基于select、poll、epoll的区别详解

int do_select(int n, fd_set_bits *fds, struct timespec *end_time){    ktime_t expire, *to = NULL;    struct poll_wqueues table;    poll_table *wait;    int retval, i, timed_out = 0;    unsigned long slack = 0;    rcu_read_lock();    retval = max_select_fd(n, fds);    rcu_read_unlock();    if (retval < 0)        return retval;    n = retval;    poll_initwait(&table);    wait = &table.pt;    if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {        wait = NULL;        timed_out = 1;    }    if (end_time && !timed_out)        slack = estimate_accuracy(end_time);    retval = 0;    for (;;) {        unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;        set_current_state(TASK_INTERRUPTIBLE);        inp = fds->in; outp = fds->out; exp = fds->ex;        rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;        for (i = 0; i < n; ++rinp, ++routp, ++rexp) {            unsigned long in, out, ex, all_bits, bit = 1, mask, j;            unsigned long res_in = 0, res_out = 0, res_ex = 0;            const struct file_operations *f_op = NULL;            struct file *file = NULL;            in = *inp++; out = *outp++; ex = *exp++;            all_bits = in | out | ex;            if (all_bits == 0) {                i += __NFDBITS;                continue;            }            for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) {                int fput_needed;                if (i >= n)                    break;                if (!(bit & all_bits))                    continue;                file = fget_light(i, &fput_needed);                if (file) {                    f_op = file->f_op;                    mask = DEFAULT_POLLMASK;                    if (f_op && f_op->poll)                        mask = (*f_op->poll)(file, retval ? NULL : wait);                    fput_light(file, fput_needed);                    if ((mask & POLLIN_SET) && (in & bit)) {                        res_in |= bit;                        retval++;                    }                    if ((mask & POLLOUT_SET) && (out & bit)) {                        res_out |= bit;                        retval++;                    }                    if ((mask & POLLEX_SET) && (ex & bit)) {                        res_ex |= bit;                        retval++;                    }                }            }            if (res_in)                *rinp = res_in;            if (res_out)                *routp = res_out;            if (res_ex)                *rexp = res_ex;            cond_resched();        }        wait = NULL;        if (retval || timed_out || signal_pending(current))            break;        if (table.error) {            retval = table.error;            break;        }        /*         * If this is the first loop and we have a timeout         * given, then we convert to ktime_t and set the to         * pointer to the expiry value.         */        if (end_time && !to) {            expire = timespec_to_ktime(*end_time);            to = &expire;        }        if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))            timed_out = 1;    }    __set_current_state(TASK_RUNNING);    poll_freewait(&table);    return retval;}