std::shared_mutex::lock

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_mutex 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 specifications. 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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Member functions
shared_mutex::shared_mutex
shared_mutex::~shared_mutex
Exclusive locking
shared_mutex::lock
shared_mutex::try_lock
shared_mutex::unlock
Shared locking
shared_mutex::lock_shared
shared_mutex::try_lock_shared
shared_mutex::unlock_shared
Native handle
shared_mutex::native_handle

try_lock	tries to lock the mutex, returns if the mutex is not available (public member function)
unlock	unlocks the mutex (public member function)
lock_shared	locks the mutex for shared ownership, blocks if the mutex is not available (public member function)