Child process becomes Defunct after fork and exec

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?

execlp failed. If execlp returns, it will return -1, and you need to check errno to figure out what the error was. My guess is that it can't find the 'child' binary. Is it in a directory in your PATH? — Miles Budnek
If execlp succeeds, it does not return – there is nothing there for it to return to. — molbdnilo

Paul Evans Paul Evans · Accepted Answer · 2019-08-18T11:03:15

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.

Child process becomes Defunct after fork and exec

2 Answers