C++ 实现线程安全的map(OpenHarmony源码实现版)

概述

STL容器不是线程安全的。比如对于vector，即使写方（生产者）是单线程写入，但是并发读的时候，由于潜在的内存重新申请和对象复制问题，会导致读方（消费者）的迭代器失效。实际表现也就是招致了core dump。另外一种情况，如果是多个写方，并发的push_back()，也会导致core dump。但可以通过固定vector的大小（调用resize()）避免动态扩容（无push_back）来做到lock-free。

c++的map的并发操作也是不安全的，c++里边有红黑树实现的std::map和hash表 unordered_map。在《C++并发编程实战》一书中的162页提供了一个细粒度锁的MAP数据结构，使用了 boost的shared_mutex (C++14已经支持，C++11没有)，那上面的实现代码挺长的，这里给出个鸿蒙源码实现的safe_map，代码精简，值得学习。

接上篇欣赏了OpenHarmony源码中实现的ThreadPool的实现，链接在这里：

c++的ThreadPool,OpenHarmony源码实现版赏析和使用

源码实现

源码位置：code-v3.0-LTS\OpenHarmony\utils\native\base\include\safe_map.h

/* * Copyright (c) 2021 Huawei Device Co., Ltd. * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *     http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */#ifndef UTILS_BASE_SAFE_MAP_H#define UTILS_BASE_SAFE_MAP_H#include 

源码欣赏

使用模板语法template 让这个map的实现更通用。这是c++模板泛型的强大之处，不用针对每个类型都实现一遍，复用性更强。且模板是在编译期检查的，也降低的出错的可能性。内部实现上，倒是没啥特别的，就是对相应的操作加了锁。锁使用的RAII模型的std::lock_guard写法，这种很常见也很常用。

自定义实现了几个常用的操作方法如Find,Erase和Clear，每个里面的操作都相应的加了锁。操作符重载实现了[]和赋值=操作。注意这两处的地方没有用锁，你知道为什么吗？如果多个线程只访问容器但不更改其结构，则不需要对容器进行同步。另外一个原因是，对于map,如果只是通过[]的方式修改而不是新插入，则多线程下也不会core dump。

单元测试

源码中同样有safe_map的单元测试，单元测试框架使用的是google的gtest。看来gtest还是很强大的，华为也选择使用了它。以下给出源码，可以熟悉下gtest单元测试的用法。

