Recieve a message from server asynchronously

Question

I have a client program and a server program. There could be multiple servers and multiple
clients that can connect to multiple servers of there choice
The client program lists a menu

1. connect 4000 // connects to server on port 4000
2. bid 1000 4000 // send a bid value of 1000 to the server at port 4000

Now a server may recieve bids from several clients connected to it and keeps track of the highest
bid till now. Whenever a new bid is placed the server sends a broadcast to each client connected
to it one by one like - write(users[i].sock_fd, msg, size). How do I listen to this message on the client side ? There are two things here

1. The client needs to listen to the message sent by server.
2. The client is also reading the text or menu items (connect and bid) from command line from the user.

I have coded the part 2) But confused how to code 1) into client and simultaneously make the 2) also working

Client code :

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

#define BUF_SIZE 128

#define MAX_AUCTIONS 5
#ifndef VERBOSE
#define VERBOSE 0
#endif

#define ADD 0
#define SHOW 1
#define BID 2
#define QUIT 3

/* Auction struct - this is different than the struct in the server program
 */
typedef struct auction_data
{
    int sock_fd;
    char item[BUF_SIZE];
    int current_bid;
} auction_data;
auction_data *auction_data_ptr;

/* Displays the command options available for the user.
 * The user will type these commands on stdin.
 */

void print_menu()
{
    printf("The following operations are available:\n");
    printf("    show\n");
    printf("    add <server address> <port number>\n");
    printf("    bid <item index> <bid value>\n");
    printf("    quit\n");
}

/* Prompt the user for the next command 
 */
void print_prompt()
{
    printf("Enter new command: ");
    fflush(stdout);
}

/* Unpack buf which contains the input entered by the user.
 * Return the command that is found as the first word in the line, or -1
 * for an invalid command.
 * If the command has arguments (add and bid), then copy these values to
 * arg1 and arg2.
 */
int parse_command(char *buf, int size, char *arg1, char *arg2)
{
    int result = -1;
    char *ptr = NULL;
    if (strncmp(buf, "show", strlen("show")) == 0)
    {
        return SHOW;
    }
    else if (strncmp(buf, "quit", strlen("quit")) == 0)
    {
        return QUIT;
    }
    else if (strncmp(buf, "add", strlen("add")) == 0)
    {
        result = ADD;
    }
    else if (strncmp(buf, "bid", strlen("bid")) == 0)
    {
        result = BID;
    }

    ptr = strtok(buf, " ");  // first word in buf
    ptr = strtok(NULL, " "); // second word in buf

    if (ptr != NULL)
    {
        strncpy(arg1, ptr, BUF_SIZE);
    }
    else
    {
        return -1;
    }
    ptr = strtok(NULL, " "); // third word in buf

    if (ptr != NULL)
    {
        strncpy(arg2, ptr, BUF_SIZE);
        return result;
    }
    else
    {
        return -1;
    }
    return -1;
}

/* Connect to a server given a hostname and port number.
 * Return the socket for this server
 */
int add_server(char *hostname, int port)
{
    // Create the socket FD.
    int sock_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (sock_fd < 0)
    {
        perror("client: socket");
        exit(1);
    }

    // Set the IP and port of the server to connect to.
    struct sockaddr_in server;
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    struct addrinfo *ai;

    /* this call declares memory and populates ailist */
    if (getaddrinfo(hostname, NULL, NULL, &ai) != 0)
    {
        close(sock_fd);
        return -1;
    }
    /* we only make use of the first element in the list */
    server.sin_addr = ((struct sockaddr_in *)ai->ai_addr)->sin_addr;

    // free the memory that was allocated by getaddrinfo for this list
    freeaddrinfo(ai);

    // Connect to the server.
    if (connect(sock_fd, (struct sockaddr *)&server, sizeof(server)) == -1)
    {
        perror("client: connect");
        close(sock_fd);
        return -1;
    }
    if (VERBOSE)
    {
        fprintf(stderr, "\nDebug: New server connected on socket %d. Awaiting item\n", sock_fd);
    }
    return sock_fd;
}
/* ========================= Add helper functions below ========================
 * Please add helper functions below to make it easier for the TAs to find the 
 * work that you have done.  Helper functions that you need to complete are also
 * given below.
 */

