UPDATE My other answer addresses scheduling resources among threads (after the question was clarified).
It shows both a semaphore approach to coordinate work among (many) workers, and a thread_pool
to limit workers in the first place and queue the work.
On linux (and perhaps other OSes?) you can use a lock file idiom (but it's not supported with some file-systems and old kernels).
I would suggest to use Interprocess synchronisation objects.
E.g., using a Boost Interprocess named semaphore:
#include <boost/interprocess/sync/named_semaphore.hpp>
#include <boost/thread.hpp>
#include <cassert>
int main()
{
using namespace boost::interprocess;
named_semaphore sem(open_or_create, "ffed38bd-f0fc-4f79-8838-5301c328268c", 0ul);
if (sem.try_wait())
{
std::cout << "Oops, second instance
";
}
else
{
sem.post();
// feign hard work for 30s
boost::this_thread::sleep_for(boost::chrono::seconds(30));
if (sem.try_wait())
{
sem.remove("ffed38bd-f0fc-4f79-8838-5301c328268c");
}
}
}
If you start one copy in the back ground, new copies will "refuse" to start ("Oops, second instance") for about 30s.
I have a feeling it might be easier to reverse the logic here. Mmm. Lemme try.
some time passes
Hehe. That was more tricky than I thought.
The thing is, you want to make sure that the lock doesn't remain when your application is interrupted or killed. In the interest of sharing the techniques for portably handling the signals:
#include <boost/interprocess/sync/named_semaphore.hpp>
#include <boost/thread.hpp>
#include <cassert>
#include <boost/asio.hpp>
#define MAX_PROCESS_INSTANCES 3
boost::interprocess::named_semaphore sem(
boost::interprocess::open_or_create,
"4de7ddfe-2bd5-428f-b74d-080970f980be",
MAX_PROCESS_INSTANCES);
// to handle signals:
boost::asio::io_service service;
boost::asio::signal_set sig(service);
int main()
{
if (sem.try_wait())
{
sig.add(SIGINT);
sig.add(SIGTERM);
sig.add(SIGABRT);
sig.async_wait([](boost::system::error_code,int sig){
std::cerr << "Exiting with signal " << sig << "...
";
sem.post();
});
boost::thread sig_listener([&] { service.run(); });
boost::this_thread::sleep_for(boost::chrono::seconds(3));
service.post([&] { sig.cancel(); });
sig_listener.join();
}
else
{
std::cout << "More than " << MAX_PROCESS_INSTANCES << " instances not allowed
";
}
}
There's a lot that could be explained there. Let me know if you're interested.
NOTE It should be quite obvious that if kill -9
is used on your application (forced termination) then all bets are off and you'll have to either remove the Name Semaphore object or explicitly unlock it (post()
).
Here's a testrun on my system:
sehe@desktop:/tmp$ (for a in {1..6}; do ./test& done; time wait)
More than 3 instances not allowed
More than 3 instances not allowed
More than 3 instances not allowed
Exiting with signal 0...
Exiting with signal 0...
Exiting with signal 0...
real 0m3.005s
user 0m0.013s
sys 0m0.012s