能否通過信號對其它進程裏的事件進行計數？--csapp 3.e homework 8.23

#include "../csapp.h"

volatile int counter = 0;

void handler(int sig)
{
        counter++;
        sleep(1);   //do some work in the handler
        return;
}

int main()
{
        int i;
        Signal(SIGUSR2, handler);
        if(Fork() == 0) {
                for(i =  0; i < 10; i++){
                        Kill(getppid(), SIGUSR2);
                        printf("sent SIGUSR2 to parent \n");
                }
                exit(0);
        }
        Wait(NULL);
        printf("counter = %d\n", counter);
        exit(0);
}

$ ./counterprob 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
sent SIGUSR2 to parent 
counter = 2

答案似乎是不能的，原因是信號是不排隊的，只能同時存在一個pending的信號，一個正在處理，後來的都會被cancel 掉

程序執行圖：

 SIGUSR2      SIGUSR2  SIGUSR2  SIGUSR2  SIGUSR2
      |                        |                 |                |               |
being handled   Pending  Canceld  Canceld  Canceld
 need 1 sec