请给出互斥锁、读写锁以及自旋锁和递归锁的用法实例

1. 互斥锁

互斥锁用于保护共享资源，确保同一时刻只有一个线程能够访问该资源。实例代码如下：

#include <mutex>
#include <thread>
#include <iostream>

std::mutex mtx;

void print_func(int num) {
    mtx.lock();
    std::cout << "Thread " << num << " is printing." << std::endl;
    mtx.unlock();
}

int main() {
    std::thread t1(print_func, 1);
    std::thread t2(print_func, 2);
    t1.join();
    t2.join();
    return 0;
}

2. 读写锁

读写锁用于在读多写少的场景中提高效率，允许多个线程同时读取共享资源，但只允许一个线程写入共享资源。实例代码如下：

#include <iostream>
#include <thread>
#include <shared_mutex>

std::shared_mutex rw_mutex;
int shared_data = 0;

void read_func(int num) {
    rw_mutex.lock_shared();
    std::cout << "Thread " << num << " read shared_data: " << shared_data << std::endl;
    rw_mutex.unlock_shared();
}

void write_func(int num) {
    rw_mutex.lock();
    shared_data = num;
    std::cout << "Thread " << num << " wrote shared_data: " << shared_data << std::endl;
    rw_mutex.unlock();
}

int main() {
    std::thread t1(write_func, 1);
    std::thread t2(read_func, 2);
    std::thread t3(read_func, 3);
    t1.join();
    t2.join();
    t3.join();
    return 0;
}

3. 自旋锁

自旋锁适用于短时间内需要等待的场景，可以减少线程切换带来的开销。实例代码如下：

#include <iostream>
#include <thread>
#include <atomic>

std::atomic_flag spin_lock = ATOMIC_FLAG_INIT;
int shared_data = 0;

void add_func(int num) {
    while (spin_lock.test_and_set(std::memory_order_acquire));
    shared_data += num;
    spin_lock.clear(std::memory_order_release);
}

int main() {
    std::thread t1(add_func, 1);
    std::thread t2(add_func, 2);
    t1.join();
    t2.join();
    std::cout << "shared_data: " << shared_data << std::endl;
    return 0;
}

4. 递归锁

递归锁允许同一线程多次获取锁，避免死锁。实例代码如下：

#include <iostream>
#include <thread>
#include <mutex>

std::recursive_mutex mtx;
int shared_data = 0;

void add_func(int num) {
    mtx.lock();
    shared_data += num;
    if (num > 1) {
        add_func(num - 1);
    }
    mtx.unlock();
}

int main() {
    std::thread t1(add_func, 3);
    t1.join();
    std::cout << "shared_data: " << shared_data << std::endl;
    return 0;
}