#include <stdio.h>
#include <signal.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <asm/ptrace.h>
#include <asm/unistd.h>

static char stack[65536];

int child(void *arg)
{
  if(ptrace(PTRACE_TRACEME, 0, 0, 0) < 0){
    perror("ptrace");
    exit(1);
  }
  kill(getpid(), SIGSTOP);
  while(1){
    printf("getpid() returned %d\n", getpid());
    sleep(3);
  }
  return(0);
}

int main(int argc, char **argv)
{
  int pid, status, syscall;

  printf("Parent pid = %d\n", getpid());
  if((pid = clone(child, &stack[65532], SIGCHLD, NULL)) < 0){
    perror("clone");
    exit(1);
  }
  if((pid = waitpid(pid, &status, WUNTRACED)) < 0){
    perror("Waiting for stop");
    exit(1);
  }
  if(ptrace(PTRACE_SYSCALL, pid, 0, 0) < 0){
    perror("continuing");
    exit(1);
  }
  while(1){
    if((pid = waitpid(-1, &status, WUNTRACED)) <= 0){
      perror("wait");
      exit(1);
    }
    if(WIFSTOPPED(status) && (WSTOPSIG(status) == SIGTRAP)){
      syscall = ptrace(PTRACE_PEEKUSER, pid, 4 * ORIG_EAX, 0);
      if(syscall == __NR_getpid){
	if(ptrace(PTRACE_POKEUSER, pid, 4 * ORIG_EAX, __NR_getppid) < 0){
	  perror("ptrace");
	  exit(1);
	}
      }
      if(ptrace(PTRACE_SYSCALL, pid, 0, 0) < 0){
	perror("continuing");
	exit(1);
      }
    }
    else printf("wait failed - pid = %d, status = %d\n", pid, status);
  }
}