/* * Copyright (c) 2021 Huawei Device Co., Ltd. * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *     http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */#include "safe_map.h"#include #include #include #include #include #include #include  // std::chrono::seconds#include  // std::cout#include  // std::thread, std::this_thread::sleep_forusing namespace testing::ext;using namespace OHOS;using namespace std;class UtilsSafeMap : public testing::Test {};/* * @tc.name: testUtilsCopyAndAssign001 * @tc.desc: single thread test the normal feature insert and erase and EnsureInsert */HWTEST_F(UtilsSafeMap, testUtilsCopyAndAssign001, TestSize.Level1){    SafeMap demoData;    // insert new    demoData.Insert("A", 1);    ASSERT_FALSE(demoData.IsEmpty());    ASSERT_EQ(demoData.Size(), 1);    SafeMap newdemo = demoData;    int tar = -1;    ASSERT_TRUE(newdemo.Find("A", tar));    ASSERT_EQ(1, tar);    tar = -1;    SafeMap newdemo2;    newdemo2 = demoData;    ASSERT_TRUE(newdemo2.Find("A", tar));    ASSERT_EQ(1, tar);}/* * @tc.name: testUtilsoperator001 * @tc.desc: SafeMap */HWTEST_F(UtilsSafeMap, testUtilsoperator001, TestSize.Level1){    SafeMap demoData;    // insert new    demoData.Insert("A", 1);    ASSERT_FALSE(demoData.IsEmpty());    ASSERT_EQ(demoData.Size(), 1);    ASSERT_EQ(demoData["A"], 1);    SafeMap newdemo = demoData;    ASSERT_EQ(newdemo["A"], 1);    int tar = -1;    newdemo["B"] = 6;    ASSERT_TRUE(newdemo.Find("B", tar));    ASSERT_EQ(6, tar);    SafeMap newdemo2;    newdemo2 = newdemo;    ASSERT_EQ(newdemo2["A"], 1);}/* * @tc.name: testUtilsNormalFeatureInsert001 * @tc.desc: SafeMap */HWTEST_F(UtilsSafeMap, testUtilsNormalFeatureInsert001, TestSize.Level1){    SafeMap demoData;    ASSERT_TRUE(demoData.IsEmpty());    // insert new    demoData.Insert("A", 1);    ASSERT_FALSE(demoData.IsEmpty());    ASSERT_EQ(demoData.Size(), 1);    // insert copy one should fail    ASSERT_FALSE(demoData.Insert("A", 2));    ASSERT_EQ(demoData.Size(), 1);}/* * @tc.name: testUtilsNormalFeatureEnsureInsert001 * @tc.desc: SafeMap */HWTEST_F(UtilsSafeMap, testUtilsNormalFeatureEnsureInsert001, TestSize.Level1){    SafeMap demoData;    ASSERT_TRUE(demoData.IsEmpty());    demoData.Insert("A", 1);    demoData.EnsureInsert("B", 2);    ASSERT_FALSE(demoData.IsEmpty());    ASSERT_EQ(demoData.Size(), 2);    // insert copy one and new one    demoData.EnsureInsert("B", 5);    demoData.EnsureInsert("C", 6);    ASSERT_EQ(demoData.Size(), 3);}/* * @tc.name: testUtilsNormalFeatureFind001 * @tc.desc: SafeMap */HWTEST_F(UtilsSafeMap, testUtilsNormalFeatureFind001, TestSize.Level1){    SafeMap demoData;    ASSERT_TRUE(demoData.IsEmpty());    demoData.Insert("A", 1);    demoData.Insert("B", 10000);    demoData.EnsureInsert("B", 2);    demoData.EnsureInsert("C", 6);    ASSERT_FALSE(demoData.IsEmpty());    ASSERT_EQ(demoData.Size(), 3);    int i = 0;    ASSERT_TRUE(demoData.Find("A", i));    ASSERT_EQ(i, 1);    ASSERT_TRUE(demoData.Find("B", i));    ASSERT_EQ(i, 2);    ASSERT_TRUE(demoData.Find("C", i));    ASSERT_EQ(i, 6);}/* * @tc.name: testUtilsNormalFeatureFindAndSet001 * @tc.desc: SafeMap */HWTEST_F(UtilsSafeMap, testUtilsNormalFeatureFindAndSet001, TestSize.Level1){    SafeMap demoData;    ASSERT_TRUE(demoData.IsEmpty());    demoData.Insert("A", 1);    demoData.EnsureInsert("B", 2);    int oldvalue = 0;    int newvalue = 3;    ASSERT_TRUE(demoData.FindOldAndSetNew("A", oldvalue, newvalue));    // old value    ASSERT_EQ(oldvalue, 1);    newvalue = 4;    ASSERT_TRUE(demoData.FindOldAndSetNew("B", oldvalue, newvalue));    // old value    ASSERT_EQ(oldvalue, 2);    int i = -1;    ASSERT_TRUE(demoData.Find("A", i));    // new value    ASSERT_EQ(i, 3);    ASSERT_TRUE(demoData.Find("B", i));    // new value    ASSERT_EQ(i, 4);}/* * @tc.name: testUtilsNormalFeatureEraseAndClear001 * @tc.desc: SafeMap */HWTEST_F(UtilsSafeMap, testUtilsNormalFeatureEraseAndClear001, TestSize.Level1){    SafeMap demoData;    ASSERT_TRUE(demoData.IsEmpty());    demoData.Insert("A", 1);    demoData.EnsureInsert("B", 2);    ASSERT_EQ(demoData.Size(), 2);    demoData.Erase("A");    ASSERT_EQ(demoData.Size(), 1);    demoData.Clear();    ASSERT_EQ(demoData.Size(), 0);}/* * @tc.name: testUtilsConcurrentWriteAndRead001 * @tc.desc: 100 threads test in writein to the same key of the map, while read at same time  and no throw */const int THREAD_NUM = 100;HWTEST_F(UtilsSafeMap, testUtilsConcurrentWriteAndRead001, TestSize.Level1){    SafeMap demoData;    std::thread threads[THREAD_NUM];    std::thread checkThread[THREAD_NUM];    ASSERT_NO_THROW({ auto lamfuncInsert = [](SafeMap& data, const string& key,     const int& value, const std::chrono::system_clock::time_point& absTime) {     std::this_thread::sleep_until(absTime);     data.EnsureInsert(key, value); }; auto