[置顶] std::mutex std::unique_lock std::lock_guard std::recursive_mutex的理解

std::mutex 称为互斥量；

std::unique_lock, std::lock_guard, std::recursive_mutex可以简单理解为对std::mutex的封装，且对互斥量的unlock是在对象（比如std::unique_lock对象）销毁时执行。

下面举两个例子，注意红色标注的部分：

Example 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

// mutex example #include <iostream> // std::cout #include <thread> // std::thread #include <mutex> // std::mutex std::mutex mtx; // mutex for critical section void print_block (int n, char c) { // critical section (exclusive access to std::cout signaled by locking mtx): mtx.lock(); for (int i=0; i<n; ++i) { std::cout << c; } std::cout << '\n'; mtx.unlock(); } int main () { std::thread th1 (print_block,50,'*'); std::thread th2 (print_block,50,'$'); th1.join(); th2.join(); return 0; }

Possible output (order of lines may vary, but characters are never mixed):

************************************************** $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Example 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

// unique_lock example #include <iostream> // std::cout #include <thread> // std::thread #include <mutex> // std::mutex, std::unique_lock std::mutex mtx; // mutex for critical section void print_block (int n, char c) { // critical section (exclusive access to std::cout signaled by lifetime of lck): std::unique_lock<std::mutex> lck (mtx); for (int i=0; i<n; ++i) { std::cout << c; } std::cout << '\n'; } int main () { std::thread th1 (print_block,50,'*'); std::thread th2 (print_block,50,'$'); th1.join(); th2.join(); return 0; }

Edit & Run

Possible output (order of lines may vary, but characters are never mixed):

************************************************** $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Example 3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

// lock_guard example #include <iostream> // std::cout #include <thread> // std::thread #include <mutex> // std::mutex, std::lock_guard #include <stdexcept> // std::logic_error std::mutex mtx; void print_even (int x) { if (x%2==0) std::cout << x << " is even\n"; else throw (std::logic_error("not even")); } void print_thread_id (int id) { try { // using a local lock_guard to lock mtx guarantees unlocking on destruction / exception: std::lock_guard<std::mutex> lck (mtx); print_even(id); } catch (std::logic_error&) { std::cout << "[exception caught]\n"; } } 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; }

Edit & Run

Possible output:

[exception caught] 2 is even [exception caught] 4 is even [exception caught] 6 is even [exception caught] 8 is even [exception caught] 10 is even