Question

I’m working on some legacy code that uses win32 WriteFile() to write to a random location in a binary file. The offset of the write can be past the end of the file in which case WriteFile() seems to automatically extend the file size to the offset and then write the data to the file.

I’d like to use std::fstream to do the same, but when I try to seekp() to the appropriate location, past the end of the file, the seekp() fails and the subsequent write() fails as well.

So it seems to me that I have to ‘manually’ fill in the space between the current EOF and the location I want to write to.

The code looks like this:

void Save(size_t offset, const Element& element)
{
    m_File.seekp(offset, std::ios_base::beg);
    m_File.write(reinterpret_cast<const char*>(&element), sizeof(Element));
    if (m_File.fail()) {
        // ... error handling
    }
}

So is my only option to ‘manually’ write 0s from the current EOF up to offset?

Answer 1

Here is an example I picked up verbatim from MSDN:

// basic_ostream_seekp.cpp
// compile with: /EHsc
#include <fstream>
#include <iostream>

int main()
{
    using namespace std;
    ofstream x("basic_ostream_seekp.txt");
    streamoff i = x.tellp();
    cout << i << endl;
    x << "testing";
    i = x.tellp();
    cout << i << endl;
    x.seekp(2);   // Put char in third char position in file
    x << " ";

    x.seekp(2, ios::end);   // Put char two after end of file
    x << "z";
}

The file “basic_ostream_seekp.txt” has te ting\0\0z at the end of the program, i.e., you are allowed to seek past the end of the file.

In any case, if write does fail for you, you could check and see if seekp does too. If it does, you can detect the failure earlier.