text: #include "kernel/types.h"
  #include "kernel/signo.h"
  #include "user/user.h"
  
  static void handler(int signo) {
      wait(0);
      printf("handle child exit: wait\n");
  }
  
  int main() {
      char ch;
      sigaction(SIGCHLD, handler, sigrestorer);
  
      if (fork() == 0) {
          read(0, &ch, 1);
          exit(0);
      }
  
      while(1);
      exit(0);
  }
  