Interprocess reader/writer lock with Boost - c++

Question

This thread is gold when it comes to explaining how to implement reader/writer locks with Boost. It seems relatively simple and I really love it but it also seems to be using a non-named lock and I need an interprocess solution (doesn't need to be portable, can be Windows-only).
Is there a way to have an interprocess shared_mutex? I see there is a named_mutex but I can't get it to work with shared_lock ot other locks.
Any pointers are appreciated.
[EDIT]
In the meantime, I have come across this thread which almost hits the nail on the head. I have two issues:
it doesn't show complete code (I am guessing I need to use named_upgradable_mutex but I am not quite sure) and
I don't like the answer for the modified "writer" which uses no off the shelf class that does unlocking in destructor but a sequence of 3 raw calls on the mutex.
Comments or good solutions are still welcome.

Answer 1

The Boost.Interprocess documentation describes the so-called upgradable mutexes it supports and the upgradable mutex operations for the two supported upgradable mutex types:
boost::interprocess::interprocess_upgradable_mutex, a non-recursive, anonymous upgradable mutex that can be placed in shared memory or memory mapped files.
boost::interprocess::named_upgradable_mutex, a non-recursive, named upgradable mutex.
EDIT: I believe this works:
#include <iostream>
#include <string>
#include <unistd.h>
#include <boost/scope_exit.hpp>
#include <boost/interprocess/mapped_region.hpp>
#include <boost/interprocess/shared_memory_object.hpp>
#include <boost/interprocess/sync/interprocess_upgradable_mutex.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/sharable_lock.hpp>
#include <boost/interprocess/sync/upgradable_lock.hpp>
// http://stackoverflow.com/questions/12439099/interprocess-reader-writer-lock-with-boost/
#define SHARED_MEMORY_NAME "SO12439099-MySharedMemory"
struct shared_data {
private:
typedef boost::interprocess::interprocess_upgradable_mutex upgradable_mutex_type;
mutable upgradable_mutex_type mutex;
volatile int counter;
public:
shared_data()
: counter(0)
{
}
int count() const {
boost::interprocess::sharable_lock<upgradable_mutex_type> lock(mutex);
return counter;
}
void set_counter(int counter) {
boost::interprocess::scoped_lock<upgradable_mutex_type> lock(mutex);
this->counter = counter;
}
};
int main(int argc, char *argv[])
{
using namespace boost::interprocess;
if (argc != 2) {
std::cerr << "Usage: " << argv[0] << " WHICH" << std::endl;
return 1;
}
const std::string which = argv[1];
if (which == "parent") {
shared_memory_object::remove(SHARED_MEMORY_NAME);
shared_memory_object shm(create_only, SHARED_MEMORY_NAME, read_write);
BOOST_SCOPE_EXIT(argc) {
shared_memory_object::remove(SHARED_MEMORY_NAME);
} BOOST_SCOPE_EXIT_END;
shm.truncate(sizeof (shared_data));
// Map the whole shared memory into this process.
mapped_region region(shm, read_write);
// Construct the shared_data.
new (region.get_address()) shared_data;
// Go to sleep for a minute.
sleep(60);
return 0;
} else if (which == "reader_child") {
shared_memory_object shm(open_only, SHARED_MEMORY_NAME, read_write);
mapped_region region(shm, read_write);
shared_data& d = *static_cast<shared_data *>(region.get_address());
for (int i = 0; i < 100000; ++i) {
std::cout << "reader_child: " << d.count() << std::endl;
}
} else if (which == "writer_child") {
shared_memory_object shm(open_only, SHARED_MEMORY_NAME, read_write);
mapped_region region(shm, read_write);
shared_data& d = *static_cast<shared_data *>(region.get_address());
for (int i = 0; i < 100000; ++i) {
d.set_counter(i);
std::cout << "writer_child: " << i << std::endl;
}
}
}
I tried this on a Mac with the following script:
#!/usr/bin/env sh
./a.out reader_child &
./a.out reader_child &
./a.out writer_child &
./a.out reader_child &
./a.out reader_child &
(You have to start the parent first: ./a.out parent)
The output showed interleaving of "reader_child" and "writer_child" lines (with all of the "reader_child" lines showing a non-zero value after the first "writer_child" line), so it appears to be working.