/* Print to standard output information about the auction
 */
void print_auctions(struct auction_data *a, int size)
{
    printf("Current Auctions:\n");

    for (int i = 0; i < size; i++)
    {
        struct auction_data auction_data = a[i];
        printf("(%d) %s bid = %d\n", i, auction_data.item, auction_data.current_bid);
    }

    /* TODO Print the auction data for each currently connected 
     * server.  Use the follosing format string:
     *     "(%d) %s bid = %d\n", index, item, current bid
     * The array may have some elements where the auction has closed and
     * should not be printed.
     */
}

/* Process the input that was sent from the auction server at a[index].
 * If it is the first message from the server, then copy the item name
 * to the item field.  (Note that an item cannot have a space character in it.)
 */
void update_auction(char *buf, int size, struct auction_data *a, int index)
{

    // TODO: Complete this function

    // fprintf(stderr, "ERROR malformed bid: %s", buf);
    // printf("\nNew bid for %s [%d] is %d (%d seconds left)\n",           );
}

int main(void)
{

    char name[BUF_SIZE];
    int size = 0;
    // Declare and initialize necessary variables
    // TODO

    // Get the user to provide a name.
    printf("Please enter a username: ");
    fflush(stdout);
    int num_read = read(STDIN_FILENO, name, BUF_SIZE);
    printf("%s-name\n", name);
    if (num_read <= 0)
    {
        fprintf(stderr, "ERROR: read from stdin failed\n");
        exit(1);
    }
    print_menu();
    // TODO
    char server_reply[2000];

    while (1)
    {

        print_prompt();
        char *command;
        scanf("%m[^\n]s", &command);
        getchar();
        char arg1[100];
        char arg2[100];
        int commandNumber = parse_command(command, 1000, arg1, arg2);
        char dest[100] = "";
        strcpy(dest, name);
        dest[strlen(dest) - 1] = '\0';
        if (commandNumber == ADD)
        {
            printf("%s-name4\n", dest);

            int port = atoi(arg2);
            int sock_fd = add_server(arg1, port);
            printf("%s-server\n", server_reply);
            write(sock_fd, dest, strlen(dest));
            auction_data_ptr = (auction_data *)realloc(auction_data_ptr, (size + 1) * sizeof(auction_data_ptr));
            auction_data_ptr[size].sock_fd = sock_fd;
            size++;
        }
        else if (commandNumber == SHOW)
        {
            print_auctions(auction_data_ptr, size);
        }
        else if (commandNumber == BID)
        {
            int itemIndex = atoi(arg1);
            int bidValue = atoi(arg2);
            printf("%d-test\n", auction_data_ptr[itemIndex].sock_fd);
            send(auction_data_ptr[itemIndex].sock_fd, arg2, strlen(arg2), 0);
        }
        else if (commandNumber == QUIT)
        {
        }

        // TODO
    }
    return 0; // Shoud never get here
}

Server Code :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#ifndef PORT
#define PORT 30000
#endif
#define MAX_BACKLOG 5
#define MAX_CONNECTIONS 20
#define BUF_SIZE 128
#define MAX_NAME 56

int verbose = 0;

struct user
{
    int sock_fd;
    char name[MAX_NAME];
    int bid;
};

typedef struct
{
    char *item;
    int highest_bid; // value of the highest bid so far
    int client;      // index into the users array of the top bidder
} Auction;

