text: /* Select()-based ae.c module.
   *
   * Copyright (c) 2009-2012, Salvatore Sanfilippo <antirez at gmail dot com>
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
   *   * Redistributions of source code must retain the above copyright notice,
   *     this list of conditions and the following disclaimer.
   *   * Redistributions in binary form must reproduce the above copyright
   *     notice, this list of conditions and the following disclaimer in the
   *     documentation and/or other materials provided with the distribution.
   *   * Neither the name of Redis nor the names of its contributors may be used
   *     to endorse or promote products derived from this software without
   *     specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
   * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
   * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
   * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
   * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   * POSSIBILITY OF SUCH DAMAGE.
   */
  
  
  #include <sys/select.h>
  #include <string.h>
  
  typedef struct aeApiState {
      fd_set rfds, wfds;
      /* We need to have a copy of the fd sets as it's not safe to reuse
       * FD sets after select(). */
      fd_set _rfds, _wfds;
  } aeApiState;
  
  static int aeApiCreate(aeEventLoop *eventLoop) {
      aeApiState *state = zmalloc(sizeof(aeApiState));
  
      if (!state) return -1;
  
      // 每次都要清空 fd_set
      FD_ZERO(&state->rfds);
      FD_ZERO(&state->wfds);
      eventLoop->apidata = state;
      return 0;
  }
  
  static int aeApiResize(aeEventLoop *eventLoop, int setsize) {
      /* Just ensure we have enough room in the fd_set type. */
      if (setsize >= FD_SETSIZE) return -1;
      return 0;
  }
  
  static void aeApiFree(aeEventLoop *eventLoop) {
      zfree(eventLoop->apidata);
  }
  
  static int aeApiAddEvent(aeEventLoop *eventLoop, int fd, int mask) {
      aeApiState *state = eventLoop->apidata;
  
      if (mask & AE_READABLE) FD_SET(fd,&state->rfds);
      if (mask & AE_WRITABLE) FD_SET(fd,&state->wfds);
      return 0;
  }
  
  static void aeApiDelEvent(aeEventLoop *eventLoop, int fd, int mask) {
      aeApiState *state = eventLoop->apidata;
  
      if (mask & AE_READABLE) FD_CLR(fd,&state->rfds);
      if (mask & AE_WRITABLE) FD_CLR(fd,&state->wfds);
  }
  
  static int aeApiPoll(aeEventLoop *eventLoop, struct timeval *tvp) {
      aeApiState *state = eventLoop->apidata;
      int retval, j, numevents = 0;
  
      // 拷贝
      // 因为一次 select 会清空列表
      memcpy(&state->_rfds,&state->rfds,sizeof(fd_set));
      memcpy(&state->_wfds,&state->wfds,sizeof(fd_set));
  
      // 准备就绪的套接字个数
      retval = select(eventLoop->maxfd+1,
                  &state->_rfds,&state->_wfds,NULL,tvp);
      if (retval > 0) {
          for (j = 0; j <= eventLoop->maxfd; j++) {
              int mask = 0;
              aeFileEvent *fe = &eventLoop->events[j];
  
              if (fe->mask == AE_NONE) continue;
  
              // 调用 FD_ISSET 判断之前写入的 fd_set 中的 fd 是否设置，如果被设置说明是就绪的，可以执行对应的操作
              if (fe->mask & AE_READABLE && FD_ISSET(j,&state->_rfds))
                  mask |= AE_READABLE;
              if (fe->mask & AE_WRITABLE && FD_ISSET(j,&state->_wfds))
                  mask |= AE_WRITABLE;
              eventLoop->fired[numevents].fd = j;
              eventLoop->fired[numevents].mask = mask;
              numevents++;
          }
      }
      return numevents;
  }
  
  static char *aeApiName(void) {
      return "select";
  }
  