void lock(); | (since C++14) |
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
in any mode (shared or exclusive), the behavior is undefined.
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
and std::lock_guard
are used to manage exclusive locking.
Shared mutexes do not support direct transition from shared to unique ownership mode: the shared lock has to be relinquished with unlock_shared()
before exclusive ownership may be obtained with lock()
. boost::upgrade_mutex may be used for this purpose.
Example
This example shows how lock
and unlock
can be used to protect shared data.
#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; std::cout << id << " => " << g_num << '\n'; g_num_mutex.unlock(); std::this_thread::sleep_for(std::chrono::seconds(1)); } } int main() { std::thread t1(slow_increment, 0); std::thread t2(slow_increment, 1); t1.join(); t2.join(); }
Possible output:
0 => 1 1 => 2 0 => 3 1 => 4 0 => 5 1 => 6
See also
tries to lock the mutex, returns if the mutex is not available (public member function) | |
unlocks the mutex (public member function) |
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