/*
 * Accept a connection. Note that a new file descriptor is created for
 * communication with the client. The initial socket descriptor is used
 * to accept connections, but the new socket is used to communicate.
 * Return the new client's file descriptor or -1 on error.
 */
int accept_connection(int fd, struct user *users)
{
    int user_index = 0;
    while (user_index < MAX_CONNECTIONS && users[user_index].sock_fd != -1)
    {
        user_index++;
    }

    if (user_index == MAX_CONNECTIONS)
    {
        fprintf(stderr, "server: max concurrent connections\n");
        return -1;
    }

    int client_fd = accept(fd, NULL, NULL);
    if (client_fd < 0)
    {
        perror("server: accept");
        close(fd);
        exit(1);
    }

    users[user_index].sock_fd = client_fd;
    users[user_index].name[0] = '\0';
    return client_fd;
}

/* Remove \r\n from str if the characters are at the end of the string.
 * Defensively assuming that \r could be the last or second last character.
 */
void strip_newline(char *str)
{
    if (str[strlen(str) - 1] == '\n' || str[strlen(str) - 1] == '\r')
    {
        if (str[strlen(str) - 2] == '\r')
        {
            str[strlen(str) - 2] = '\0';
        }
        else
        {
            str[strlen(str) - 1] = '\0';
        }
    }
}

/*
 * Read a name from a client and store in users.
 * Return the fd if it has been closed or 0 otherwise.
 */
int read_name(int client_index, struct user *users)
{
    int fd = users[client_index].sock_fd;

    /* Note: This is not the best way to do this.  We are counting
     * on the client not to send more than BUF_SIZE bytes for the
     * name.
     */
    int num_read = read(fd, users[client_index].name, MAX_NAME);
    if (num_read == 0)
    {
        users[client_index].sock_fd = -1;
        return fd;
    }
    users[client_index].name[num_read] = '\0';
    strip_newline(users[client_index].name);

    if (verbose)
    {
        fprintf(stderr, "[%d] Name: %s\n", fd, users[client_index].name);
    }

    /*
    if (num_read == 0 || write(fd, buf, strlen(buf)) != strlen(buf)) {
        users[client_index].sock_fd = -1;
        return fd;
    }
*/

    return 0;
}

/* Read a bid from a client and store it in bid.
 * If the client does not send a number, bid will be set to -1
 * Return fd if the socket is closed, or 0 otherwise.
 */
int read_bid(int client_index, struct user *users, int *bid)
{
    printf("inside bid\n");
    int fd = users[client_index].sock_fd;
    char buf[BUF_SIZE];
    char *endptr;
    int num_read = read(fd, buf, BUF_SIZE);
    if (num_read == 0)
    {
        return fd;
    }
    buf[num_read] = '\0';

    if (verbose)
    {
        fprintf(stderr, "[%d] bid: %s", fd, buf);
    }

    // Check if the client sent a valid number
    // (We are not checking for a good bid here.)
    errno = 0;
    *bid = strtol(buf, &endptr, 10);
    if (errno != 0 || endptr == buf)
    {
        *bid = -1;
    }

    return 0;
}

void broadcast(struct user *users, char *msg, int size)
{
    for (int i = 0; i < MAX_CONNECTIONS; i++)
    {
        if (users[i].sock_fd != -1)
        {
            if (write(users[i].sock_fd, msg, size) == -1)
            {
                // Design flaw: can't remove this socket from select set
                close(users[i].sock_fd);
                users[i].sock_fd = -1;
            }
        }
    }
}

int prep_bid(char *buf, Auction *a, struct timeval *t)
{
    // send item, current bid, time left in seconds
    printf("robin2-%s-%d\n", a->item, a->highest_bid);

    printf("robin-%ld\n", t->tv_sec);
    sprintf(buf, "%s %d %ld", a->item, a->highest_bid, t->tv_sec);
    printf("robin-bid2\n");

    return 0;
}

/* Update auction if new_bid is higher than current bid.  
 * Write to the client who made the bid if it is lower
 * Broadcast to all clients if the bid is higher
 */
int update_bids(int client_index, struct user *users,
                int new_bid, Auction *auction, struct timeval *t)
{
    char buf[BUF_SIZE];

    if (new_bid > auction->highest_bid)
    {
        auction->highest_bid = new_bid;
        auction->client = client_index;

