Linux环境下C++如何进行异步I/O操作

在Linux环境下，C++可以通过多种方式实现异步I/O操作。以下是一些常用的方法：

1. 使用libaio

libaio是Linux提供的异步I/O库，可以用于执行异步读写操作。

示例代码：

#include <
    iostream>
    
#include <
    fcntl.h>
    
#include <
    unistd.h>
    
#include <
    libaio.h>


void async_read(int fd, off_t offset, size_t length) {
    
    io_context_t ctx = 0;
    
    io_setup(1, &
    ctx);
    

    struct iocb cb;
    
    io_prep_pread(&
    cb, fd, nullptr, length, offset);
    
    cb.data = (void*)1;
    

    if (io_submit(ctx, 1, &
cb) != 1) {
    
        std::cerr <
    <
     "io_submit failed" <
    <
     std::endl;
    
        return;

    }
    

    struct io_event events[1];
    
    while (io_getevents(ctx, 1, 1, events, nullptr) >
 0) {
    
        if (events[0].res <
 0) {
    
            std::cerr <
    <
     "io_getevents failed" <
    <
     std::endl;
    
            break;

        }
    
        std::cout <
    <
     "Read " <
    <
     events[0].res <
    <
     " bytes" <
    <
     std::endl;

    }
    

    io_destroy(ctx);

}


int main() {
    
    int fd = open("test.txt", O_RDONLY);
    
    if (fd <
 0) {
    
        std::cerr <
    <
     "Failed to open file" <
    <
     std::endl;
    
        return 1;

    }
    

    async_read(fd, 0, 1024);
    

    close(fd);
    
    return 0;

}
    

2. 使用epoll

epoll是Linux提供的I/O事件通知机制，可以用于异步I/O操作。

示例代码：

#include <
    iostream>
    
#include <
    fcntl.h>
    
#include <
    unistd.h>
    
#include <
    sys/epoll.h>


void async_read(int fd) {
    
    int epoll_fd = epoll_create1(0);
    
    if (epoll_fd <
 0) {
    
        std::cerr <
    <
     "epoll_create1 failed" <
    <
     std::endl;
    
        return;

    }
    

    struct epoll_event event;
    
    event.events = EPOLLIN;
    
    event.data.fd = fd;
    

    if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, fd, &
    event) <
 0) {
    
        std::cerr <
    <
     "epoll_ctl failed" <
    <
     std::endl;
    
        close(epoll_fd);
    
        return;

    }
    

    char buffer[1024];

    while (true) {
    
        int num_bytes = read(fd, buffer, sizeof(buffer));
    
        if (num_bytes <
 0) {
    
            std::cerr <
    <
     "read failed" <
    <
     std::endl;
    
            break;

        }

        if (num_bytes == 0) {
    
            std::cout <
    <
     "EOF reached" <
    <
     std::endl;
    
            break;

        }
    
        std::cout <
    <
     "Read " <
    <
     num_bytes <
    <
     " bytes" <
    <
     std::endl;

    }
    

    close(epoll_fd);

}


int main() {
    
    int fd = open("test.txt", O_RDONLY);
    
    if (fd <
 0) {
    
        std::cerr <
    <
     "Failed to open file" <
    <
     std::endl;
    
        return 1;

    }
    

    async_read(fd);
    

    close(fd);
    
    return 0;

}
    

3. 使用std::future和std::async

C++11引入了std::future和std::async，可以用于实现异步操作。

示例代码：

#include <
    iostream>
    
#include <
    fstream>
    
#include <
    future>
    

std::string async_read(const std::string&
 filename) {
    
    std::ifstream file(filename);

    if (!file.is_open()) {
    
        throw std::runtime_error("Failed to open file");

    }
    

    std::string content((std::istreambuf_iterator<
    char>
    (file)), std::istreambuf_iterator<
    char>
    ());
    
    return content;

}


int main() {
    
    auto future = std::async(std::launch::async, async_read, "test.txt");


    // Do other work here

    try {
    
        std::string content = future.get();
    
        std::cout <
    <
     "Read " <
    <
     content.size() <
    <
     " bytes" <
    <
     std::endl;

    }
     catch (const std::exception&
 e) {
    
        std::cerr <
    <
     "Exception: " <
    <
     e.what() <
    <
     std::endl;

    }
    

    return 0;

}
    

4. 使用asio

asio是一个跨平台的C++库，用于网络和低层I/O编程，支持异步操作。

示例代码：

#include <
    iostream>
    
#include <
    fstream>
    
#include <
    asio.hpp>
    

using asio::ip::tcp;
    

void async_read(asio::io_context&
     io_context, const std::string&
 filename) {
    
    std::ifstream file(filename, std::ios::binary);

    if (!file.is_open()) {
    
        std::cerr <
    <
     "Failed to open file" <
    <
     std::endl;
    
        return;

    }
    

    asio::streambuf buffer;
    
    asio::async_read(file, buffer,
        [&
buffer](std::error_code ec, std::size_t bytes_transferred) {

            if (!ec) {
    
                std::cout <
    <
     "Read " <
    <
     bytes_transferred <
    <
     " bytes" <
    <
     std::endl;
    
                std::istream is(&
    buffer);
    
                std::string content((std::istreambuf_iterator<
    char>
    (is)), std::istreambuf_iterator<
    char>
    ());
    
                std::cout <
    <
     content <
    <
     std::endl;

            }
 else {
    
                std::cerr <
    <
     "Error: " <
    <
     ec.message() <
    <
     std::endl;

            }

        }
    );
    

    io_context.run();

}


int main() {
    
    asio::io_context io_context;
    
    async_read(io_context, "test.txt");
    
    return 0;

}
    

这些方法各有优缺点，选择哪种方法取决于具体的应用场景和需求。libaio和epoll更适合底层系统编程，而std::future和std::async以及asio则更适合应用层编程。

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