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- const README: rewrite the space-saving claim per issue #5 follow-ups (constant-expression const vs #define immediates; which const qualify as constant expressions) - const README: fix the class-section diagnosis per issue #338 (overload resolution picks the const overload; the restriction only surfaces once that overload is removed) - sizeof README: virtual-inheritance result is compiler-dependent (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246 - static README: new section on internal linkage / access restriction with anonymous-namespace comparison #142 - 10-day practice: full problem list table + fixed day links #136 - new design_pattern/singleton/singleton_default.hpp: reusable thread-safe CRTP singleton template, compile+concurrency tested #303
77 lines
2.0 KiB
Markdown
77 lines
2.0 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-concurrency/helgrind-concurrency-check/) 的文章，由站点同步脚本导出。
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# Helgrind 并发错误检测
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## C++那些事之helgrind并发编程检测
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通常我们在写多线程程序的时候很容易遇到两个问题：
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- 死锁了，不知道什么原因导致
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- 数据不一致，多个线程锁用错了
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那么有没有工具来检测这两种场景呢
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答案是有的，我们可以使用valgrind的helgrind工具检测这两个问题，为了使本文讲解的更加丝滑，引出了几个例子。
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**注：完整示例及修复示例已更新至星球。**
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## 死锁
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假设有两个线程互相持有对方的锁，此时我们可以模拟出死锁，例如：
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```cpp
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void thread_func1() {
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std::lock_guard<std::mutex> lock1(mutex1);
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std::this_thread::sleep_for(std::chrono::milliseconds(100));
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std::lock_guard<std::mutex> lock2(mutex2);
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}
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void thread_func2() {
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std::lock_guard<std::mutex> lock2(mutex2);
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std::this_thread::sleep_for(std::chrono::milliseconds(100));
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std::lock_guard<std::mutex> lock1(mutex1);
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}
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```
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helgrind使用的方法如下：
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```
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valgrind --tool=helgrind ./a.out
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```
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此时我们可以看到死锁的位置，例如这个示例中指出了两个线程在对应位置发生了死锁
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```
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thread_func2() (deadlock.cc:18)
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thread_func1() (deadlock.cc:12)
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```
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## 数据竞争
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假设有一个共享变量，我们在多线程对它增加10w次，这个结果是多少呢？
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很明显这里产生了多线程竞争全局变量的问题，最终的结果不一致，使用helgrind检测方法同上。
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```cpp
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int shared_var = 0;
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void increment() {
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for (int i = 0; i < 100000; ++i) {
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++shared_var;
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}
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}
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```
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此时，我们可以看到data race的行数是哪一行，然后去解决问题。
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最后，留几个问题吧：
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1.上面这两个有问题的代码，如何修复？
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2.除了上面工具，还有其他办法？
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> 本节完 |