Скачать или смотреть How to Create Sparse Files in Windows Using the Win32 API

Learn how to efficiently manage file storage and create sparse files in Windows using the `Win32` API. This guide covers the `FSCTL_SET_ZERO_DATA` function and provides clear code examples.
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Creating Sparse Files in Windows Using Win32 API

When working with files in operating systems, efficient storage management can significantly optimize performance and save disk space. One useful feature that Linux provides is the fallocate system call, which allows developers to create sparse files. Sparse files are files that allocate large amounts of space, but are actually empty within those allocated regions. This approach can be beneficial for applications that deal with large datasets.

The Problem: Sparse Files and the fallocate Equivalent in Windows

In Linux, you can use the fallocate function with the FALLOC_FL_PUNCH_HOLE and FALLOC_FL_KEEP_SIZE flags to deallocate blocks within a file, thus creating a sparse area. Specifically, an example call looks like this:

[[See Video to Reveal this Text or Code Snippet]]

However, Windows does not have a direct equivalent to this function. This raises a question: How can developers create sparse files in the Windows environment using the Win32 API?

The Solution: Using DeviceIoControl in Win32

Windows provides a way to achieve sparse file functionality using the DeviceIoControl function in combination with the FSCTL_SET_ZERO_DATA control code. Here’s how you can do it:

Step-by-Step Implementation

Preparation: Ensure you have a handle hFile to the file you want to modify. The file should already be opened for writing.

Define the FILE_ZERO_DATA_INFORMATION Structure: This structure will hold the offsets for the data you wish to zero out:

FileOffset: This is the starting point inside the file from where the zeroing should begin.

BeyondFinalZero: This is the endpoint where the zeroing action should stop.

Using DeviceIoControl: This function will perform the actual operation of zeroing out the specified data.

Example Code

Here is a ready-to-use example in C that demonstrates how to punch a hole in a file to create a sparse file:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Creating sparse files in Windows may seem challenging since there is no direct equivalent to the fallocate command in Linux. However, utilizing the DeviceIoControl function along with the FSCTL_SET_ZERO_DATA operation allows you to effectively manage file storage and create sparse files. This method not only optimizes space usage but also improves the performance of file handling operations within your applications.

Now, you can seamlessly manage large files while conserving disk space with this practical approach using the Win32 API.