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新增一节造轮子，封装 POSIX 接口编写 mq_b::thread
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Mq-b committed Aug 31, 2024
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# Linux 中封装 POSIX 接口编写 thread 类

在我们上一节内容其实就提到了 `std::thread` 的 msvc stl 的实现，本质上就是封装了 win32 的那些接口，包括其中最重要的就是利用了模板技术接受任意可调用类型，将其转发给 C 的只是接受单一函数指针的 `_beginthreadex` 去创建线程。

上一节中我们只是讲了单纯的讲了 “*模板包装C风格API进行调用*”，这一节我们就来实际一点，直接封装编写一个自己的 `std::thread`，使用 POSIX 接口。

我们将在 Ubuntu22.04 中使用 gcc11.4 开启 C++17 标准进行编写和测试。

## 实现

### 搭建框架

```cpp
namespace mq_b{
class thread{
public:
class id;

id get_id() const noexcept;
};

namespace this_thread {
[[nodiscard]] thread::id get_id() noexcept;
}

class thread::id {
public:
id() noexcept = default;

private:
explicit id(pthread_t other_id) noexcept : Id(other_id) {}

pthread_t Id;

friend thread::id thread::get_id() const noexcept;
friend thread::id this_thread::get_id() noexcept;
friend bool operator==(thread::id left, thread::id right) noexcept;

template <class Ch, class Tr>
friend std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& str, thread::id Id);
};

[[nodiscard]] inline thread::id thread::get_id() const noexcept {
return thread::id{ Id };
}

[[nodiscard]] inline thread::id this_thread::get_id() noexcept {
return thread::id{ pthread_self() };
}

template <class Ch, class Tr>
std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& str, thread::id Id){
str << Id.Id;
return str;
}
}
```

我们先搭建一个基础的架子给各位， 定义我们自己的命名空间，定义内部类 `thread::id`，以及运算符重载。这些并不涉及什么模板技术，我们先进行编写，其实重点只是构造函数而已，剩下的其它成员函数也很简单，我们一步一步来。

另外，**POSIX 的线程函数都只需要一个句柄，其实就是无符号 int 类型就能操作**，别名是 `pthread_t` 所以我们只需要保有这样一个 id 就好了。

### 实现构造函数

```cpp
template<typename Fn, typename ...Args>
thread(Fn&& func, Args&&... args) {
using Tuple = std::tuple<std::decay_t<Fn>, std::decay_t<Args>...>;
auto Decay_copied = std::make_unique<Tuple>(std::forward<Fn>(func), std::forward<Args>(args)...);
auto Invoker_proc = start<Tuple>(std::make_index_sequence<1 + sizeof...(Args)>{});
if (int result = pthread_create(&Id, nullptr, Invoker_proc, Decay_copied.get()); result == 0) {
(void)Decay_copied.release();
}else{
std::cerr << "Error creating thread: " << strerror(result) << std::endl;
throw std::runtime_error("Error creating thread");
}
}
template <typename Tuple, std::size_t... Indices>
static constexpr auto start(std::index_sequence<Indices...>) noexcept {
return &Invoke<Tuple, Indices...>;
}

template <class Tuple, std::size_t... Indices>
static void* Invoke(void* RawVals) noexcept {
const std::unique_ptr<Tuple> FnVals(static_cast<Tuple*>(RawVals));
Tuple& Tup = *FnVals.get();
std::invoke(std::move(std::get<Indices>(Tup))...);
nullptr 0;
}
```
这其实很简单，几乎是直接复制了我们上一节的内容，只是把函数改成了 `pthread_create`，然后多传了两个参数，以及修改了 Invoke 的返回类型和 return，确保类型符合 `pthread_create` 。
### 完善其它成员函数
然后再稍加完善那些简单的成员函数，也就是：
```cpp
~thread(){
if (joinable())
std::terminate();
}
thread(const thread&) = delete;
thread& operator=(const thread&) = delete;
thread(thread&& other) noexcept : Id(std::exchange(other.Id, {})) {}
thread& operator=(thread&& t) noexcept{
if (joinable())
std::terminate();
swap(t);
return *this;
}
void swap(thread& t) noexcept{
std::swap(Id, t.Id);
}
bool joinable() const noexcept{
return !(Id == 0);
}
void join() {
if (!joinable()) {
throw std::runtime_error("Thread is not joinable");
}
int result = pthread_join(Id, nullptr);
if (result != 0) {
throw std::runtime_error("Error joining thread: " + std::string(strerror(result)));
}
Id = {}; // Reset thread id
}
void detach() {
if (!joinable()) {
throw std::runtime_error("Thread is not joinable or already detached");
}
int result = pthread_detach(Id);
if (result != 0) {
throw std::runtime_error("Error detaching thread: " + std::string(strerror(result)));
}
Id = {}; // Reset thread id
}
id get_id() const noexcept;
native_handle_type native_handle() const{
return Id;
}
```

