The Linux Kernel Driver Interface (all of your questions answered and then some) Greg Kroah-Hartman This is being written to try to explain why Linux does not have a binary kernel interface, nor does it have a stable kernel interface. Please realize that this article describes the _in kernel_ interfaces, not the kernel to userspace interfaces. The kernel to userspace interface is the one that application programs use, the syscall interface. That interface is _very_ stable over time, and will not break. I have old programs that were built on a pre 0.9something kernel that still work just fine on the latest 2.6 kernel release. That interface is the one that users and application programmers can count on being stable. Executive Summary ----------------- You think you want a stable kernel interface, but you really do not, and you don't even know it. What you want is a stable running driver, and you get that only if your driver is in the main kernel tree. You also get lots of other good benefits if your driver is in the main kernel tree, all of which has made Linux into such a strong, stable, and mature operating system which is the reason you are using it in the first place. Intro ----- It's only the odd person who wants to write a kernel driver that needs to worry about the in-kernel interfaces changing. For the majority of the world, they neither see this interface, nor do they care about it at all. First off, I'm not going to address _any_ legal issues about closed source, hidden source, binary blobs, source wrappers, or any other term that describes kernel drivers that do not have their source code released under the GPL. Please consult a lawyer if you have any legal questions, I'm a programmer and hence, I'm just going to be describing the technical issues here (not to make light of the legal issues, they are real, and you do need to be aware of them at all times.) So, there are two main topics here, binary kernel interfaces and stable kernel source interfaces. They both depend on each other, but we will discuss the binary stuff first to get it out of the way. Binary Kernel Interface ----------------------- Assuming that we had a stable kernel source interface for the kernel, a binary interface would naturally happen too, right? Wrong. Please consider the following facts about the Linux kernel: - Depending on the version of the C compiler you use, different kernel data structures will contain different alignment of structures, and possibly include different functions in different ways (putting functions inline or not.) The individual function organization isn't that important, but the different data structure padding is very important. - Depending on what kernel build options you select, a wide range of different things can be assumed by the kernel: - different structures can contain different fields - Some functions may not be implemented at all, (i.e. some locks compile away to nothing for non-SMP builds.) - Memory within the kernel can be aligned in different ways, depending on the build options. - Linux runs on a wide range of different processor architectures. There is no way that binary drivers from one architecture will run on another architecture properly. Now a number of these issues can be addressed by simply compiling your module for the exact specific kernel configuration, using the same exact C compiler that the kernel was built with. This is sufficient if you want to provide a module for a specific release version of a specific Linux distribution. But multiply that single build by the number of different Linux distributions and the number of different supported releases of the Linux distribution and you quickly have a nightmare of different build options on different releases. Also realize that each Linux distribution release contains a number of different kernels, all tuned to different hardware types (different processor types and different options), so for even a single release you will need to create multiple versions of your module. Trust me, you will go insane over time if you try to support this kind of release, I learned this the hard way a long time ago... Stable Kernel Source Interfaces ------------------------------- This is a much more "volatile" topic if you talk to people who try to keep a Linux kernel driver that is no