How do you communicate and synchronize between kernel-mode and user-mode drivers? (2024)

If you are developing device drivers for Windows, you need to understand the difference between kernel-mode and user-mode drivers. In this article, you will learn what these terms mean, why they matter, and how to communicate and synchronize between them.

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Kernel-mode drivers are software components that run in the same memory space as the operating system kernel. They have direct access to hardware resources, such as memory, CPU, and I/O devices. They can also interact with other kernel components, such as system services, device stacks, and object managers. Kernel-mode drivers are typically faster and more efficient than user-mode drivers, but they also pose more risks. If a kernel-mode driver crashes or contains a bug, it can cause a system failure or a security breach.

User-mode drivers are software components that run in a separate memory space from the operating system kernel. They have limited access to hardware resources, and they rely on system calls or APIs to communicate with the kernel. They can also use standard user-mode libraries and frameworks, such as Win32, COM, or .NET. User-mode drivers are typically safer and easier to develop than kernel-mode drivers, but they also have some drawbacks. They are slower and less efficient than kernel-mode drivers, and they may face compatibility issues with different versions of the operating system.

Communication between kernel-mode and user-mode drivers can be achieved through a variety of methods. DeviceIoControl is a function that enables an application or driver to send an IOCTL code to a kernel-mode driver and receive a response, with the data being exchanged via input/output buffers. ReadFile and WriteFile are functions that permit user-mode applications or drivers to read from or write to a device object controlled by a kernel-mode driver. Memory-mapped files, which are mapped into the virtual address space of both kernel-mode and user-mode processes, allow processes to share data by accessing the same physical memory location. Lastly, user-mode callbacks are functions registered by a user-mode driver and called by a kernel-mode driver in order to notify it of events or requests.

Synchronization mechanisms are used to ensure that the communication between kernel-mode and user-mode drivers is consistent and reliable, preventing data corruption, deadlock, or race conditions. Mutexes permit only one thread to access a shared resource at a time, protecting critical sections of code or data structures. Semaphores limit the number of threads that can access a shared resource concurrently, controlling the concurrency level or the queue length of requests. Events signal the state of a condition or an operation and can be used to notify a driver when a request is completed. Interlocked functions perform atomic operations on shared variables, such as incrementing, decrementing, or comparing and exchanging values.

Device Drivers