boost:interprocess > managed_shared_memory > different values

Question

I would like to understand the behavior of the following code.

IDAInterface is a libary with a member "myValue".

C++:

#include <boost/interprocess/managed_shared_memory.hpp>
#include <cstdlib>

#include <idainterface.h>

IDAInterface ifIDA;

int main(int argc, char *argv[])
{
   using namespace boost::interprocess;
   typedef std::pair<IDAInterface, int> MyType; // [1]

   if(argc == 1){  //Parent process
      struct shm_remove{
         shm_remove() { shared_memory_object::remove("MySharedMemory"); }
         ~shm_remove(){ shared_memory_object::remove("MySharedMemory"); }
      } remover;
      ifIDA.myValue = 15;
      managed_shared_memory segment(create_only, "MySharedMemory", 65536);
      MyType *instance = segment.construct<MyType> ("MyType instance") (ifIDA, 0);
      std::string s(argv[0]); s += " child ";
      if(0 != std::system(s.c_str())) return 1;
      std::cout<<"\nPROZESS 1  "<< ifIDA.myValue;
      std::cout.flush();
      //std::cout<<"\nPROZESS 1  "<< instance->first.myValue;
      //std::cout.flush();
      //segment.destroy<MyType>("MyType instance");
      if(segment.find<MyType>("MyType instance").first) return 1;
   }
   else{
      managed_shared_memory segment(open_only, "MySharedMemory");
      std::pair<MyType*, managed_shared_memory::size_type> res;
      res = segment.find<MyType> ("MyType instance");
      if(res.second != 1) return 1;
      IDAInterface nIFIDA;
      nIFIDA = res.first->first;
      std::cout<<"\nPROZESS 2  "<< nIFIDA.myValue;
      std::cout.flush();
      nIFIDA.EineZahl = 10;
      std::cout<<"\nPROZESS 2  "<< nIFIDA.myValue;
      std::cout.flush();
      segment.destroy<MyType>("MyType instance");
   }
   return 0;
} 

The output:

PROZESS 2 15

PROZESS 2 10

PROZESS 1 15

PROZESS 1 15

As I understood should be the value in process 1, after running process 2, also 10. Why is in process 1 the value of "myValue" always 15? And how to get the modified value of "myValue" through process 2 in process 1?

Answer 1

I believe the basic understanding of Boost.Interprocess is correct, but the implementation is wrong. In this case, process 2:

• Constructs a local copy of IDAInterface from the shared IDAInterface instance. Changes to the local copy will not be observed by other processes.
• Modifies the wrong member variable. Process 1 is checking myValue, but Process 2 modifies EineZahl.

Another point to note is it that when using segment_manager::find(), the return value's first member variable should be checked for non-null to determine if the instance was found. In the case where the instance was not found, the second member variable will be 1.

---

Here is a complete example where Process 1 creates an integer in a shared memory segment, setting the value to 15. Process 1 will then spawn Process 2, who will attach to the shared memory segment, locate the integer, and change its value to 10. Once Process 2 exits, Process 1 prints the modified value.

#include <cstdlib>
#include <iostream>

#include <boost/interprocess/managed_shared_memory.hpp>

const char* segment_name = "MySharedMemory";
const char* instance_name = "MyType instance";
typedef int my_type;

int parent_main(const std::string& process)
{
  using namespace boost::interprocess;
  struct shm_remove {
     shm_remove() { shared_memory_object::remove(segment_name); }
     ~shm_remove(){ shared_memory_object::remove(segment_name); }
  } remover;

  // Create memory segment.
  managed_shared_memory segment(create_only, segment_name, 65536);

  // Create an instance of my_type with a value of 15 in the shared segment.
  my_type* instance = segment.construct<my_type>(instance_name)(15);

  // Print value before child.
  std::cout << "p1 - before child: " << *instance << std::endl;

  // Spawn child.
  std::string command = process + " child";
  if (0 != std::system(command.c_str())) return 1;

  // Child has exited, so print the shared instance value.
  std::cout << "p1 - after child: " << *instance << std::endl;
  return 0;
}

int child_main()
{
  using namespace boost::interprocess;
  // Attach to shared memory segment.
  managed_shared_memory segment(open_only, segment_name);

  // Find the my_type instance in the segment.
  my_type* instance = segment.find<my_type>(instance_name).first;

  // If the instance was not found, then return early.
  if (!instance) return 1;

  // Value before modifying (initial value set by parent).
  std::cout << "p2 - begin child: " << *instance << std::endl;

  // Modify and print value.
  *instance = 10;
  std::cout << "p2 - end child: " << *instance << std::endl;
  return 0;
}

int main(int argc, char *argv[])
{
  return (1 == argc) ? parent_main(argv[0]) : child_main();
}

Output:

p1 - before child: 15
p2 - begin child: 15
p2 - end child: 10
p1 - after child: 10