std::mutex::lock

 

C++

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Threads thread (C++11) jthread (C++20) stop_token (C++20) stop_source (C++20) stop_callback (C++20) hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)     this_thread namespace get_id (C++11) yield (C++11) sleep_for (C++11) sleep_until (C++11) Atomic types atomic (C++11) atomic_ref (C++20) atomic_flag (C++11) Initialization of atomic types atomic_init (C++11)(deprecated in C++20) ATOMIC_VAR_INIT (C++11)(deprecated in C++20) ATOMIC_FLAG_INIT (C++11) Free functions for atomic operations atomic_storeatomic_store_explicit (C++11)(C++11) atomic_loadatomic_load_explicit (C++11)(C++11) atomic_exchangeatomic_exchange_explicit (C++11)(C++11) atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11) atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11) atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11) atomic_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20)

Free functions for atomic flags atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11) atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) atomic_flag_testatomic_flag_test_explicit (C++20)(C++20) atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20) atomic_flag_notify_one (C++20) atomic_flag_notify_all (C++20) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (C++11) Mutual exclusion mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17) timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14) Generic lock management lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11) lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11) Condition variables condition_variable (C++11) condition_variable_any (C++11) notify_all_at_thread_exit (C++11) cv_status (C++11) Semaphores counting_semaphorebinary_semaphore (C++20)(C++20) Latches and barriers latch (C++20) barrier (C++20) Futures promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11) launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

std::mutex

Member functions

mutex::mutex

mutex::~mutex

Locking

mutex::lock

mutex::try_lock

mutex::unlock

Native handle

mutex::native_handle

 

void lock();		(since C++11)

Locks the mutex. If another thread has already locked the mutex, a call to lock will block execution until the lock is acquired.

If lock is called by a thread that already owns the mutex, the behavior is undefined: for example, the program may deadlock. An implementation that can detect the invalid usage is encouraged to throw a std::system_error with error condition resource_deadlock_would_occur instead of deadlocking.

Prior unlock() operations on the same mutex synchronize-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

This example shows how lock and unlock can be used to protect shared data.

Run this code

#include <iostream>
#include <chrono>
#include <thread>
#include <mutex>
 
int g_num = 0;  // protected by g_num_mutex
std::mutex g_num_mutex;
 
void slow_increment(int id) 
{
    for (int i = 0; i < 3; ++i) {
        g_num_mutex.lock(); 
        ++g_num;
        // note, that the mutex also syncronizes the output
        std::cout << "id: " << id << ", g_num: " << g_num << '\n';
        g_num_mutex.unlock();
 
        std::this_thread::sleep_for(std::chrono::milliseconds(234));
    }
}
 
int main()
{
    std::thread t1{slow_increment, 0};
    std::thread t2{slow_increment, 1};
    t1.join();
    t2.join();
}

Possible output:

id: 0, g_num: 1
id: 1, g_num: 2
id: 1, g_num: 3
id: 0, g_num: 4
id: 0, g_num: 5
id: 1, g_num: 6

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)