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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../thread/shared_mutex/lock_shared.html below:

std::shared_mutex::lock_shared - cppreference.com

void lock_shared();

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.

(none)

(none)

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.

lock_shared() is usually not called directly: std::shared_lock is used to manage shared locking.

#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

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