c++ Synchronize shared memory when reading

Question

I want to synchronize a shared_memory_object for reading iff the shared_memory_object already exists. This is the code I am using for synchronization with a bool variable.

boost::interprocess::shared_memory_object my_shared_mat;
bool mat_ready = true;
while (mat_ready)
{
    try {
        my_shared_mat = boost::interprocess::shared_memory_object(
            boost::interprocess::open_only,  // only open
            "shared_mat",                    // name
            boost::interprocess::read_only); // read-only mode
        mat_ready = false;
    }
    catch (boost::interprocess::interprocess_exception &ex) {
        std::cout << ex.what() << std::endl;
        mat_ready = true;
    }
}

boost::interprocess::mapped_region region(my_shared_mat, boost::interprocess::read_only);

If the shared memory exists, I do not have any problems, the data is shared among the processes, but if the shared memory is not present the program crashes at the mapped_region call.

Answer 1

If you want to synchronize with another process, use a synchronization primitive: condition variable with a mutex:

#include <boost/interprocess/shared_memory_object.hpp>
#include <boost/interprocess/mapped_region.hpp>
#include <boost/interprocess/sync/named_mutex.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/named_condition.hpp>
#include <iostream>

namespace bip   = boost::interprocess;
using Mutex     = bip::named_mutex;
using Condition = bip::named_condition;

int main(int argc, char**) {
    Mutex     mx(bip::open_or_create, "shared_mat_mx");
    Condition cv(bip::open_or_create, "shared_mat_cv");

    if (argc>1) { // server
        auto mat = bip::shared_memory_object(bip::create_only, "shared_mat", bip::read_write);
        mat.truncate(10 << 10); // 10kb

        bip::mapped_region region(mat, bip::read_only);

        {
            bip::scoped_lock<Mutex> lk(mx);
            cv.notify_all(); // notify all clients we're there
        }
    } else {
        {
            bip::scoped_lock<Mutex> lk(mx);
            cv.wait(lk); // wait for server signal
        }
        auto mat = bip::shared_memory_object(bip::open_only, "shared_mat", bip::read_only);
        bip::mapped_region region(mat, bip::read_only);

        std::cout << "Mapped the region of size " << region.get_size() << "\n";
    }
}

Running a number of clients in the background:

for a in {1..10}; do ./sotest& done

Makes them all wait. Starting a server:

./sotest server

Makes them all progress, and they show:

Mapped the region of size 10240