C语言实现任务调度与定时器

代码实现是在xl2tpd的源码中get到的，感觉很有意思的一段代码。基本功能就是实现定时器，时间到后从定时队列中取出，然后完成指定的任务。

1. schedule.c代码(自己添加了main函数，用来调试)

/*
 * Layer Two Tunnelling Protocol Daemon
 * Copyright (C) 1998 Adtran, Inc.
 * Copyright (C) 2002 Jeff McAdams
 *
 * Mark Spencer
 *
 * This software is distributed under the terms
 * of the GPL, which you should have received
 * along with this source.
 *
 * Scheduler code for time based functionality
 *
 */
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include "scheduler.h"

struct schedule_entry *events;
static struct timeval zero;
static sigset_t alarm;

/*
	init_scheduler配合schedule_lock，schedule_unlock使用，用来屏蔽当前进程中特定协议的处理
*/
void init_scheduler (void)/*初始化了两个不同的信号集*/
{
   
    struct sigaction act;
    act.sa_handler = alarm_handler;/*alarm信号执行体*/
#if defined (LINUX) && (__i386__)
    act.sa_restorer = NULL;
#endif
    act.sa_flags = 0;
    sigemptyset (&act.sa_mask);
    sigaddset (&act.sa_mask, SIGALRM);/*将SIGALRM信号添加到信号集sa_mask中，SIGALRM信号会阻塞*/
    sigaction (SIGALRM, &act, NULL);/*安装登记信号*/
    events = NULL;
    zero.tv_usec = 0;
    zero.tv_sec = 0;
    sigemptyset (&alarm);
    sigaddset (&alarm, SIGALRM);/*将SIGALRM信号添加到信号集alarm中，SIGALRM信号会阻塞*/
}

void alarm_handler (int signal)
{
   
    /* Check queue for events which should be
       executed right now.  Execute them, then
       see how long we should set the next timer
     */
    struct schedule_entry *p = events;
    struct timeval now;
    struct timeval then;
    struct itimerval itv;
    static int cnt = 0;
    cnt++;
    if (cnt != 1)
    {
   
        /* Whoa, we got called from within ourselves! */
        //log (LOG_DEBUG, "%s : Whoa... cnt = %d\n", __FUNCTION__, cnt);
        return;
    }
    while (events)
    {
   
        gettimeofday (&now, NULL);
        p = events;
        if (TVLESSEQ (p->tv, now))
        {
   
            events = events->next;
            /* This needs to be executed, as it has expired.
               It is expected that p->func will free p