我读到 boost iostreams 据说支持 64 位半便携式方式访问大文件。他们的常见问题解答提到64 位偏移函数 http://www.boost.org/doc/libs/1_39_0/libs/iostreams/doc/faq.html#offsets，但没有关于如何使用它们的示例。有人使用这个库来处理大文件吗？打开两个文件、查找它们的中间部分并将一个文件复制到另一个文件的简单示例将非常有帮助。

Thanks.

简短回答

只需包括

#include <boost/iostreams/seek.hpp>

并使用seek http://www.boost.org/doc/libs/1_42_0/libs/iostreams/doc/functions/seek.html功能如

boost::iostreams::seek(device, offset, whence);

where

• device是文件、流、streambuf 或任何可转换为的对象seekable;
• offset是类型的 64 位偏移量stream_offset http://www.boost.org/doc/libs/1_42_0/libs/iostreams/doc/functions/positioning.html#synopsis;
• whence is BOOST_IOS::beg, BOOST_IOS::cur or BOOST_IOS::end.

返回值seek属于类型std::streampos，并且它可以转换为stream_offset使用position_to_offset http://www.boost.org/doc/libs/1_42_0/libs/iostreams/doc/functions/positioning.html#position_to_offset功能。

Example

这是一个冗长、乏味且重复的示例，它展示了如何打开两个文件、寻找 >4GB 的偏移量以及在它们之间复制数据。

警告：此代码将创建非常大的文件（几个 GB）。在支持稀疏文件的操作系统/文件系统上尝试此示例。 Linux 还可以；我没有在其他系统上测试它，例如Windows。

/*
 * WARNING: This creates very large files (several GB)
 * unless your OS/file system supports sparse files.
 */
#include <boost/iostreams/device/file.hpp>
#include <boost/iostreams/positioning.hpp>
#include <cstring>
#include <iostream>

using boost::iostreams::file_sink;
using boost::iostreams::file_source;
using boost::iostreams::position_to_offset;
using boost::iostreams::seek;
using boost::iostreams::stream_offset;

static const stream_offset GB = 1000*1000*1000;

void setup()
{
    file_sink out("file1", BOOST_IOS::binary);
    const char *greetings[] = {"Hello", "Boost", "World"};
    for (int i = 0; i < 3; i++) {
        out.write(greetings[i], 5);
        seek(out, 7*GB, BOOST_IOS::cur);
    }
}

void copy_file1_to_file2()
{
    file_source in("file1", BOOST_IOS::binary);
    file_sink out("file2", BOOST_IOS::binary);
    stream_offset off;

    off = position_to_offset(seek(in, -5, BOOST_IOS::end));
    std::cout << "in: seek " << off << std::endl;

    for (int i = 0; i < 3; i++) {
        char buf[6];
        std::memset(buf, '\0', sizeof buf);

        std::streamsize nr = in.read(buf, 5);
        std::streamsize nw = out.write(buf, 5);
        std::cout << "read: \"" << buf << "\"(" << nr << "), "
                  << "written: (" << nw << ")" << std::endl;

        off = position_to_offset(seek(in, -(7*GB + 10), BOOST_IOS::cur));
        std::cout << "in: seek " << off << std::endl;
        off = position_to_offset(seek(out, 7*GB, BOOST_IOS::cur));
        std::cout << "out: seek " << off << std::endl;
    }
}

int main()
{
    setup();
    copy_file1_to_file2();
}