# std::shared_mutex::lock_shared

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< cpp‎ | thread‎ | shared mutex

void lock_shared(); |  | (since C++17)

Acquires shared ownership of the mutex. If another thread is holding the mutex in exclusive ownership, a call to `lock_shared` will block execution until shared ownership can be acquired.

If `lock_shared` is called by a thread that already owns the `mutex` in any mode (exclusive or shared), the behavior is undefined.

If more than the implementation-defined maximum number of shared owners already locked the mutex in shared mode, `lock_shared` blocks execution until the number of shared owners is reduced. The maximum number of owners is guaranteed to be at least 10000.

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 specifications. The mutex is not locked in the case of any exception being thrown.

### Notes

`lock_shared()` is usually not called directly: std::shared_lock is used to manage shared 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 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);
                    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
            {
                {
                    smtx.lock_shared(); // better to use: std::shared_lock lock(smtx);
                    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

lock | locks the mutex, blocks if the mutex is not available (public member function)
try_lock_shared | tries to lock the mutex for shared ownership, returns if the mutex is not available (public member function)
unlock_shared | unlocks the mutex (shared ownership) (public member function)

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