std::mutex::try_lock (3) - Linux Manuals

std::mutex::try_lock: std::mutex::try_lock

NAME

std::mutex::try_lock - std::mutex::try_lock

Synopsis

bool try_lock(); (since C++11)

Tries to lock the mutex. Returns immediately. On successful lock acquisition returns true, otherwise returns false.
This function is allowed to fail spuriously and return false even if the mutex is not currently locked by any other thread.
If try_lock is called by a thread that already owns the mutex, the behavior is undefined.
Prior unlock() operation on the same mutex synchronizes-with (as defined in std::memory_order) this operation if it returns true. Note that prior lock() does not synchronize with this operation if it returns false.

Parameters

(none)

Return value

true if the lock was acquired successfully, otherwise false.

Exceptions

(none)

Example

// Run this code

  #include <chrono>
  #include <mutex>
  #include <thread>
  #include <iostream> // std::cout

  std::chrono::milliseconds interval(100);

  std::mutex mutex;
  int job_shared = 0; // both threads can modify 'job_shared',
      // mutex will protect this variable

  int job_exclusive = 0; // only one thread can modify 'job_exclusive'
      // no protection needed

  // this thread can modify both 'job_shared' and 'job_exclusive'
  void job_1()
  {
      std::this_thread::sleep_for(interval); // let 'job_2' take a lock

      while (true) {
          // try to lock mutex to modify 'job_shared'
          if (mutex.try_lock()) {
              std::cout << "job shared (" << job_shared << ")\n";
              mutex.unlock();
              return;
          } else {
              // can't get lock to modify 'job_shared'
              // but there is some other work to do
              ++job_exclusive;
              std::cout << "job exclusive (" << job_exclusive << ")\n";
              std::this_thread::sleep_for(interval);
          }
      }
  }

  // this thread can modify only 'job_shared'
  void job_2()
  {
      mutex.lock();
      std::this_thread::sleep_for(5 * interval);
      ++job_shared;
      mutex.unlock();
  }

  int main()
  {
      std::thread thread_1(job_1);
      std::thread thread_2(job_2);

      thread_1.join();
      thread_2.join();
  }

Possible output:

  job exclusive (1)
  job exclusive (2)
  job exclusive (3)
  job exclusive (4)
  job shared (1)