死锁产生

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <fstream>
class LofFile{
public:
    LofFile();
    ~LofFile();
    void share_print(std::string id, int value);
    void share_print2(std::string id, int value);
private:
    std::mutex m_mutex;
    std::mutex m_mutex2;
    std::ofstream f;//f是被mutex所保护的资源
};
LofFile::LofFile(){
    f.open("log.txt");
}
LofFile::~LofFile(){
}
void LofFile::share_print(std::string id, int value){
    std::lock_guard<std::mutex> locker(m_mutex);
    std::lock_guard<std::mutex> locker2(m_mutex2);
    std::cout << "from" << id << ":" << value << std::endl;
}
void LofFile::share_print2(std::string id, int value) {
    std::lock_guard<std::mutex> locker2(m_mutex2);
    std::lock_guard<std::mutex> locker(m_mutex);
    std::cout << "from" << id << ":" << value << std::endl;
}
void function_1(LofFile& log) {
    for (int i = 0; i > -100; i--)
        log.share_print("From function_1", i);
}
int main() {
    LofFile log;
    std::thread t1(function_1,std::ref(log));
    for (int i = 0; i > -100; i--)
        log.share_print2("From main", i);
    t1.join();
    return 0;
}

• 当function_1线程锁住m_mutex
• main线程锁住m_mutex2
• function_1等待m_mutex2解锁
• main等待m_mutex解锁
• 导致死锁

死锁避免

• 评估是否需要两个以上的锁
• 避免调用mutex的同时调用其他的不熟悉的函数，有可能这个函数包含了其它的另外的锁
• 使用C++标准库提供的lock函数，可以确保多个mutex的顺序是相同的（部分极端场合，lock函数不能使用）

方法1：确保每一个locker顺序相同

方法2：C++提供机制

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <fstream>
class LofFile{
public:
    LofFile();
    ~LofFile();
    void share_print(std::string id, int value);
    void share_print2(std::string id, int value);
private:
    std::mutex m_mutex;
    std::mutex m_mutex2;
    std::ofstream f;//f是被mutex所保护的资源
};
LofFile::LofFile(){
    f.open("log.txt");
}
LofFile::~LofFile(){
}
void LofFile::share_print(std::string id, int value){
    //按顺序上锁
    std::lock(m_mutex, m_mutex2);
    //locker获取mutex的所有权，析构时解锁mutex
    std::lock_guard<std::mutex> locker(m_mutex,std::adopt_lock);
    std::lock_guard<std::mutex> locker2(m_mutex2, std::adopt_lock);
    std::cout << "from" << id << ":" << value << std::endl;
}
void LofFile::share_print2(std::string id, int value) {
    //按顺序上锁
    std::lock(m_mutex, m_mutex2);
    //locker获取mutex的所有权，析构时解锁mutex
    std::lock_guard<std::mutex> locker2(m_mutex2, std::adopt_lock);
    std::lock_guard<std::mutex> locker(m_mutex, std::adopt_lock);
    std::cout << "from" << id << ":" << value << std::endl;
}
void function_1(LofFile& log) {
    for (int i = 0; i > -100; i--)
        log.share_print("From function_1", i);
}
int main() {
    LofFile log;
    std::thread t1(function_1,std::ref(log));
    for (int i = 0; i > -100; i--)
        log.share_print2("From main", i);
    t1.join();
    return 0;
}