How to calculate offset for containing file inside orphaned child VHD?

Question

I have an orphaned child vhd(differencing Vhd). In c++ code, I wanted to parse the byte stream and identify the files in it. Is there any way to locate offset for file?

Answer 1

Hello ad1601,

Thank you for your question and for reaching out with your question today.

To locate the offset for a file within an orphaned child VHD (differencing VHD) and parse its byte stream, you can follow these general steps:

1. Understand the VHD File Structure: Familiarize yourself with the Virtual Hard Disk (VHD) file format specification to understand its structure. The VHD file format consists of various structures, such as the header, footer, BAT (Block Allocation Table), and data blocks.
2. Read the VHD Header: Read the VHD header to obtain important information about the VHD, such as the disk geometry, block size, and the location of the BAT.
3. Read the BAT: The BAT contains information about the location of each block in the VHD. Traverse the BAT to find the blocks associated with the orphaned child VHD.
4. Identify the Parent VHD: If the orphaned child VHD is a differencing VHD, it will have a reference to its parent VHD. Retrieve the parent VHD and repeat the steps to locate the desired file within the parent VHD.
5. Locate the File Offset: Once you have identified the correct block(s) containing the file you want to extract, you can calculate the offset within the VHD. The offset will be based on the block size and the location of the specific block(s) within the VHD file.
6. Read and Parse the File: Using the calculated offset, you can read the required data from the VHD file at that offset. Then, parse the byte stream according to the file format you expect within the VHD.

It's important to note that the process of locating the file offset within the VHD and parsing the byte stream can be complex, as it involves understanding the VHD file format and handling various structures. You may need to use low-level file handling and binary parsing techniques in your C++ code to achieve this.

To help you in this process, you can refer to the VHD file format specification documentation provided by Microsoft. Additionally, there are open-source libraries and tools available that can assist in reading and manipulating VHD files, which you may find helpful in your C++ development efforts.

I used AI provided by ChatGPT to formulate part of this response. I have verified that the information is accurate before sharing it with you.

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