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有人能提供一个使用boost-iostreams查找、读取和写入大于4GB文件的示例吗

boost-iostreams large-files c++

7

Queueless · 技术社区 · 16 年前

64 bit offset functions ,但没有关于如何使用它们的例子。有人用这个库来处理大文件吗?打开两个文件,找到它们的中间位置,然后将一个文件复制到另一个文件的简单示例将非常有用。

谢谢。

1 回复 | 直到 16 年前

1

7

Danilo Piazzalunga 16 年前

简答

只需包括

#include <boost/iostreams/seek.hpp>

seek 功能如

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

哪里

device 是一个文件、流、streambuf或任何可转换为 seekable ;
offset 是类型为64位的偏移量 stream_offset ;
whence 是 BOOST_IOS::beg , BOOST_IOS::cur 或 BOOST_IOS::end .

返回值为 寻求 std::streampos ,它可以转换为 stream_offset 使用 position_to_offset 功能。

/*
 * 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();
}