Question

我想使用标准C ++ fstream类从串口（/ dev / ttyGS0）读取和写入数据。

两小时前我认为这是一项微不足道的任务（它只是一个文件（嗯，这是一个特殊的...）），直到写入串口引发错误并且perror（）显示“非法搜寻” 。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，可能导致错误。

如何使用标准c ++库写入串口？

它可能完全或者我必须回退到像open（）这样的C函数，还是使用像LibSerial这样的附加库？

Answer 1

与此同时，我自己想出了一个解决方案。

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。

答案很长：可以从std :: filebuf派生并更改实现。从理论上讲，它实际上非常简单，因为只需要在函数 std :: filebuf :: _ M_seek 中注释掉一行代码。

实际上并不是那么简单，因为 std :: filebuf :: _ M_seek 本身不是虚拟的，这意味着它不能在派生类中被覆盖。幸运的是（在这种情况下）仅由另一个函数 std :: filebuf :: overflow 使用，它是虚拟的。

所以这是我的班级 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_ */

我在gcc源代码中找到的libstdc ++实现100％复制了实现。出于版权原因，我还会发布原始文件的标题。

您可以轻松找到注释掉的一行 - 这是“非法搜寻”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;
}

以下是如何使用 FilebufNoSeek 类。只需将其传递给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();

希望这会有所帮助 - 祝你好运！

阅读＆amp;使用标准C ++ fstream在串行设备（/ dev / ttyGS0）上写入？

1 个答案: