Boost.Asio深度解析：核心技术与网络编程

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Boost.Asio 是一个强大的 C++ 库，专为网络和系统级I/O编程设计，提供了一套高效、异步和事件驱动的API，使得在没有阻塞线程的情况下实现并发处理成为可能。本PDF教程由官方网站内容整理，旨在帮助读者深入了解和掌握Boost.Asio的核心概念、功能和用法。 1. **基本概念与功能**: - 提供了诸如`USINGBOOST.ASIO`这样的宏，简化了库的使用。核心功能包括异步I/O操作，如网络通信、文件I/O、定时器和信号处理等。 2. **设计模式与并发**: - **Proactor设计模式**：基于事件循环而非线程，允许在不创建或管理大量线程的情况下进行并发操作。 - **Strands**：一种用于管理异步操作的工具，避免了线程锁的繁琐管理，提高代码的执行效率。 3. **内存管理与并发策略**: - **缓冲区管理**：通过高效的缓冲机制支持流式数据传输。 - **流处理与异常处理**：处理短读和短写问题，确保数据的一致性和完整性。 - **并发提示**：提供工具帮助开发者合理安排任务并优化性能。 4. **高级特性**: - **协程（Coroutines）**：包括栈上和栈下两种实现方式，以及C++2011标准支持下的特性。 - **网络编程**： - **TCP、UDP和ICMP**：基础网络协议的支持。 - **其他协议支持**：如SSL/TLS、对象柄（Object HANDLEs）、UNIX域套接字和流式文件描述符。 - **特定平台支持**：针对Windows的特定功能，如HANDELS、随机访问HANDELS和Windows信号处理。 5. **现代C++特性**: - **C++11及后续版本**：库兼容新版本的C++语言特性，如可移动对象（Move Semantics）、可移动处理器（Move Handlers）、变参模板、数组容器和原子操作等。 - **智能指针**：通过`shared_ptr`和其他智能指针类型处理内存管理和所有权。通过学习和实践这份PDF教程，读者可以掌握Boost.Asio的强大功能，并将其应用于高效的C++网络和系统编程中。理解这些核心概念和功能有助于开发者编写出高并发、低开销的代码，适应不断变化的软件需求。

 A library user may need to perform initialisation in a thread shortly after the thread

starts and before any other application code is executed. For example, users of

Microsoft's COM must call CoInitializeEx before any other COM operations can be

called from that thread.

 The library interface is decoupled from interfaces for thread creation and

management, and permits implementations on platforms where threads are not

available.

Strands: Use Threads Without Explicit Locking

A strand is defined as a strictly sequential invocation of event handlers (i.e. no concurrent

invocation). Use of strands allows execution of code in a multithreaded program without the

need for explicit locking (e.g. using mutexes).

Strands may be either implicit or explicit, as illustrated by the following alternative

approaches:

 Calling io_context::run() from only one thread means all event handlers execute in an

implicit strand, due to the io_context's guarantee that handlers are only invoked from

inside run().

 Where there is a single chain of asynchronous operations associated with a

connection (e.g. in a half duplex protocol implementation like HTTP) there is no

possibility of concurrent execution of the handlers. This is an implicit strand.

 An explicit strand is an instance of strand<> or io_context::strand. All event

handler function objects need to be bound to the strand

using boost::asio::bind_executor() or otherwise posted/dispatched through

the strand object.

In the case of composed asynchronous operations, such

as async_read() or async_read_until(), if a completion handler goes through a

strand, then all intermediate handlers should also go through the same strand. This is needed

to ensure thread safe access for any objects that are shared between the caller and the

composed operation (in the case of async_read() it's the socket, which the caller

can close() to cancel the operation).

To achieve this, all asynchronous operations obtain the handler's associated executor by

using the get_associated_executor function. For example:

boost::asio::associated_executor_t<Handler> a =

boost::asio::get_associated_executor(h);

my_socket.async_read_some(my_buffer,

boost::asio::bind_executor(my_strand,

[](error_code ec, size_t length)

{

// ...

}));

Buffers

Fundamentally, I/O involves the transfer of data to and from contiguous regions of memory,

called buffers. These buffers can be simply expressed as a tuple consisting of a pointer and a

size in bytes. However, to allow the development of efficient network applications, Boost.Asio

includes support for scatter-gather operations. These operations involve one or more buffers:

 A scatter-read receives data into multiple buffers.

 A gather-write transmits multiple buffers.

Therefore we require an abstraction to represent a collection of buffers. The approach used in

Boost.Asio is to define a type (actually two types) to represent a single buffer. These can be

stored in a container, which may be passed to the scatter-gather operations.

In addition to specifying buffers as a pointer and size in bytes, Boost.Asio makes a distinction

between modifiable memory (called mutable) and non-modifiable memory (where the latter is

created from the storage for a const-qualified variable). These two types could therefore be

defined as follows:

typedef std::pair<void*, std::size_t> mutable_buffer;

typedef std::pair<const void*, std::size_t> const_buffer;

Here, a mutable_buffer would be convertible to a const_buffer, but conversion in the opposite

direction is not valid.

However, Boost.Asio does not use the above definitions as-is, but instead defines two

classes: mutable_buffer and const_buffer. The goal of these is to provide an opaque

representation of contiguous memory, where:

 Types behave as std::pair would in conversions. That is, a mutable_buffer is

convertible to a const_buffer, but the opposite conversion is disallowed.

 There is protection against buffer overruns. Given a buffer instance, a user can only

create another buffer representing the same range of memory or a sub-range of it. To

provide further safety, the library also includes mechanisms for automatically

determining the size of a buffer from an array, boost::arrayor std::vector of POD

elements, or from a std::string.

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