使用标准 C++ fstream 在串行设备 (/dev/ttyGS0) 上读写？

Question

我想使用标准 C++ fstream 从串行端口 (/dev/ttyGS0) 读取和写入数据 class。

两个小时前我认为这是一个微不足道的任务（它只是一个文件（好吧，它是一个特殊的......）），直到写入串口引发错误并且 perror() 揭示了一个 "Illegal seek"。进一步调查显示 /dev/ttyGS0（与每个串行设备一样）是一个不可搜索的文件，这可能导致了错误。

如何使用标准 C++ 库写入串行端口？

完全有可能还是我必须回退到像 open() 这样的 C 函数或使用像 LibSerial 这样的附加库？

Answer 1

同时，我自己想出了一个解决办法。

简短回答：使用标准 C++ 库读取或写入不可搜索的设备并非易事。

长答案：可以通过从 std::filebuf 派生并更改实现来实现。理论上其实很简单，因为只需要在函数std::filebuf::_M_seek.[=13=中注释掉一行代码]

实际上并没有这么简单，因为std::filebuf::_M_seek本身不是虚拟的，也就是说在派生中不能被覆盖class。幸运的是，它（在这种情况下）仅被另一个虚拟函数 std::filebuf::overflow 使用。

所以这是我的 class FilebufNoSeek 可用于访问不可搜索的设备：

/*
 * filebufNoSeek.hpp
 */

#ifndef INCLUDE_SYSTEMIO_FILEBUFNOSEEK_HPP_
#define INCLUDE_SYSTEMIO_FILEBUFNOSEEK_HPP_

#include <fstream>

using namespace std;


class FilebufNoSeek : public filebuf
{
public:
    FilebufNoSeek() {};
protected:
    filebuf::pos_type _M_seek(filebuf::off_type __off, ios_base::seekdir __way, filebuf::__state_type __state);
    virtual filebuf::int_type overflow(filebuf::int_type __c);
};


#endif /* INCLUDE_SYSTEMIO_FILEBUFNOSEEK_HPP_ */

该实现是 100% 从我在 gcc 源代码中找到的 libstdc++ 实现复制的。由于版权原因，我还发布了原始文件的header。

您可以轻松找到被注释掉的一行 - 这是 "illegal seek" 错误的来源。

// File based streams -*- C++ -*-

// Copyright (C) 1997-2014 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file bits/fstream.tcc
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{fstream}
 */

/*
 * FilebufNoSeek.cpp
 *
 *  Created / copied / modified on: 21.12.2015 by ChristophK
 */

#include "FilebufNoSeek.hpp"


filebuf::pos_type FilebufNoSeek::_M_seek(filebuf::off_type __off, ios_base::seekdir __way, filebuf::__state_type __state)
{
    pos_type __ret = pos_type(off_type(-1));
    if (_M_terminate_output())
    {
//      off_type __file_off = _M_file.seekoff(__off, __way); // Causes "illegal seek"
        off_type __file_off = off_type(0); // New - no seek
        if (__file_off != off_type(-1))
        {
            _M_reading = false;
            _M_writing = false;
            _M_ext_next = _M_ext_end = _M_ext_buf;
            _M_set_buffer(-1);
            _M_state_cur = __state;
            __ret = __file_off;
            __ret.state(_M_state_cur);
        }
    }
    return __ret;
}



filebuf::int_type FilebufNoSeek::overflow(filebuf::int_type __c)
{
    int_type __ret = traits_type::eof();
    const bool __testeof = traits_type::eq_int_type(__c, __ret);
    const bool __testout = (_M_mode & ios_base::out
            || _M_mode & ios_base::app);
    if (__testout)
    {
        if (_M_reading)
        {
            _M_destroy_pback();
            const int __gptr_off = _M_get_ext_pos(_M_state_last);
            if (_M_seek(__gptr_off, ios_base::cur, _M_state_last)
                    == pos_type(off_type(-1)))
                return __ret;
        }
        if (this->pbase() < this->pptr())
        {
            // If appropriate, append the overflow char.
            if (!__testeof)
            {
                *this->pptr() = traits_type::to_char_type(__c);
                this->pbump(1);
            }

            // Convert pending sequence to external representation,
            // and output.
            if (_M_convert_to_external(this->pbase(),
                    this->pptr() - this->pbase()))
            {
                _M_set_buffer(0);
                __ret = traits_type::not_eof(__c);
            }
        }
        else if (_M_buf_size > 1)
        {
            // Overflow in 'uncommitted' mode: set _M_writing, set
            // the buffer to the initial 'write' mode, and put __c
            // into the buffer.
            _M_set_buffer(0);
            _M_writing = true;
            if (!__testeof)
            {
                *this->pptr() = traits_type::to_char_type(__c);
                this->pbump(1);
            }
            __ret = traits_type::not_eof(__c);
        }
        else
        {
            // Unbuffered.
            char_type __conv = traits_type::to_char_type(__c);
            if (__testeof || _M_convert_to_external(&__conv, 1))
            {
                _M_writing = true;
                __ret = traits_type::not_eof(__c);
            }
        }
    }
    return __ret;
}

下面是如何使用 class FilebufNoSeek。只需将它传递给 std::iostream:

的构造函数

FilebufNoSeek serialFilebuf;
serialFilebuf.open("/dev/ttyGS0");

std::iostream serialStream(serialFilebuf);
serialStream << "I'm writing to a nonseekable file" << std::endl;

serialFilebuf.close();

希望这对您有所帮助 - 祝您好运！

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