sigaction()中第二个结构(*oldact)的用法是什么

int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);

这是我的密码

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

void CustomHandler(int signo)
{
    printf("Inside custom handler");
    switch(signo)
    {
    case SIGFPE:
        printf("ERROR: Illegal arithmatic operation.\n");
        break;

    }

    exit(signo);
}

void newCustomHandler(int signo)
{
    printf("Inside new custom handler");
    switch(signo)
    {
    case SIGINT:
        printf("ERROR: Illegal arithmatic operation.\n");
        break;

    }

    exit(signo);
}

int main(void)
{
    long value;
    int i;
    struct sigaction act = {CustomHandler};
    struct sigaction newact = {newCustomHandler};


    newact = act;
    sigaction(SIGINT, &newact, NULL); //whats the difference between this

    /*sigaction(SIGINT, &act, NULL); // and this?
    sigaction(SIGINT, NULL, &newact);*/


    for(i = 0; i < 5; i++)
    {
        printf("Value: ");
        scanf("%ld", &value);
        printf("Result = %ld\n", 2520 / value);
    }
}

现在,当我运行程序并按Ctrl+c时,它显示在自定义处理程序中。

我读过sigaction的文档,上面写着

如果act为非null,则 如果oldact不为空,则前一个 动作保存在旧动作中。

当我可以直接赋值时,为什么需要传递第二个结构

newact = act

谢谢。