问题描述
我有一个由 boost.asio 编写的服务器。该服务器从客户端获取文件并将其写入磁盘。我只是有一个问题。当服务器获取文件时,它在完全接收文件时将其写入磁盘。我希望服务器以实时方式将缓冲区写入磁盘。例如,服务器每 100kb 大小的从客户端获取的文件就会写入磁盘。我已经编写了以下代码,但我不知道如何编辑才能达到这个目标。
void Session::DoReadFileContent(size_t arg_bytes_transferred)
{
if (arg_bytes_transferred > 0)
{
m_outputFile.write(m_buffer.data(),static_cast<std::streamsize>(arg_bytes_transferred));
if (m_outputFile.tellp() >= static_cast<std::streamsize>(m_fileSize))
{
std::cout << "Received file: " << m_fileName << std::endl;
return;
}
}
auto self = shared_from_this();
m_socket.async_read_some(boost::asio::buffer(m_buffer.data(),m_buffer.size()),[this,self](boost::system::error_code arg_error_code,size_t arg_bytes)
{
DoReadFileContent(arg_bytes);
});
}
解决方法
首先,在这种情况下,最好读取明确的数据大小,而不是读取任何可用数据的 read_some
。
在这种模式下,跟踪“剩余可接收字节数”比m_fileSize
更容易。
这里有一些小的改组,使您的代码成为一个独立的示例。它期望服务器发送一行文本,给出有效负载大小和输出文件名,然后是该文件的内容。可以使用 netcat 运行示例服务器,例如:
(stat -c '%soutput.dat' main.cpp; cat main.cpp) | netcat -l -p 6969
#include <boost/asio.hpp>
#include <fstream>
#include <iostream>
using boost::system::error_code;
using boost::asio::ip::tcp;
struct Session : std::enable_shared_from_this<Session> {
Session(boost::asio::io_context& io,uint16_t port)
: m_socket(io)
{
m_socket.connect({{},port});
}
void Start();
void DoReadFileContent(size_t transferred = 0);
private:
std::array<char,1024> m_buffer;
std::streamsize m_remainingSize = 0;
std::string m_fileName = "noname.dat";
std::ofstream m_outputFile;
tcp::socket m_socket;
};
void Session::Start() {
// Reading a size (in text for simplicity) and subsequently receive as many bytes
//
// I'm keeping this sync for simplicity,because you probably already have
// this coded somehwere
boost::asio::streambuf buf;
error_code ec;
auto n = read_until(m_socket,buf,"\n",ec);
std::istream is(&buf);
if (is >> m_remainingSize && getline(is,m_fileName)) {
std::cerr << "Protocol trace: n:" << n << ",fileName:" << m_fileName << " payload_size:" << m_remainingSize << "\n";
m_outputFile.exceptions(std::ios::failbit | std::ios::badbit);
m_outputFile.open(m_fileName,std::ios::binary);
// write excess buffer contents as part of payload
if (buf.size()) {
std::cerr << "Writing " << buf.size() << " bytes\n";
m_remainingSize -= buf.size();
m_outputFile << &buf;
}
DoReadFileContent();
} else {
std::cerr << "Protocol error,payload_size expected\n";
}
}
void Session::DoReadFileContent(size_t transferred) {
if (transferred > 0) {
std::cerr << "Writing " << transferred << " bytes\n";
m_remainingSize -= transferred;
m_outputFile.write(m_buffer.data(),transferred);
}
if (m_remainingSize <= 0) {
std::cout << "Completed file: " << m_fileName << std::endl;
return;
}
auto self = shared_from_this();
auto expect = std::min(size_t(m_remainingSize),m_buffer.size());
std::cout << "Trying to receive next " << expect << " bytes" << std::endl;
async_read(m_socket,boost::asio::buffer(m_buffer.data(),expect),[this,self](error_code ec,size_t arg_bytes) {
std::cerr << "async_read: " << ec.message() << " - " << arg_bytes << " bytes\n";
if (!ec) {
DoReadFileContent(arg_bytes);
}
});
}
int main() {
boost::asio::io_context io;
std::make_shared<Session>(io,6868) // download from port 6868
->Start();
io.run(); // complete
}
测试
(stat -c '%soutput.dat' main.cpp; cat main.cpp) | netcat -l -p 6868&
./a.out
md5sum main.cpp output.dat
打印,例如:
Protocol trace: n:15,fileName:output.dat payload_size:2654
Trying to receive next 1024 bytes
async_read: Success - 1024 bytes
Writing 1024 bytes
Trying to receive next 1024 bytes
async_read: Success - 1024 bytes
Writing 1024 bytes
Trying to receive next 606 bytes
async_read: Success - 606 bytes
Writing 606 bytes
Completed file: output.dat
最后两行
b4eec7203f6a1dcbfbf3d298c7ec0832 main.cpp
b4eec7203f6a1dcbfbf3d298c7ec0832 output.dat
表示接收到的文件与原始文件相同。
注意事项:
-
数据包在我的系统上以未指定的大小传送,例如接收到的文件相同:
Protocol trace: n:15,fileName:output.dat payload_size:2654 Writing 497 bytes Trying to receive next 1024 bytes async_read: Success - 1024 bytes Writing 1024 bytes Trying to receive next 1024 bytes async_read: Success - 1024 bytes Writing 1024 bytes Trying to receive next 109 bytes async_read: Success - 109 bytes Writing 109 bytes Completed file: output.dat b4eec7203f6a1dcbfbf3d298c7ec0832 main.cpp b4eec7203f6a1dcbfbf3d298c7ec0832 output.dat
请注意,它以来自
read_until
的输入缓冲区中已经的 497 个字节开始。
- 协议不安全:
- 应该验证文件名。想象一下,如果文件是“/home/sehe/myimportant_file.txt”或更糟,比如说 /dev/sde1 并且我们有权限进行原始块设备访问会发生什么......
- 您可能希望为 streambuf 指定一个最大大小,这样如果您得到一个永远不会发送
'\n'
的模糊器,您就不会只是吞噬所有 RAM
- 文件 IO 的错误处理非常粗糙。我使用了 io 异常,但您可能想在不同的地方检查
m_outputFile.good()