        prep_bid(buf, auction, t);
        if (verbose)
        {
            fprintf(stderr, "[%d] Sending to %d:\n    %s\n",
                    getpid(), users[client_index].sock_fd, buf);
        }

        broadcast(users, buf, strlen(buf) + 1);
    }
    else
    {
        fprintf(stderr, "Client %d sent bid that was too low.  Ignored\n",
                client_index);
    }
    return 0;
}

int main(int argc, char **argv)
{
    argc = 7;
    argv[1] = "-v";
    argv[2] = "-t";
    argv[3] = "5";
    argv[4] = "-p";
    argv[5] = "4000";
    argv[6] = "robin";
    Auction auction;
    int opt;
    int port = PORT;
    struct timeval timeout;
    struct timeval *time_ptr = NULL;
    int minutes = 0;
    while ((opt = getopt(argc, argv, "vt:p:")) != -1)
    {
        switch (opt)
        {
        case 'v':
            verbose = 1;
            break;
        case 't':
            minutes = atoi(optarg);
            timeout.tv_sec = minutes * 60;
            timeout.tv_usec = 0;
            time_ptr = &timeout;
            break;
        case 'p':
            port = atoi(optarg);
            break;
        default:
            fprintf(stderr, "Usage: auction_server [-v] [-t timeout] [-p port] item\n");
            exit(1);
        }
    }
    if (optind >= argc)
    {
        fprintf(stderr, "Expected argument after options\n");
        exit(1);
    }

    auction.item = argv[optind];
    auction.client = -1;
    auction.highest_bid = -1;

    struct user users[MAX_CONNECTIONS];
    for (int index = 0; index < MAX_CONNECTIONS; index++)
    {
        users[index].sock_fd = -1;
        users[index].name[0] = '\0';
    }

    // Create the socket FD.
    int sock_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (sock_fd < 0)
    {
        perror("server: socket");
        exit(1);
    }

    // Set information about the port (and IP) we want to be connected to.
    struct sockaddr_in server;
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr.s_addr = INADDR_ANY;

    // This sets an option on the socket so that its port can be reused right
    // away. Since you are likely to run, stop, edit, compile and rerun your
    // server fairly quickly, this will mean you can reuse the same port.
    int on = 1;
    int status = setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR,
                            (const char *)&on, sizeof(on));
    if (status == -1)
    {
        perror("setsockopt -- REUSEADDR");
    }

    // This should always be zero. On some systems, it won't error if you
    // forget, but on others, you'll get mysterious errors. So zero it.
    memset(&server.sin_zero, 0, 8);

    // Bind the selected port to the socket.
    if (bind(sock_fd, (struct sockaddr *)&server, sizeof(server)) < 0)
    {
        perror("server: bind");
        close(sock_fd);
        exit(1);
    }

    // Announce willingness to accept connections on this socket.
    if (listen(sock_fd, MAX_BACKLOG) < 0)
    {
        perror("server: listen");
        close(sock_fd);
        exit(1);
    }

    if (verbose)
    {
        fprintf(stderr, "[%d] Ready to accept connections on %d\n",
                getpid(), port);
    }

    // The client accept - message accept loop. First, we prepare to listen
    // to multiple file descriptors by initializing a set of file descriptors.
    int max_fd = sock_fd;
    fd_set all_fds;
    FD_ZERO(&all_fds);
    FD_SET(sock_fd, &all_fds);

    while (1)
    {
        // select updates the fd_set it receives, so we always use a copy
        // and retain the original.
        fd_set listen_fds = all_fds;
        int nready;
        if ((nready = select(max_fd + 1, &listen_fds, NULL, NULL, time_ptr)) == -1)
        {
            perror("server: select");
            exit(1);
        }
        if (nready == 0)
        {
            char buf[BUF_SIZE];
            sprintf(buf, "Auction closed: %s wins with a bid of %d\r\n",
                    users[auction.client].name, auction.highest_bid);
            printf("%s", buf);
            broadcast(users, buf, BUF_SIZE);
            exit(0);
        }
        // Is it the original socket? Create a new connection ...
        if (FD_ISSET(sock_fd, &listen_fds))
        {

            int client_fd = accept_connection(sock_fd, users);
            if (client_fd != -1)
            {
                if (client_fd > max_fd)
                {
                    max_fd = client_fd;
                }
                FD_SET(client_fd, &all_fds);
                if (verbose)
                {
                    fprintf(stderr, "[%d] Accepted connection on %d\n",
                            getpid(), client_fd);
                }
            }
        }