lamfuncCheck = [](SafeMap& data, const string& key,     std::chrono::system_clock::time_point absTime) {     std::this_thread::sleep_until(absTime);     thread_local int i = -1;     data.Find(key, i); }; using std::chrono::system_clock; std::time_t timeT = system_clock::to_time_t(system_clock::now()); timeT += 2; string key("A"); for (int i = 0; i < THREAD_NUM; ++i) {     threads[i] = std::thread(lamfuncInsert, std::ref(demoData), key, i, system_clock::from_time_t(timeT));     checkThread[i] = std::thread(lamfuncCheck, std::ref(demoData), key, system_clock::from_time_t(timeT)); } std::this_thread::sleep_for(std::chrono::seconds(3)); for (auto& t : threads) {     t.join(); } for (auto& t : checkThread) {     t.join(); }    });}/* * @tc.name: testUtilsConcurrentWriteAndFind001 * @tc.desc: 100 threads test in writein to the corresponding key of the map, * while read at same time  and check the results */HWTEST_F(UtilsSafeMap, testUtilsConcurrentWriteAndFind001, TestSize.Level1){    SafeMap demoData;    std::thread threads[THREAD_NUM];    std::vector<std::future> vcfi;    ASSERT_NO_THROW({ auto lamfuncInsert = [](SafeMap& data, const string& key,     const int& value, const std::chrono::system_clock::time_point& absTime) {     std::this_thread::sleep_until(absTime);     data.EnsureInsert(key, value); }; auto lamfuncCheckLoop = [](SafeMap& data, const string& key,     std::chrono::system_clock::time_point absTime) {     std::this_thread::sleep_until(absTime);     thread_local int i = -1;     while (!data.Find(key, i)) {  std::this_thread::sleep_for(std::chrono::microseconds(10));     }     return i; }; using std::chrono::system_clock; std::time_t timeT = system_clock::to_time_t(system_clock::now()); timeT += 2; string key("A"); for (int i = 0; i < THREAD_NUM; ++i) {     threads[i] = std::thread(lamfuncInsert, std::ref(demoData),  key + std::to_string(i), i, system_clock::from_time_t(timeT));     vcfi.push_back(std::async(std::launch::async, lamfuncCheckLoop,  std::ref(demoData), key + std::to_string(i), system_clock::from_time_t(timeT))); } std::this_thread::sleep_for(std::chrono::seconds(4)); for (auto& t : threads) {     t.join(); } vector result; for (auto& t : vcfi) {     result.push_back(t.get()); } std::sort(result.begin(), result.end()); for (int i = 0; i < THREAD_NUM; ++i) {     ASSERT_EQ(i, result[i]); }    });}/* * @tc.name: testUtilsConcurrentWriteAndFindAndSet001 * @tc.desc: 100 threads test in writein to the corresponding key of the map, * while findandfix at same time  and check the results */HWTEST_F(UtilsSafeMap, testUtilsConcurrentWriteAndFindAndSet001, TestSize.Level1){    SafeMap demoData;    std::thread threads[THREAD_NUM];    std::vector<std::future> vcfi;    ASSERT_NO_THROW({ auto lamfuncInsert = [](SafeMap& data, const string& key,     const int& value, const std::chrono::system_clock::time_point& absTime) {     std::this_thread::sleep_until(absTime);     data.EnsureInsert(key, value); }; auto lamfuncCheckLoop = [](SafeMap& data, const string& key,     const int& newvalue, std::chrono::system_clock::time_point absTime) {     std::this_thread::sleep_until(absTime);     thread_local int i = -1;     while (!data.FindOldAndSetNew(key, i, newvalue)) {  std::this_thread::sleep_for(std::chrono::microseconds(10));     }     return i; }; using std::chrono::system_clock; std::time_t timeT = system_clock::to_time_t(system_clock::now()); timeT += 2; string key("A"); for (int i = 0; i < THREAD_NUM; ++i) {     threads[i] = std::thread(lamfuncInsert, std::ref(demoData),  key + std::to_string(i), i, system_clock::from_time_t(timeT));     vcfi.push_back(std::async(std::launch::async, lamfuncCheckLoop,  std::ref(demoData), key + std::to_string(i), i + 1, system_clock::from_time_t(timeT))); } std::this_thread::sleep_for(std::chrono::seconds(4)); for (auto& t : threads) {     t.join(); } vector result; for (auto& t : vcfi) {     result.push_back(t.get()); } std::sort(result.begin(), result.end()); for (int i = 0; i < THREAD_NUM; ++i) {     ASSERT_EQ(i, result[i]); } int t = 0; result.clear(); for (int i = 0; i < THREAD_NUM; ++i) {     t = -1;     ASSERT_TRUE(demoData.Find("A" + std::to_string(i), t));     result.push_back(t); } std::sort(result.begin(), result.end()); for (int i = 0; i < THREAD_NUM; ++i) {     ASSERT_EQ(i + 1, result[i]); }    });}

引用

C++线程安全map （低效率） | 码农家园

C++11:基于std::queue和std::mutex构建一个线程安全的队列_10km的博客-CSDN博客_std::queue 线程安全

C++11:基于std::unordered_map和共享锁构建线程安全的map_10km的博客-CSDN博客_c++ map 无锁

如何设计并实现一个线程安全的 Map ？(上篇) - JavaShuo

c++线程安全的map_clh01s的博客-CSDN博客_c++ 线程安全的map

C++ 20 线程安全的Map_学习好烦啊的博客-CSDN博客_c++ map 线程安全

c++ map 多线程_线程安全原理简析及HashMap多线程并发5种场景异常分析_sumilao的博客-CSDN博客

C++ STL容器如何解决线程安全的问题？ - 腾讯云开发者社区-腾讯云

RISC-V MCU中文社区_致力于RISC-V技术的推广，提供一个交流学习的开放平台