null_file.hpp

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/*********************************************************************/
// dar - disk archive - a backup/restoration program
// Copyright (C) 2002-2025 Denis Corbin
//
// This program 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 2
// of the License, or (at your option) any later version.
//
// This program 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.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
//
// to contact the author, see the AUTHOR file
/*********************************************************************/
 
 
#ifndef NULL_FILE_HPP
#define NULL_FILE_HPP
 
#include "../my_config.h"
#include "generic_file.hpp"
#include "thread_cancellation.hpp"
 
namespace libdar
{
 
 
 
 
    class null_file : public generic_file, public thread_cancellation
    {
    public :
        null_file(gf_mode m) : generic_file(m) { set_offset(0); };
    null_file(const null_file & ref) = default;
    null_file(null_file && ref) noexcept = default;
    null_file & operator = (const null_file & ref) = default;
    null_file & operator = (null_file && ref) noexcept = default;
    ~null_file() = default;
 
    virtual bool skippable(skippability direction, const infinint & amount) override { return true; };
        virtual bool skip(const infinint &pos) override { set_offset(pos); return true; };
        virtual bool skip_to_eof() override { offset = max_offset; return true; };
        virtual bool skip_relative(signed int x) override { return set_rel_offset(x); };
    virtual bool truncatable(const infinint & pos) const override { return true; };
        virtual infinint get_position() const override { return offset; };
 
    protected :
    virtual void inherited_read_ahead(const infinint & amount) override {};
 
        virtual U_I inherited_read(char *a, U_I size) override
    {
#ifdef MUTEX_WORKS
        check_self_cancellation();
#endif
        return 0;
    };
 
        virtual void inherited_write(const char *a, U_I siz) override
    {
#ifdef MUTEX_WORKS
        check_self_cancellation();
#endif
        set_offset(offset + siz);
    };
 
    virtual void inherited_truncate(const infinint & pos) override { if(pos < offset) offset = pos; };
    virtual void inherited_sync_write() override {};
    virtual void inherited_flush_read() override {};
    virtual void inherited_terminate() override {};
 
    private:
    infinint offset;
    infinint max_offset;
 
    void set_offset(const infinint & x)
    {
        if(x > max_offset)
        max_offset = x;
        offset = x;
    }
 
    bool set_rel_offset(signed int x)
    {
        if(x >= 0)
        {
        set_offset(offset + x);
        return true;
        }
        else // x < 0
        {
        infinint tmp = -x;
        if(tmp > offset)
        {
            offset = 0;
            return false;
        }
        else
        {
            offset -= tmp;
            return true;
        }
        }
    }
 
    };
 
 
} // end of namespace
 
#endif