An ECHO Server using a Single Process

int main(int argc, char *argv[])
{
    char *service = "echo";
    struct sockaddr_in fsin;
    int msock;
    fd_set rfds;
    fd_set afds;
    int alen;
    int fd, nfds;
        
    msock = passiveTCP(service, QLEN);

    nfds = getdtablesize();
    FD_ZERO(&afds);
    FD_SET(msock, &afds);

    while (1) {
        memcpy(&rfds, &afds, sizeof(rfds));

        if (select(nfds, &rfds, (fd_set *)0, (fd_set *)0,
                (struct timeval *)0) < 0)
            errexit("select: %s\n", strerror(errno));
        if (FD_ISSET(msock, &rfds)) {
            int ssock;

            alen = sizeof(fsin);
            ssock = accept(msock, (struct sockaddr *)&fsin,
                &alen);
            if (ssock < 0)
                errexit("accept: %s\n", strerror(errno));
            FD_SET(ssock, &afds);
        }
        for (fd=0; fd < nfds; ++fd)
            if (fd != msock && FD_ISSET(fd, &rfds))
                if (echo(fd) == 0) {
                    (void) close(fd);
                    FD_CLR(fd, &afds);
                }
    }
}

int echo(int fd)
{
    char buf[BUFSIZ];
    int cc;

    cc = read(fd, buf, sizeof buf);
    if (cc < 0)
        errexit("echo read: %s\n", strerror(errno));
    if (cc && write(fd, buf, cc) < 0)
        errexit("echo write: %s\n", strerror(errno));
    return cc;
}