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_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

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 mutex is not
lock_shared available
(public member function)