Producer-Consumer implemented with clone2 and semaphores in c++

Question

I am implementing a producer consumer with c++ using clone2() and semaphores but it has an unexpected behavior.

Here is the code:

#include <iostream>
#include <semaphore.h>
#include <unistd.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sched.h>
#include <errno.h>
#include <mutex>
#include <sys/wait.h>

using namespace std;

sem_t empty, full;
mutex mutexSem;

int var = 1;
int items[10];
int in = 0;
int out = 0;

static int produce(void * arg) {

  while (true) {
      sem_wait(&empty);
      mutexSem.lock();
      cout << "produce position: " << in << endl;
      items[in] = var++;
      in = (in+1) % 10;
      mutexSem.unlock();
      sem_post(&full);
    }

  }

  static int consume(void *arg) {

    while (true) {
      sem_wait(&full);
      mutexSem.lock();
      cout << "consume position: " << out << endl;
      items[out] = 0;
      out = (out + 1 ) % 10;
      mutexSem.unlock();
      sem_post(&empty);
    }

  }

int
main(int argc, char *argv[]) {

  int emptyResponse = sem_init(&empty, 0, 10);
  int fullResponse = sem_init(&full, 0, 0);
  void ** child_stack;

  child_stack = (void **) malloc(16384) + 16384 / sizeof(*child_stack);

  if (errno != 0 || emptyResponse != 0
     || fullResponse != 0) {
    cout << "errNo->" << errno << endl;
  }

  clone(produce, child_stack, CLONE_VM | CLONE_FILES , NULL);
  sleep(4);
  clone(consume, child_stack, CLONE_VM | CLONE_FILES, NULL);

  return 0;

}

I compile the code like

g++ -std=c++11 clone2.cpp -o clone2 -lpthread

And each time i run the code it produces a different result. I want to produce an infinite producer/consumer that each time an item is produced, then i will be consumed. I don't know why after producing 10 items, then it consumes 10 items and the process finish. I don't know also why i have to use a sleep(anyNumber) between the two threads, even though it won't do anything. Someone can tell me pleae if the FLAGS that i am setting are ok, and how this clone work.

This is an example of an output:

produce: 0

produce: 1

produce: 2

produce: 3

produce: 4

produce: 5

produce: 6

produce: 7

produce: 8

produce: 9

consume: 0

consume: 1

consume: 2

consume: 3

consume: 1515067019

consume: 5

consume: 6

consume: 7

consume: 8

consume: 9

Sometimes is like it, sometimes it has a segmentation fault, sometimes it just don't print any number so it will appear like

consume:

consume: 5

And so on...

NOTE: It's MANDATORY to use clone2() and i am using ubuntu 14.04 LTS , Intel core i7 64 bits

Thanks!

Answer 1

When you create your new "threads" you tell them to share the memory with the parent process. There are however two big problems with that:

1. The stack you create for the children is shared as well, so that both "threads" will have the same stack.
2. The parent process exits, and I'm not sure what happens with the allocated shared memory but it might be released.

Both these things leads to undefined behavior.

The first problem can be solved by having two stacks, one for each thread. The second problem can be solved by the parent process waiting for the children to exit.