Boost.Asio：发送太大消息时出现分段错误

Question

我的程序将一些内部日志保存到 .txt 文件中。 如果您通过 TCP（SSL 加密）连接到它，程序将发送日志文件的内容。

这是发送数据的代码：

void NIUserSession::write(std::string message)
{
    std::cout << "Writing message" << std::endl;
    message.append("<EOF>");
    boost::system::error_code ec;
    boost::asio::async_write(this->socket_, boost::asio::buffer(message),
                       boost::asio::transfer_all(), boost::bind(&NIUserSession::writeHandler,
                                                                this, boost::asio::placeholders::error,
                                                                boost::asio::placeholders::bytes_transferred()));
}

void NIUserSession::writeHandler(const boost::system::error_code &error, std::size_t bytes_transferred)
{
    std::cout << "Write Handler" << std::endl;
    if(error)
    {
        std::cout << "Write handler error: " << error.message() << std::endl;
        this->disconnect();
    }
}

所以NIUserSession::write 将日志文件内容作为字符串传递。

如果程序很长时间没有运行，则日志文件很短并且一切正常。但是，如果它运行一段时间并且日志文件变得越来越长，则程序在尝试发送数据时会收到SIGSEGV。这是 gdb 日志：

Writing message

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff4fe1700 (LWP 21047)]
__memcpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S:33
33      ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S: No such file or directory.
(gdb) where
#0  __memcpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S:33
#1  0x00007ffff7998454 in ?? () from /lib/x86_64-linux-gnu/libssl.so.1.0.0
#2  0x00007ffff79985c3 in ?? () from /lib/x86_64-linux-gnu/libssl.so.1.0.0
#3  0x00000000004b7eca in boost::asio::ssl::detail::io_op<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::ssl::detail::write_op<boost::asio::const_buffers_1>, boost::asio::detail::write_op<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> > >, boost::asio::const_buffers_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boost::_mfi::mf2<void, NIUserSession, boost::system::error_code const&, unsigned long>, boost::_bi::list3<boost::_bi::value<NIUserSession*>, boost::arg<1> (*)(), boost::arg<2> > > > >::operator()(boost::system::error_code, unsigned long, int) ()
#4  0x00000000004b8bf8 in boost::asio::detail::write_op<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> > >, boost::asio::const_buffers_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boost::_mfi::mf2<void, NIUserSession, boost::system::error_code const&, unsigned long>, boost::_bi::list3<boost::_bi::value<NIUserSession*>, boost::arg<1> (*)(), boost::arg<2> > > >::operator()(boost::system::error_code const&, unsigned long, int) ()
#5  0x00000000004b7e6c in boost::asio::ssl::detail::io_op<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::ssl::detail::write_op<boost::asio::const_buffers_1>, boost::asio::detail::write_op<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> > >, boost::asio::const_buffers_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boost::_mfi::mf2<void, NIUserSession, boost::system::error_code const&, unsigned long>, boost::_bi::list3<boost::_bi::value<NIUserSession*>, boost::arg<1> (*)(), boost::arg<2> > > > >::operator()(boost::system::error_code, unsigned long, int) ()
#6  0x00000000004b965c in boost::asio::detail::reactive_socket_send_op<boost::asio::mutable_buffers_1, boost::asio::detail::write_op<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::mutable_buffers_1, boost::asio::detail::transfer_all_t, boost::asio::ssl::detail::io_op<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::ssl::detail::write_op<boost::asio::const_buffers_1>, boost::asio::detail::write_op<boost::asio::ssl::stream<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> > >, boost::asio::const_buffers_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boost::_mfi::mf2<void, NIUserSession, boost::system::error_code const&, unsigned long>, boost::_bi::list3<boost::_bi::value<NIUserSession*>, boost::arg<1> (*)(), boost::arg<2> > > > > > >::do_complete(boost::asio::detail::task_io_service*, boost::asio::detail::task_io_service_operation*, boost::system::error_code const&, unsigned long) ()
#7  0x00000000004add59 in boost::asio::detail::epoll_reactor::descriptor_state::do_complete(boost::asio::detail::task_io_service*, boost::asio::detail::task_io_service_operation*, boost::system::error_code const&, unsigned long) ()
#8  0x00000000004ad911 in boost::asio::detail::task_io_service::run(boost::system::error_code&) ()
#9  0x00000000004a9c1f in NetInterface::init() ()
#10 0x00007ffff641aa60 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#11 0x00007ffff566e184 in start_thread (arg=0x7ffff4fe1700) at pthread_create.c:312
#12 0x00007ffff5b8237d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

我不明白为什么会这样。我可能需要给boost::asio::buffer 一个尺寸吗？

另外，io_service::run() 正在其自己的分离线程中运行。这会是个问题吗？

Answer 1

这是 asio 使用的常见问题。您正在使用boost::asio::buffer(message) 发送缓冲区，它不会复制数据。它只是为数据创建一个引用，你负责这个对象（你的message）的生命周期，直到操作结束。但是当您退出函数NIUserSession::write 时，所有堆栈变量都会被销毁，包括您的message。

要解决此问题，您应该将数据放入生命周期较长的对象中，例如放入 shared_ptr。有效的例子可以是这样的：

void NIUserSession::write(std::string &message_orig)
{
    std::cout << "Writing message" << std::endl;

    std::shared_ptr message = std::make_shared<std::string>( message_orig );
    message->append("<EOF>");
    boost::system::error_code ec;
    boost::asio::async_write(this->socket_, boost::asio::buffer(*message),
        boost::asio::transfer_all(), boost::bind(&NIUserSession::writeHandler,
               this, boost::asio::placeholders::error,
               boost::asio::placeholders::bytes_transferred(),
               message /* <- capture it into callback to guarantee lifetime */
               ));
}