How to truncate a file in C?

As opposed to trying to do this yourself, I'd really recommend using something like log4net; it has a lot of this sort of useful functionality built in.

When the file is over 500000 bytes, it will cut the beginning 250000 bytes off from the file so the remaining file is 250000 bytes long.

FileStream fs = new FileStream(strFileName, FileMode.OpenOrCreate);
        if (fs.Length > 500000)
        {
            // Set the length to 250Kb
            Byte[] bytes = new byte[fs.Length];
            fs.Read(bytes, 0, (int)fs.Length);
            fs.Close();
            FileStream fs2 = new FileStream(strFileName, FileMode.Create);
            fs2.Write(bytes, (int)bytes.Length - 250000, 250000);
            fs2.Flush();
        } // end if (fs.Length > 500000) 

Try to play around with FileStream.SetLength

FileStream fileStream = new FileStream(...);
fileStream.SetLength(sizeInBytesNotChars);

Close the file and then reopen it using FileMode.Truncate.

Some log implementations archive the old file under an old name before reopening it, to preserve a larger set of data without any file getting too big.