CODEWITHSUNDEEP
clinuxsocketsposix
Joshun
Joshun

Reputation: 248

Posix Sockets: Find out greatest number of file descriptors

How can I keep track of the greatest number of file descriptors each time instead of using FD_SETSIZE (which may be very large)? So far the code is (adapted from Beginning Linux Programming, 2nd Edition):

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <unistd.h>

#define SERVER_PORT 9734
#define ALLOWED_CLIENTS INADDR_ANY
#define BACKLOG 5
#define DELAY 0

int main()
{
    int server_sockfd, client_sockfd;
    socklen_t server_len, client_len;
    struct sockaddr_in server_address, client_address;
    int result;
    fd_set readfds, testfds;

    server_sockfd = socket(AF_INET, SOCK_STREAM, 0);
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = htonl(ALLOWED_CLIENTS);
    server_address.sin_port = htons(SERVER_PORT);
    server_len = sizeof(server_address);
    bind(server_sockfd, (struct sockaddr *)&server_address, server_len);

    listen(server_sockfd, BACKLOG);
    FD_ZERO(&readfds); /* Initialise readfds fd_set struct */
    FD_SET(server_sockfd, &readfds); /* Initialise readfds to handle input from server_sockfd */

    while(1) {
        char ch;
        int fd;
        int nread;

        testfds = readfds;
        printf("Server waiting...\n");
        /* Wait indefinitely for client request (input) using testfds */
        result = select(FD_SETSIZE, &testfds, (fd_set *)0, (fd_set *)0, (struct timeval *)0);

        if(result < 1) {
            perror("Server 5");
            exit(1);
        }

        /* At this stage, activity of a client trying to connect has been found.
         * We will find which descriptor it is on by checking each in turn. */
         for(fd = 0; fd < FD_SETSIZE; fd++)
         {
             if(FD_ISSET(fd, &testfds)) { /* If activity occurs on the given file descriptor... */

                    if(fd == server_sockfd) { /* If activity occurs on server_sockfd, it must be
                                               * a request for a new connection */
                        client_len = sizeof(client_address);

                        /* Extract connection request - set client_sockfd equal to this */
                        client_sockfd = accept(server_sockfd, (struct sockaddr *)&client_address, &client_len);

                        /* Add client_sockfd to the descriptor set */
                        FD_SET(client_sockfd, &readfds);
                        printf("    -Added client (fd %d)\n", fd);
                    }
                    else
                    {
                        ioctl(fd, FIONREAD, &nread); /* Find out how much data needs to be read in */

                        if(nread == 0) { /* No data left - finished with this client */
                            close(fd);
                            FD_CLR(fd, &readfds);
                            printf("    -Removed client (fd %d)\n", fd);
                        }
                        else {
                            read(fd, &ch, 1); /* Carry out the server's actual function */
                            sleep(DELAY);
                            printf("    -Serving client (fd %d)\n", fd);
                            ch++;
                            write(fd, &ch, 1);
                        }
                    }
                }
            }
        }
}

The book went on to say that this would make it much less efficient, which makes sense, and that a variable should be used to keep track of the largest fd number connected, but I just can't figure out how to implement this, have spent ages experimenting. Thanks in advance.

Upvotes: 1

Views: 633

Answers (1)

thuovila
thuovila

Reputation: 2020

You should have a variable, e.g. int maxfd, which you adjust every time your code contains FD_SET() or FD_CLR(). The answer to this question contains an example of adjusting maxfd properly.

Unlike the comments suggest, I dont think you need to make "the" (which the?) variable static. The comments are right about poll and epoll, but knowing how to use select is useful as well.

Upvotes: 2

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