How get list of named objects in boost interprocess shared memory

Question

According to guide on Boost docs

It is possible to create named vectors of some type (for instance double)

  using namespace boost::interprocess;
  typedef allocator<int, managed_shared_memory::segment_manager>  ShmemAllocator;
  managed_shared_memory segment(create_only, "MySharedMemory", 65536);

  //Initialize shared memory STL-compatible allocator
  const ShmemAllocator alloc_inst (segment.get_segment_manager());

  //Construct a vector named "values_A" in shared memory with argument alloc_inst
  MyVector *vA= segment.construct<MyVector>("values_A")(alloc_inst);
  MyVector *vB= segment.construct<MyVector>("values_B")(alloc_inst);

  for(int i = 0; i < 100; ++i)  //Insert data in the vector
     vA->push_back(i);

And then if client process knows the names of shared objects ("values_A" and "values_B") it is easy to access them.

  managed_shared_memory segment(open_only, "MySharedMemory");
  MyVector *vA_client = segment.find<MyVector>("values_A").first;
  MyVector *vB_client = segment.find<MyVector>("values_B").first;

  //Use vector in reverse order
  std::sort(vA_client->rbegin(), vA_client->rend());

But if client doesn't know the names of those objects?

How to get list of those objects? {"values_A" , "values_B"}.

If there was some objects of other type ("MyVector2") registered (named "intA", "intB") - How would you filter only those, whose type is MyVector?

I have a suspicion that it has to do with method named_begin and named_end but I don't know how to use it.

Thank You for your help :)

score 2 · Answer 1 · answered Dec 06 '17 at 16:55

These methods are part of the segment manager interface.

Beware of race conditions when using these directly. find_or_construct does something else than find and construct sequentially in a multi-process situation.

Example:

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#include <boost/interprocess/managed_mapped_file.hpp>
#include <boost/interprocess/containers/vector.hpp>
#include <iostream>
namespace bip = boost::interprocess;

using Shared = bip::managed_mapped_file;
using Manager = Shared::segment_manager;
template <typename T> using Alloc = bip::allocator<T, Manager>;
template <typename T> using V = boost::container::vector<T, Alloc<T> >;

int main() {
    bip::managed_mapped_file mmf(bip::open_or_create, "test.bin", 10ull<<10);
    auto* segment = mmf.get_segment_manager();

    for (auto it = segment->named_begin(); it != segment->named_end(); ++it) {
        std::cout << "before: " << std::string(it->name(), it->name_length()) << "\n";
    }

    for (auto next : { "foo", "bar", "qux" }) {
        if (!mmf.find<V<int> >(next).first)
        {
            mmf.find_or_construct<V<int> >(next)(segment);
            break;
        }
    }

    for (auto it = segment->named_begin(); it != segment->named_end(); ++it) {
        std::cout << "after:  " << std::string(it->name(), it->name_length()) << "\n";
    }
}

Prints (first run):

after:  foo

Second run:

before: foo
after:  bar
after:  foo

Third run:

before: bar
before: foo
after:  bar
after:  foo
after:  qux

Fourth (and later) run:

before: bar
before: foo
before: qux
after:  bar
after:  foo
after:  qux

How get list of named objects in boost interprocess shared memory

1 Answers1