C++使用GCONTTYER（）方法读写随机存取文件

以代码为例：

const int length = 1024 * 1024; //     1048576
char buffer[length];

fstream f;

int main(int argc, char *argv[])
{
    f.open("file.bin", ios::in | ios::out | ios::binary);

    f.read(buffer, length);

    int k = 0;
    while (f.gcount() > 0)
    {
        k++;
        cout << "Block #" << k << ": " << f.gcount() << " bytes" << endl;

        f.read(buffer, f.gcount());
    } // while

    f.close();

    return 0;

} // main

现在，假设我想做一件无用的事情：读取每个块，然后在同一个文件中再次写入。实际上，这是一个什么都不做的操作

const int length = 1024 * 1024; //     1048576
char buffer[length];

fstream f;

int main(int argc, char *argv[])
{
    f.open("file.bin", ios::in | ios::out | ios::binary);

    f.read(buffer, length);

    int k = 0;
    while (f.gcount() > 0)
    {
        k++;
        cout << "Block #" << k << ": " << f.gcount() << " bytes" << endl;

// this is the code I added
        f.seekp(-f.gcount(), ios_base::cur); // move file pointer backwards
        f.write(buffer, f.gcount()); // write the buffer again <=> do nothing
// end of the code I added

        f.read(buffer, f.gcount());
    } // while

    f.close();

    return 0;

} // main

为什么块2和块3没有列出

谢谢

函数在输出序列上搜索，但输出序列没有改变，因为您刚才读取的内容改变了输入序列

我认为最好在每次执行读取时更新输出序列，我不确定它是否有效，但您可以尝试：

// ...

f.read(buffer, length);
f.seekp(f.gcount(), ios_base::cur); // update output sequence

int k = 0;
while (f.gcount() > 0)
{
    k++;
    cout << "Block #" << k << ": " << f.gcount() << " bytes" << endl;

// this is the code I added
    f.seekp(-f.gcount(), ios_base::cur); // move file pointer backwards
    f.write(buffer, f.gcount()); // write the buffer again <=> do nothing
// end of the code I added

    f.read(buffer, f.gcount());
    f.seekp(f.gcount(), ios_base::cur); // update output sequence
}

// ...

---

没用。C++中最大的困惑之一是在文件中既有输入也有输出，Sekp和SeCKG是一样的东西，就像Telp和Telg一样。

Block #1: 1048576 bytes

// ...

f.read(buffer, length);
f.seekp(f.gcount(), ios_base::cur); // update output sequence

int k = 0;
while (f.gcount() > 0)
{
    k++;
    cout << "Block #" << k << ": " << f.gcount() << " bytes" << endl;

// this is the code I added
    f.seekp(-f.gcount(), ios_base::cur); // move file pointer backwards
    f.write(buffer, f.gcount()); // write the buffer again <=> do nothing
// end of the code I added

    f.read(buffer, f.gcount());
    f.seekp(f.gcount(), ios_base::cur); // update output sequence
}

// ...