如何使用C ++在> 4GB文件中写入数据? [英] How to write data in >4GB file with C++?

问题描述

我正在尝试写一个大文件，但是遇到了问题.

I'm trying to write a large file, but ran into a problem.

我花了很长时间寻找写入位置，但是写入的文件不能超过4,2Gb.我忘记了什么?

I use long long for seeking a place to write, but can't write file more than 4,2Gb. What I forgot?

更多详细信息: 我打开4Gb文件:

More details: I open 4Gb-file:

ifstream ifs(db_name.c_str(), ios_base::binary);
if (!ifs) 
    throw Bad_archive();
ifs.read(as_bytes(seg_size), sizeof(int));
ifs.read(as_bytes(last_idx), sizeof(int));
ifs.read(as_bytes(free_segs), sizeof(int));
if (free_segs > 0)
{
    long long seek_g = 3 * sizeof(int) + (long long)last_idx * seg_size;
    ifs.seekg(seek_g, ios_base::beg);
    for (int i = 0; i < free_segs; i++)
    {
        int tmp = 0;
        ifs.read(as_bytes(tmp), sizeof(int));
        free_spaces.push_back(tmp);
    }
}
ifs.close();

在那之后，我读取了400Mb文件，我想将其添加到数据库中.并编写(这是短代码):

After that, I read 400Mb-file, which I want to add to the db. And write (here is the short code):

// write object
ofstream ofs(db_name.c_str(), ios_base::binary | ios_base::in | ios_base::ate);
for (int i = 0; ; i++)
{
    // set stream position
    long long write_position = sizeof(int) * 3;

    ...

    write_position += (long long) seg_size * last_idx;
    ofs.seekp(write_position, ios::beg);

            ...

    // write sizeof object
    if (i == 0)
        ofs.write(as_bytes(object_size), sizeof(object_size)); 
    else
    {
        int null = 0;
        ofs.write(as_bytes(null), sizeof(null));
    }

    // write data
    for (int j = 0; j < (seg_size - sizeof(int) * 2); j++)
    {
        ofs.write(as_bytes(obj_content[j + i * (seg_size - sizeof(int) * 2)]), 
        sizeof(char));

    }
    if (write_new_seg)
        last_idx++;

    ...

    ofs.write(as_bytes(cont_seg), sizeof(cont_seg));
}
ofs.close();

之后，我保存数据库信息:

After that, I save db info:

if (last_idx == 0)
{
    ofstream ofs(db_name.c_str());
    ofs.close();
}
ofstream ofs(db_name.c_str(), ios_base::binary | ios_base::in | 
            ios_base::out | ios_base::ate);
long long seek_p = 0;
ofs.seekp(seek_p, ios_base::beg);
ofs.write(as_bytes(seg_size), sizeof(seg_size));
ofs.write(as_bytes(last_idx), sizeof(last_idx));
ofs.write(as_bytes(free_segs), sizeof(free_segs));
ofs.close();

但是此代码有效:

ofstream ofs2("huge2");
ofs2.close();
ofstream ofs("huge2", ios_base::in | ios_base::ate);
long long sp = 10000000000;
ofs.seekp(10000000000, ios_base::beg);

ofs.write(as_bytes(sp), sizeof(sp));

ofs.close();

推荐答案

    fseeko64 (Filename, 0, SEEK_END);
    long size = ftello64 (Filename);
    fclose (Filename);

要访问大文件，不能使用fseek，而必须使用fseeko64();

For accessing the large files , fseek cannot be used , instead have to go for fseeko64();

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