54 lines
1.6 KiB
C++
54 lines
1.6 KiB
C++
#include <cstdio>
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#include <memory>
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#include <thread>
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unsigned const max_hazard_pointers = 100;
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struct hazard_pointer {
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std::atomic<std::thread::id> id;
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std::atomic<void*> pointer;
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};
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hazard_pointer hazard_pointers[max_hazard_pointers];
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class hp_owner {
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hazard_pointer* hp;
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public:
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hp_owner(hp_owner const&) = delete;
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hp_owner operator=(hp_owner const&) = delete;
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hp_owner() : hp(nullptr)
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{
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for (unsigned i = 0; i < max_hazard_pointers; ++i) {
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std::thread::id old_id;
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// try to claim ownership of a hazard pointer
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if (hazard_pointers[i].id.compare_exchange_strong(
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old_id, std::this_thread::get_id())) {
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// successfully claimed the entry for the current thread,
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// store it and stop the search
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hp = &hazard_pointers[i];
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printf("hp: %ld\n", hp);
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break;
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}
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// another threads owns this entry, move on to the next
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}
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// if you get to the end of the list without finding a free entry,
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// there are too many threads using hazard pointers, so throw an
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// exception
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if (!hp) {
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throw std::runtime_error("No hazard pointers available");
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}
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}
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std::atomic<void*>& get_pointer() { return hp->pointer; }
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~hp_owner()
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{
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// when each thread exits, if an instance of hp_owner was created
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// for the thread, then it's destryoed. The destructor then resets
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// the actual pointer to nullptr before setting the owner ID to
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// std::thread::id(), allowing another thread to reuse the entry later.
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// TODO: this causes crash
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// hp->pointer.store(nullptr);
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// hp->id.store(std::thread::id());
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}
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};
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