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Showing content from https://cplusplus.com/reference/mutex/unique_lock/lock/ below:

public member function

<mutex>

std::unique_lock::lock

Lock mutex

Calls member lock of the managed mutex object.

Calling lock on a mutex object that has already been locked by other threads causes the current thread to block (wait) until it can own a lock to it.

When the function returns, the object owns a lock on the mutex.

If the call to lock fails, a system_error exception is thrown.



Parameters none

Return value none

Example 
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// unique_lock::lock/unlock
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::unique_lock, std::defer_lock

std::mutex mtx;           // mutex for critical section

void print_thread_id (int id) {
  std::unique_lock<std::mutex> lck (mtx,std::defer_lock);
  // critical section (exclusive access to std::cout signaled by locking lck):
  lck.lock();
  std::cout << "thread #" << id << '\n';
  lck.unlock();
}

int main ()
{
  std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(print_thread_id,i+1);

  for (auto& th : threads) th.join();

  return 0;
}

Possible output (order of lines may vary, but they are never intermingled): 
thread #1
thread #2
thread #3
thread #4
thread #5
thread #6
thread #7
thread #8
thread #9
thread #10


Data races The unique_lock object is modified.
The managed mutex object is accessed and modified (as an atomic operation, causing no data races).

Exception safetyBasic guarantee: if an exception is thrown by this member function, the unique_lock object is left in a valid state.

If the call fails, a system_error exception is thrown:


An exception is also thrown if the call to lock on the managed mutex object fails, and on any other condition reported with such mechanism by the library implementation.

See also
unique_lock::try_lock
Lock mutex if not locked (public member function)
unique_lock::unlock
Unlock mutex (public member function)

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