子进程在fork和exec之后变为Defunct

Question

I am learning fork and exec and creating multiple child processes using fork and execlp and all I do in the child process is let it sleep. Basically I just want all my child to be alive. But as soon as i start my monitor.cpp which creates processes all of the child exit immediately and they do defunct!

Monitor which forks multiple children

#include <iostream>
#include <thread>
#include <chrono>
#include <string>

#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char* argv[])
{
 for(size_t i=0; i<std::stoi(argv[1]) ; ++i)
 {
  int pid = fork();
  if(pid == 0)
  {
    execlp("child", "child", std::string(std::to_string(i)).c_str(), (char *)0);
    std::cout << "child exiting " << std::endl;
    exit(1);
  }
  else if(pid > 0)
  {
   std::cout <<"child started with " << pid << std::endl;
  }
  else
  {
   std::cout << "fork failed" << std::endl;
  }
 }

 while(true)
 {
  std::this_thread::sleep_for(std::chrono::seconds(100000));
 }

 return 0;
}

Child Code

#include <iostream>
#include <thread>
#include <chrono>

int main(int argc, char* argv[])
{
  std::cout << " child started with id " << argv[1] << std::endl;

  std::cout <<"child sleeping " << argv[1] << std::endl;
  std::this_thread::sleep_for(std::chrono::seconds(1000));

  std::cout << "child exiting " << argv[1] << std::endl;
  return 0;
}

Output:

 child started with 1834 child started with 1835 child exiting child started with 1836 child exiting child started with 1837 child started with 1838 child started with 1839 child exiting child started with 1840 child started with 1841 child exiting child started with 1842 child started with 1843 child exiting child exiting child exiting child exiting child exiting child exiting 

ps -ef shows all of my child processes as Defunct even though my parent is still alive.

Can you please explain what am I missing?

Answer 1

From the 'execlp' man page:

The exec() functions only return if an error has occurred. The return value is -1, and errno is set to indicate the error.

Since "child exiting" is being printed in two places, it's not obvious if it's exiting. You need to check it's return value and errno .

Answer 2

You need to reap the child-process as they exit. This is done using wait or waitpid calls.

Until the parent has done this, they will be visible as defunc / zombie processes. (init, process 1, is responsible for reaping all process that do not have a parent after they exit)