理解SIGALRM信号

#include <unistd.h>
unsigned alarm(unsigned seconds);

int sys_alarm(long seconds)
{
    int old = current->alarm;
    // 算出原来的时间
    if (old)
        old = (old - jiffies) / HZ;
    /*
        系统每隔n秒触发一次时钟中断。jiffies是系统发生时钟中断的次数。
        1秒等于HZ个jiffies，这里就是算出jiffies数，即时钟中断发生多少次后触发SIGALRM信号
    */
    current->alarm = (seconds>0)?(jiffies+HZ*seconds):0;
    // 返回旧的
    return (old);
}

 */
void schedule(void)
{
    int i,next,c;
    struct task_struct ** p;

    // 遍历进程
    for(p = &LAST_TASK ; p > &FIRST_TASK ; --p)
        if (*p) {
            /*
                判断是否设置了alarm并且过期了
                alarm < jiffies说明过期了。设置alarm信号，清除设置标记
            */
            if ((*p)->alarm && (*p)->alarm < jiffies) {
                    (*p)->signal |= (1<<(SIGALRM-1));
                    (*p)->alarm = 0;
                }
        }
        ...
}

#include <unistd.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <signal.h> 

void handle_alarm() 
{ 
    exit(0); 
} 
int main(int argc, char *argv[]) 
{ 
    signal(SIGALRM, handle_alarm); 
    alarm(10); 
    while(1) {} 
}