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std::shared_mutex::lock(3) C++ Standard Libary std::shared_mutex::lock(3) NAME std::shared_mutex::lock - std::shared_mutex::lock Synopsis void lock(); (since C++17) Acquires an exclusive ownership of the shared_mutex. If another thread is holding an exclusive lock or a shared lock on the same shared_mutex the a call to lock will block execution until all such locks are released. While shared_mu- tex is locked in an exclusive mode, no other lock of any kind can also be held. If lock is called by a thread that already owns the shared_mutex in any mode (exclusive or shared), the behavior is undefined. A prior unlock() operation on the same mutex synchronizes-with (as defined in std::memory_order) this operation. Parameters (none) Return value (none) Exceptions Throws std::system_error when errors occur, including errors from the underlying operating system that would prevent lock from meeting its specifica- tions. The mutex is not locked in the case of any exception being thrown. Notes lock() is usually not called directly: std::unique_lock, std::scoped_lock, and std::lock_guard are used to manage exclusive locking. Example // Run this code #include <chrono> #include <iostream> #include <mutex> #include <shared_mutex> #include <syncstream> #include <thread> #include <vector> std::mutex stream_mutx; void print(auto const& v) { std::unique_lock<std::mutex> lock(stream_mutx); std::cout << std::this_thread::get_id() << " saw: "; for (auto e : v) std::cout << e << ' '; std::cout << '\n'; } int main() { using namespace std::chrono_literals; constexpr int N_READERS = 5; constexpr int LAST = -999; std::shared_mutex smtx; int product = 0; auto writer = [&smtx, &product](int start, int end) { for (int i = start; i < end; ++i) { auto data = i; { std::unique_lock<std::shared_mutex> lock(smtx); // better than: // smtx.lock(); product = data; } std::this_thread::sleep_for(3ms); } smtx.lock(); // lock manually product = LAST; smtx.unlock(); }; auto reader = [&smtx, &product] { int data = 0; std::vector<int> seen; do { { // better to use: std::shared_lock lock(smtx); // smtx.lock_shared(); data = product; } // smtx.unlock_shared(); seen.push_back(data); std::this_thread::sleep_for(2ms); } while (data != LAST); print(seen); }; std::vector<std::thread> threads; threads.emplace_back(writer, 1, 13); threads.emplace_back(writer, 42, 52); for (int i = 0; i < N_READERS; ++i) threads.emplace_back(reader); for (auto&& t : threads) t.join(); } Possible output: 127755840 saw: 43 3 3 4 46 5 6 7 7 8 9 51 10 11 11 12 -999 144541248 saw: 2 44 3 4 46 5 6 7 7 8 9 51 10 11 11 12 -999 110970432 saw: 42 2 3 45 4 5 47 6 7 8 8 9 10 11 11 12 -999 119363136 saw: 42 2 3 4 46 5 6 7 7 8 9 9 10 11 11 12 12 -999 136148544 saw: 2 44 3 4 46 5 6 48 7 8 9 51 10 11 11 12 12 -999 See also try_lock tries to lock the mutex, returns if the mutex is not available (public member function) unlock unlocks the mutex (public member function) locks the mutex for shared ownership, blocks if the mu- tex is not lock_shared available (public member function) http://cppreference.com 2024.06.10 std::shared_mutex::lock(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Notes | Example | Possible output: | See also
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