为什么不能触发子进程中的信号处理程序?

问题描述

我的目的是为 fgets 创建一个 1 秒以内的时间.如果在 1 秒内没有收到任何输入，则程序终止.

My intention is to create a time out of 1 sec for fgets. If no input is received in 1 sec, then the program terminates.

我提出的设计是:父级为 SIGALRM 注册一个信号处理程序.然后它派生一个将触发 SIGALRM 的孩子，它继续调用 fgets.SIGALRM 将触发杀死父进程的处理程序.但是当我在 ubuntu 14.04 64 位上执行这个时，处理程序没有被触发，程序只是等待用户永远输入 fgets.

The design I come up with is:the parent registers a signal handler for SIGALRM. Then it forks a child which will trigger SIGALRM and it goes ahead and call fgets. The SIGALRM will trigger the handler which kills the parent process. But when I execute this on a ubuntu 14.04 64-bit, the handler is not triggered and the program just waits for user to input for fgets forever.

为什么会发生这种情况，我该如何解决?

Why would this happen and how can I fix this?

#include "csapp.h"

void handler() {
  printf("lazy man\n");
  kill(SIGKILL, getppid());
  exit(0);
}

int main() {
  char buf[100];
  signal(SIGALRM, handler);
  pid_t pid;
  if ((pid = fork()) == 0) {
    alarm(1);
  } else {
    fgets(buf, 100, stdin);
    printf("%s", buf);
  }
  return 0;
}

~

推荐答案

如果您花时间阅读的手册 kill()，你会发现你错过了参数的顺序.

If you will take time to read the manual of kill(), you will see that you miss the order of the arguments.

int kill(pid_t pid, int sig);

修复:

kill(getppid(), SIGKILL);

另外，您的孩子在发出信号之前终止他的执行，您必须在调用 alarm() 之后添加一个 sleep() 以使其等待:

Plus, your child terminate his execution before that the signal is raise you must add a sleep() after your call to alarm() to make it wait:

alarm(1);
sleep(2);

最后，printf() 在信号处理程序中是不安全的，安全功能列表.

Finally, printf() is not safe in a signal handler, list of safe function.

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