin_port));
          
            //send new connection greeting message 
            if( send(new_socket, message, strlen(message), 0) != strlen(message) )  
            {  
                perror("send");
            }  
                
            puts("Welcome message sent successfully");
                
            //add new socket to array of sockets 
            for (i = 0; i < max_clients; i++)  
            {  
                //if position is empty 
                if( client_socket[i] == 0 )  
                {  
                    client_socket[i] = new_socket;
                    printf("Adding to list of sockets as %d\n" , i);
                        
                    break;
                }  
            }  
        }  
            
        //else its some IO operation on some other socket
        for (i = 0; i < max_clients; i++)  
        {  
            sd = client_socket[i];
                
            if (FD_ISSET( sd , &readfds))  
            {  
                //Somebody disconnected
                if ((valread = read( sd , buffer, 1024)) == 0)  
                {
                    printf("Socket %d closed\n", sd);
                    //Close the socket and mark as 0 in list for reuse 
                    close(sd);
                    client_socket[i] = 0;
                }
                    
                //Echo back the message that came in 
                else
                {  
                    //set the string terminating NULL byte on the end 
                    //of the data read 
                    buffer[valread] = '\0';
                    send(sd , buffer , strlen(buffer) , 0);
                }  
            }  
        }  
    }  
        
    return 0;
}