select源码剖析

select只有一个系统调用
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *expectfds, struct timeval *timeout);

nfds指定被监听文件描述符的总数，一般为socket监听的所有文件描述符中的最大值+1。

readfds, writefds, expectfds分别指向可读可写异常事件的文件描述符集合，应用程序调用select时，将这些参数传给内核空间，内核保存用户想监听的事件，当监听的fd上有事件发生后，内核通过修改这些参数告知应用程序哪些已经就绪。每次再调用select时都要重新设置readfds, writefds, expectfds，因为文件描述符集合被内核修改了。

timeout参数设置select函数的超时时间，提供了秒和微秒的事件单位。如果是0，则select立刻返回。如果是NULL，select将一直阻塞，直到某个文件描述符上有时间发生。

select中重要的数据结构fd_set_bits，poll_table_page，poll_tablle_entry，poll_wqueues。

以下是select的源码，重要的地方我都做了注释，相信大家可以看懂

typedef struct {    unsigned long *in, *out, *ex;    unsigned long *res_in, *res_out, *res_ex;}fd_set_bits;

asmlinkage long sys_select(int n, fd_set __user *inp, fd_set __user *outp, fd_set __user *exp, struct timeval __user *tvp){    fd_set_bits fds;    char *bits;    long timeout;    int ret, size, max_fdset;    //将用户的等待事件拷贝到内核态    timeout = MAX_SCHEDULE_TIMEOUT;    if (tvp) {        time_t sec, usec;        if ((ret = verify_area(VERIFY_READ, tvp, sizeof(*tvp)))            || (ret = __get_user(sec, &tvp->tv_sec))            || (ret = __get_user(usec, &tvp->tv_usec)))            goto out_nofds;        ret = -EINVAL;        if (sec < 0 || usec < 0)            goto out_nofds;        //进行单位换算        if ((unsigned long) sec < MAX_SELECT_SECONDS) {            timeout = ROUND_UP(usec, 1000000/HZ);            timeout += sec * (unsigned long) HZ;        }    }    ret = -EINVAL;    if (n < 0)        goto out_nofds;    /* max_fdset can increase, so grab it once to avoid race */    max_fdset = current->files->max_fdset;    if (n > max_fdset)        n = max_fdset;    /*     * We need 6 bitmaps (in/out/ex for both incoming and outgoing),     * since we used fdset we need to allocate memory in units of     * long-words.      */    ret = -ENOMEM;    size = FDS_BYTES(n);    //为内核的fd_set_bits申请空间并初始化    bits = select_bits_alloc(size);    if (!bits)        goto out_nofds;    fds.in      = (unsigned long *)  bits;    fds.out     = (unsigned long *) (bits +   size);    fds.ex      = (unsigned long *) (bits + 2*size);    fds.res_in  = (unsigned long *) (bits + 3*size);    fds.res_out = (unsigned long *) (bits + 4*size);    fds.res_ex  = (unsigned long *) (bits + 5*size);    //拷贝用户空间感兴趣的事件    if ((ret = get_fd_set(n, inp, fds.in)) ||        (ret = get_fd_set(n, outp, fds.out)) ||        (ret = get_fd_set(n, exp, fds.ex)))        goto out;    //清空存放事件的位数组    zero_fd_set(n, fds.res_in);    zero_fd_set(n, fds.res_out);    zero_fd_set(n, fds.res_ex);    //监听用户感兴趣的事件    ret = do_select(n, &fds, &timeout);    if (tvp && !(current->personality & STICKY_TIMEOUTS)) {        time_t sec = 0, usec = 0;        if (timeout) {            sec = timeout / HZ;            usec = timeout % HZ;            usec *= (1000000/HZ);        }        put_user(sec, &tvp->tv_sec);        put_user(usec, &tvp->tv_usec);    }    if (ret < 0)        goto out;    if (!ret) {        ret = -ERESTARTNOHAND;        if (signal_pending(current))            goto out;        ret = 0;    }    //将存放事件结果的位数组拷贝给用户参数位数组，通过修改用户传进来的位数组将事件返回给用户    if (set_fd_set(n, inp, fds.res_in) ||        set_fd_set(n, outp, fds.res_out) ||        set_fd_set(n, exp, fds.res_ex))        ret = -EFAULT;out:    select_bits_free(bits, size);out_nofds:    return ret;}

int do_select(int n, fd_set_bits *fds, long *timeout){    struct poll_wqueues table;    poll_table *wait;    int retval, i;    long __timeout = *timeout;    spin_lock(&current->files->file_lock);    retval = max_select_fd(n, fds);    spin_unlock(&current->files->file_lock);    if (retval < 0)        return retval;    n = retval;    //设置回调函数，该回调函数将当前进程挂到等待对队中    poll_initwait(&table);    wait = &table.pt;    if (!__timeout)        wait = NULL;    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;            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) {                if (i >= n)                    break;                if (!(bit & all_bits))                    continue;                file = fget(i);                if (file) {                    f_op = file->f_op;                    mask = DEFAULT_POLLMASK;                    if (f_op && f_op->poll)                        //调用回调函数，为每一个fd分配一个poll_table_page，用来保存当前fd监听的事件                                                mask = (*f_op->poll)(file, retval ? NULL : wait);                    fput(file);                    //判断当前fd是否有事件发生，然后设置事件返回位数组                    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++;                    }                }                cond_resched();            }            //将返回事件位数组的值拷贝给传进来的参数            if (res_in)                *rinp = res_in;            if (res_out)                *routp = res_out;            if (res_ex)                *rexp = res_ex;        }        wait = NULL;        if (retval || !__timeout || signal_pending(current))            break;        if(table.error) {            retval = table.error;            break;        }        __timeout = schedule_timeout(__timeout);    }    __set_current_state(TASK_RUNNING);    //删除fd的poll_table_page    poll_freewait(&table);    *timeout = __timeout;    return retval;}

一张图帮你理解select

select的事件处理就说到这儿，留下几个问题，也是面试中常问的问题：
1.select和epoll有什么不同？
2.为什么epoll比select高效？
3.不论哪种情况，epoll永远比select高效吗？