select的调用径路sys_select()->core_sys_select()-> do_select()

最主要的作工就是在 do_select中成完，面前为数参的断判及准备作工

{
     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);             对   poll_wqueues结构停止一些初始化

wait = &table.pt;                  得取poll_table结构
     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;

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) {                历遍每一个描述符
               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;                     如果这个字没有待查找的描述符, 跳过这个长字(32位)
                    continue;
               }

for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) {           历遍每一个长字里的每一位个
                    int fput_needed;
                    if (i >= n)
                         break;
                    if (!(bit & all_bits))
                         continue;
                    file = fget_light(i, &fput_needed);               失掉指位定对应的fd的file结构
                    if (file) {
                         f_op = file->f_op;
                         mask = DEFAULT_POLLMASK;
                         if (f_op && f_op->poll) {
                              wait_key_set(wait, in, out, bit);
                              mask = (*f_op->poll)(file, wait);  在这里循环调用所监测的fd_set内的有所件文描述符对应的poll数函

              还记得socket创立那一文中说到：

init_file(file, sock_mnt, dentry, FMODE_READ | FMODE_WRITE,  & socket_file_ops );           

                                 对file结构成员赋值，并将file->f_op 更新为 socket_file_ops socket类型件文的作操数函集    

那poll数函为： .poll = sock_poll,

sock_poll()见上面分析
                         }
                         fput_light(file, fput_needed); 
                          if ((mask & POLLIN_SET) && (in & bit)) {             
                              res_in |= bit;        如果是这个描述符可读, 将这位个置位
                              retval++;             返回描述符个数加1
                              wait = NULL;
                         }
                         if ((mask & POLLOUT_SET) && (out & bit)) {   
                              res_out |= bit;         如果是这个描述符有异常错误, 将这位个置位
                              retval++;                返回描述符个数加1
                              wait = NULL;
                         }
                         if ((mask & POLLEX_SET) && (ex & bit)) {   
                              res_ex |= bit;                如果是这个描述符有异常错误, 将这位个置位
                              retval++;                      返回描述符个数加1
                              wait = NULL;
                         }
                    }
               }
               if (res_in)
                    *rinp = res_in;
               if (res_out)
                    *routp = res_out;
               if (res_ex)
                    *rexp = res_ex;
               cond_resched();
          }

     到这里历遍结束。retval保存了检测到的可作操的件文描述符的个数。如果有件文可作操，则跳出for(;;)循环，直接返回。

    若没有件文可作操且timeout间时未到同时没有收到signal,则执行                

     schedule_timeout就寝。就寝间时是非由__timeout定决，直一等到该进程被唤醒。

          wait = NULL;
          if (retval || timed_out || signal_pending(current))
               break;                                                                    跳出循环 返回结果
          if (table.error) {
               retval = table.error;
               break;
          }

if (end_time && !to) {
               expire = timespec_to_ktime(*end_time);
               to = &expire;
          }

if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,            就寝，可由信号唤醒
                            to, slack))
               timed_out = 1;
     }

poll_freewait(&table);

return retval;
}

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typedef struct {
     unsigned long *in, *out, *ex;
     unsigned long *res_in, *res_out, *res_ex;
}  fd_set_bits;           这个结构体保存了select在户用态的数参