server.c

/*
** server.c -- streaming tcp server that forks stockfish processes and proxies them over tcp
*/

#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#include <netinet/in.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define PORT "4000" // the port users will be connecting to

#define BACKLOG 10 // how many pending connections queue will hold

#define CHECKED(expr)            \
    if (expr < 0)                \
    {                            \
        perror("error: " #expr); \
        return 1;                \
    }

void sigchld_handler(int s)
{
    // waitpid() might overwrite errno, so we save and restore it:
    int saved_errno = errno;

    while (waitpid(-1, NULL, WNOHANG) > 0)
        ;

    errno = saved_errno;
}

// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
    if (sa->sa_family == AF_INET)
    {
        return &(((struct sockaddr_in *)sa)->sin_addr);
    }

    return &(((struct sockaddr_in6 *)sa)->sin6_addr);
}

void termination_handler(int signum)
{
    exit(1);
}

int main(void)
{
    signal(SIGINT, termination_handler);
    signal(SIGTERM, termination_handler);

    int sockfd, new_fd; // listen on sock_fd, new connection on new_fd
    struct addrinfo hints, *servinfo, *p;
    struct sockaddr_storage their_addr; // connector's address information
    socklen_t sin_size;
    struct sigaction sa;
    int yes = 1;
    char s[INET6_ADDRSTRLEN];
    int rv;

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE; // use my IP

    rv = getaddrinfo(NULL, PORT, &hints, &servinfo);
    CHECKED(rv);

    // loop through all the results and bind to the first we can
    for (p = servinfo; p != NULL; p = p->ai_next)
    {
        sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
        CHECKED(sockfd);

        CHECKED(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)));

        if (bind(sockfd, p->ai_addr, p->ai_addrlen) == -1)
        {
            close(sockfd);
            perror("server: bind");
            continue;
        }

        break;
    }

    freeaddrinfo(servinfo); // all done with this structure

    if (p == NULL)
    {
        fprintf(stderr, "server: failed to bind\n");
        exit(1);
    }

    CHECKED(listen(sockfd, BACKLOG));

    sa.sa_handler = sigchld_handler; // reap all dead processes
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    CHECKED(sigaction(SIGCHLD, &sa, NULL));

    printf("server: waiting for connections...\n");

    while (1)
    { // main accept() loop
        sin_size = sizeof their_addr;
        new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
        if (new_fd == -1)
        {
            perror("accept");
            continue;
        }

        inet_ntop(their_addr.ss_family, get_in_addr((struct sockaddr *)&their_addr),
                  s, sizeof s);
        printf("server: got connection from %s\n", s);

        if (!fork())
        {                  // this is the child process
            close(sockfd); // child doesn't need the listener
            signal(SIGINT, SIG_DFL);
            signal(SIGQUIT, SIG_DFL);
            signal(SIGTSTP, SIG_DFL);
            signal(SIGTTIN, SIG_DFL);
            signal(SIGTTOU, SIG_DFL);
            signal(SIGCHLD, SIG_DFL);

            close(STDOUT_FILENO);
            close(STDIN_FILENO);
            close(STDERR_FILENO);

            dup2(new_fd, STDOUT_FILENO);
            dup2(new_fd, STDIN_FILENO);

            close(new_fd);

            execlp("/stockfish", "stockfish", NULL);
            perror("execlp");
            exit(1);
        }
        close(new_fd); // parent doesn't need this
    }

    return 0;
}