        // Next, check the clients.
        for (int index = 0; index < MAX_CONNECTIONS; index++)
        {
            if (users[index].sock_fd > -1 && FD_ISSET(users[index].sock_fd, &listen_fds))
            {
                int client_closed = 0;
                int new_bid = 0;

                if (users[index].name[0] == '\0')
                {
                    client_closed = read_name(index, users);
                    if (client_closed == 0)
                    {
                        char buf[BUF_SIZE];
                        prep_bid(buf, &auction, time_ptr);
                        if (verbose)
                        {
                            fprintf(stderr, "[%d] Sending to %d:\n    %s\n",
                                    getpid(), users[index].sock_fd, buf);
                        }
                        if (write(users[index].sock_fd, buf, strlen(buf) + 1) == -1)
                        {
                            fprintf(stderr, "Write to %d failed\n", sock_fd);
                            close(sock_fd);
                        }
                    }
                }
                else
                { // read a bid
                    client_closed = read_bid(index, users, &new_bid);
                    if (client_closed == 0)
                    {
                        update_bids(index, users, new_bid, &auction, time_ptr);
                    }
                }

                if (client_closed > 0)
                {
                    FD_CLR(client_closed, &all_fds);
                    printf("Client %d disconnected\n", client_closed);
                }
            }
        }
    }

    // Should never get here.
    return 1;
}

Answer 1

Caveat: Because you've only posted partial code for server and client, this will be some suggestions.

Your client can attach/connect to multiple bid servers simultaneously. As such, it must be able to keep track of the multiple connections in a manner similar to a server.

Your main [stated] issue is that you're blocking the client on a user prompt (e.g. from stdin via scanf et. al.). Presently, this means that the client is "stuck" at user input prompt and can not field messages from the servers it is connected to. More on how to fix this below.

So, you'll have a bunch of code from the server that needs to be in the client with some minor differences. You may wish to generalize some of the server code a bit, so it can work both in server and client (e.g. you may want to move it to common.c).

You already have code in the server to handle multiple connections. The server needs a select mask that is the OR of the listen fd and all active client fds.

Likewise, your client needs a select mask that is the OR of the fd for user input (e.g. 0) and all active server connections.

Doing select on fd 0 and using stdio.h streams won't work too well. So, replace access to stdin with (e.g.) read(0,line_buffer,sizeof(line_buffer)). You do this if fd 0 is set in the select mask. The role is very similar to what your server does for the accept on sock_fd.

You'll need to allow for partial reads and append to the buffer until you see a newline. So, you'll have to do the work that fgets would normally do in assembling a whole line. Then, you can call parse_command.

Because read doesn't understand newline demarcations, the user could enter more than one line before you can do a read.

So, for user input of:

connect 4000\n
bid 100 4000\n
connect 5000\n

You may get partial reads of:

conn
ect
 4000\nbid 100 4000
\nconnect
 5000\n

You may also need to use the FIONREAD ioctl on the fd 0 to prevent blocking. And, you may need to set the kernel TTY layer into raw mode via termios calls.

The client now becomes very similar to your server code. It will handle [asynchronously] actions by any connected servers and user input.

A tip: Under the DRY principle ["don't repeat yourself"] ...

You already have a struct user in the server. The client will need something similar/identical, such as struct server. When generalizing the code, rather than having two distinct structs that do essentially the same thing, consider renaming the existing struct to (e.g.) struct connection