Socket Programming in C/C++: Handling multiple clients on server without multi threading

// 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& lt; stdio.h & gt;
#include& lt; string.h & gt; //strlen
#include& lt; stdlib.h & gt;
#include& lt; errno.h & gt;
#include& lt; unistd.h & gt; //close
#include& lt; arpa / inet.h & gt; //close
#include& lt; sys / types.h & gt;
#include& lt; sys / socket.h & gt;
#include& lt; netinet / in.h & gt;
#include& lt; sys / time.h & gt; //FD_SET, FD_ISSET, FD_ZERO macros

#define TRUE 1
#define FALSE 0
#define PORT 8888

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

    // set of socket descriptors
    fd_set readfds;

    // a message
    char* message = "
    ECHO Daemon v1 .0 \r\n& quot;
    ;

    // initialise all client_socket[] to 0 so not checked
    for (i = 0; i & lt; max_clients; i++) {
        client_socket[i] = 0;
    }

    // create a master socket
    if ((master_socket = socket(AF_INET, SOCK_STREAM, 0))
        == 0) {
        perror(" socket failed & quot;);
        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))
            & lt;
        0) {
        perror(" setsockopt & quot;);
        exit(EXIT_FAILURE);
    }

    // type of socket created
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(PORT);

    // bind the socket to localhost port 8888
    if (bind(master_socket, (struct sockaddr*)&
             address, sizeof(address))
            & lt;
        0) {
        perror(" bind failed & quot;);
        exit(EXIT_FAILURE);
    }
    printf(" Listener on port % d \n & quot;, PORT);

    // try to specify maximum of 3 pending connections for
    // the master socket
    if (listen(master_socket, 3) & lt; 0) {
        perror(" listen & quot;);
        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 & lt; max_clients; i++) {
            // socket descriptor
            sd = client_socket[i];

            // if valid socket descriptor then add to read
            // list
            if (sd & gt; 0)
                FD_SET(sd, & readfds);

            // highest file descriptor number, need it for
            // the select function
            if (sd & gt; 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 & lt; 0) & amp; &
            (errno != EINTR)) {
            printf(" select error & quot;);
        }

        // 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))
                    & lt;
                0) {
                perror(" accept & quot;);
                exit(EXIT_FAILURE);
            }

            // inform user of socket number - used in send
            // and receive commands
            printf("
                   New connection, socket fd is % d, ip is
                   :
                   % s, port
                   :
                   % d 
                  \n & quot;, 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 & quot;);
            }

            puts("
                 Welcome message sent successfully & quot;);

            // add new socket to array of sockets
            for (i = 0; i & lt; 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
                           & quot;
                           , i);

                    break;
                }
            }
        }

        // else its some IO operation on some other socket
        for (i = 0; i & lt; max_clients; i++) {
            sd = client_socket[i];

            if (FD_ISSET(sd, & readfds)) {
                // Check if it was for closing , and also
                // read the incoming message
                if ((valread = read(sd, buffer, 1024))
                    == 0) {
                    // Somebody disconnected , get his
                    // details and print
                    getpeername(sd, (struct sockaddr*)&
                                address, (socklen_t*)&
                                addrlen);
                    printf(" Host disconnected, ip % s,
                                  port % d \n & quot;
                           , inet_ntoa(address.sin_addr),
                           ntohs(address.sin_port));

                    // 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;
}