我觉得无需多言，这些都十分的简单。然后就完成了，对，就是这么简单。

### 完整代码与测试

**完整实现代码**：

```cpp
namespace mq_b{
class thread{
public:
class id;
using native_handle_type = pthread_t;

thread() noexcept :Id{} {}

template<typename Fn, typename ...Args>
thread(Fn&& func, Args&&... args) {
using Tuple = std::tuple<std::decay_t<Fn>, std::decay_t<Args>...>;
auto Decay_copied = std::make_unique<Tuple>(std::forward<Fn>(func), std::forward<Args>(args)...);
auto Invoker_proc = start<Tuple>(std::make_index_sequence<1 + sizeof...(Args)>{});
if (int result = pthread_create(&Id, nullptr, Invoker_proc, Decay_copied.get()); result == 0) {
(void)Decay_copied.release();
}else{
std::cerr << "Error creating thread: " << strerror(result) << std::endl;
throw std::runtime_error("Error creating thread");
}
}
template <typename Tuple, std::size_t... Indices>
static constexpr auto start(std::index_sequence<Indices...>) noexcept {
return &Invoke<Tuple, Indices...>;
}

template <class Tuple, std::size_t... Indices>
static void* Invoke(void* RawVals) noexcept {
const std::unique_ptr<Tuple> FnVals(static_cast<Tuple*>(RawVals));
Tuple& Tup = *FnVals.get();
std::invoke(std::move(std::get<Indices>(Tup))...);
return nullptr;
}

~thread(){
if (joinable())
std::terminate();
}

thread(const thread&) = delete;

thread& operator=(const thread&) = delete;

thread(thread&& other) noexcept : Id(std::exchange(other.Id, {})) {}

thread& operator=(thread&& t) noexcept{
if (joinable())
std::terminate();
swap(t);
return *this;
}

void swap(thread& t) noexcept{
std::swap(Id, t.Id);
}

bool joinable() const noexcept{
return !(Id == 0);
}

void join() {
if (!joinable()) {
throw std::runtime_error("Thread is not joinable");
}
int result = pthread_join(Id, nullptr);
if (result != 0) {
throw std::runtime_error("Error joining thread: " + std::string(strerror(result)));
}
Id = {}; // Reset thread id
}

void detach() {
if (!joinable()) {
throw std::runtime_error("Thread is not joinable or already detached");
}
int result = pthread_detach(Id);
if (result != 0) {
throw std::runtime_error("Error detaching thread: " + std::string(strerror(result)));
}
Id = {}; // Reset thread id
}

id get_id() const noexcept;

native_handle_type native_handle() const{
return Id;
}

pthread_t Id;
};

namespace this_thread {
[[nodiscard]] thread::id get_id() noexcept;
}

class thread::id {
public:
id() noexcept = default;

private:
explicit id(pthread_t other_id) noexcept : Id(other_id) {}

pthread_t Id;

friend thread::id thread::get_id() const noexcept;
friend thread::id this_thread::get_id() noexcept;
friend bool operator==(thread::id left, thread::id right) noexcept;

template <class Ch, class Tr>
friend std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& str, thread::id Id);
};

[[nodiscard]] inline thread::id thread::get_id() const noexcept {
return thread::id{ Id };
}

[[nodiscard]] inline thread::id this_thread::get_id() noexcept {
return thread::id{ pthread_self() };
}

template <class Ch, class Tr>
std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& str, thread::id Id){
str << Id.Id;
return str;
}
}
```
**标头**：
```cpp
#include <iostream>
#include <thread>
#include <functional>
#include <tuple>
#include <utility>
#include <cstring>
#include <pthread.h>
#include <unistd.h>
```

**测试**：

```cpp
void func(int& a) {
std::cout << &a << '\n';
}
void func2(const int& a){
std::cout << &a << '\n';
}
struct X{
void f() { std::cout << "X::f\n"; }
};

int main(){
std::cout << "main thread id: " << mq_b::this_thread::get_id() << '\n';

int a = 10;
std::cout << &a << '\n';
mq_b::thread t{ func,std::ref(a) };
t.join();

mq_b::thread t2{ func2,a };
t2.join();

mq_b::thread t3{ [] {std::cout << "thread id: " << mq_b::this_thread::get_id() << '\n'; } };
t3.join();

X x;
mq_b::thread t4{ &X::f,&x };
t4.join();

mq_b::thread{ [] {std::cout << "👉🤣\n"; } }.detach();
sleep(1);
}
```
> [运行](https://godbolt.org/z/1j48Mh89x)测试。
## 总结
其实这玩意没多少难度，唯一的难度就只有那个构造函数而已，剩下的代码和成员函数，甚至可以照着标准库抄一些，或者就是调用 POSIX 接口罢了。
不过如果各位能完全理解明白，那也足以自傲，毕竟的确没多少人懂。简单是相对而言的，如果你跟着视频一直学习了前面的模板，并且有基本的并发的知识，对 `POSIX` 接口有基本的认识，以及看了前面提到的 [**《`std::thread` 的构造-源码解析》**](https://mq-b.github.io/ModernCpp-ConcurrentProgramming-Tutorial/md/%E8%AF%A6%E7%BB%86%E5%88%86%E6%9E%90/01thread%E7%9A%84%E6%9E%84%E9%80%A0%E4%B8%8E%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90.html)，那么本节的内容对你，肯定不构成难度。

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