Linux kernel interfaces
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The Linux kernel provides several interfaces to user-space applications that are used for different purposes and that have different properties by design. There are two types of application programming interface (API) in the Linux kernel that are not to be confused: the "kernel–user space" API and the "kernel internal" API.
The Linux API is the kernel–user space API. It is the API that allows programs in user space to access system resources and services of the Linux kernel. It is composed out of the System Call Interface of the Linux kernel and the subroutines in the GNU C Library (glibc). The focus of the development of the Linux API has been to provide the usable features of the specifications defined in POSIX in a way which is reasonably compatible, robust and performant, and to provide additional useful features not defined in POSIX, just as the kernel–user space APIs of other systems implementing the POSIX API also provide additional features not defined in POSIX. The Linux kernel developers have been the most conservative and meticulous about introducing new system calls.
The Linux API, by choice, has been kept stable over the decades and never breaks; this stability guarantees the portability of source code.
Much available free and open-source software is written for the POSIX API. Since so much more development flows into the Linux kernel as compared to the other POSIX-compliant combinations of kernel and C standard library, the Linux kernel and its API have been augmented with additional features. As far as these additional features provide a technical advantage, programming for the Linux API is preferred over the POSIX-API. Well-known current examples are udev, systemd and Weston. People such as Lennart Poettering openly advocate to prefer the Linux API over the POSIX API, where this offers advantages.
System Call Interface of the Linux kernel
System Call Interface is the denomination for the entirety of all implemented and available system calls in a kernel. Various subsystems, such as e.g. the DRM define their own system calls and the entirety is called System Call Interface.
The C standard library
The GNU C Library is a wrapper around the system calls of the Linux kernel. It is this combination of the Linux kernel System Call Interface and glibc that builds the Linux API.
Differences from POSIX
There are some deviations from POSIX, together with some POSIX augmentations:
- cgroups subsystem, the system calls it introduces and libcgroup
- The system calls of the Direct Rendering Manager, especially the driver-private ioctls for the the command submission are not part of the POSIX specifications.
- Advanced Linux Sound Architecture could set system calls, which are not part of the POSIX specifications
- The system calls
futex(fast userspace mutex),
inotifyhave been exclusive to the Linux kernel so far.
- The system call
getrandomis currently being discussed
memfdwas proposed by the kdbus developers
DRM has been paramount for the development and implementations of well-defined and performant free and open-source graphics device drivers without which no rendering acceleration would be available at all, or even worse, only the 2D drivers would be available in the X.Org Server. DRM was developed for Linux, and since has been ported to other operating systems as well.
POSIX was not a designed set of specifications, instead it was written ex post facto to describe a loosely similar set of competing systems solely for procurement purposes. The deviations and augmentations section should prove some of the inconsistencies and cavities in POSIX, as it has never been the intention of the Linux kernel developers to gather as many system calls as possible, but to define such a clean and efficient Kernel-to-userspace API, while achieving and maintaining a certain compatibility with POSIX.
The term Linux ABI refers to a kernel–user space ABI. The Application binary interface refers to the compiled binaries, in machine code. Any such ABI is therefore bound to the instruction set. Defining a useful ABI and keeping it stable is less the responsibility of the Linux kernel developers or of the developers of the GNU C Library, and more the task for Linux distributions and Independent software vendor (ISVs) who wish to sell and provide support for their proprietary software as binaries only for such a single Linux ABI, as opposed to supporting multiple Linux ABIs.
In the Wikipedia a category is maintained for articles on Category:Proprietary software for Linux.
It should be able to compile the software with different compilers against the definitions specified in the ABI and achieve full binary compatibility. Compilers that are free and open-source software are e.g. GNU Compiler Collection, LLVM/Clang.
There are a lot of kernel-internal APIs for all the subsystems to interface with one another. These are being kept fairly stable, but there is no guarantee for stability. In case new research or insights make a change seem favorable, an API is changed, all necessary rewrite and testing have to be done by the author.
The Linux kernel is a monolithic kernel, hence device drivers are kernel components. To ease the burden of companies maintaining their (proprietary) device drivers out-of-tree, stable APIs for the device drivers have been repeatedly requested. The Linux kernel developers have repeatedly denied guaranteeing stable in-kernel APIs for device drivers. Guaranteeing such would have faltered the development of the Linux kernel in the past and would still in the future and, due to the nature of free and open-source software, are not necessary! Ergo, by choice, the Linux kernel has no stable in-kernel API.
Since there are no stable in–kernel APIs, there cannot be stable in–kernel ABIs.
- The Linux Programming Interface by Michael Kerrisk
- Semaphore (programming)
- system call – is a function to facilitate programs to request services from the kernel
- netlink – socket family used for IPC between kernel and user space processes, designed as the successor of ioctl; Netlink was added by Alan Cox during Linux kernel 1.3 development as a character driver interface to provide multiple kernel and user-space bidirectional communications links. Then, Alexey Kuznetsov extended it during Linux kernel 2.1 development to provide a flexible and extensible messaging interface to the new advanced routing infrastructure. Since then, Netlink sockets have become one of the main interfaces that kernel subsystems provide to user-space applications in Linux. Modern WNIC drivers use it to communicate with user-space.
- Windows API – article on various API available on Microsoft Windows operating systems
- Wine – a compatibility layer between Linux and programs written for Microsoft Windows
- libhybris – compatibility layer between Linux and programs written for Android
- "ControlGroupInterface". freedesktop.org.
- "libevdev". freedesktop.org.
- "Choosing between portability and innovation". LWN.net. 2011-03-02.
- "Interview: Lennart Poettering - Lennart Poettering will give a talk about "Systemd: beyond init" at FOSDEM 2011.". fosdem.org. 2011. Retrieved 2014-06-16. "In fact, the way I see things the Linux API has been taking the role of the POSIX API and Linux is the focal point of all Free Software development. Due to that I can only recommend developers to try to hack with only Linux in mind and experience the freedom and the opportunities this offers you. So, get yourself a copy of The Linux Programming Interface, ignore everything it says about POSIX compatibility and hack away your amazing Linux software. It's quite relieving!"
- "[PATCH, RFC] random: introduce getrandom(2) system call". LKML. 2014-07-17.
- "NetBSD 7.0 Will Finally Have DRM/KMS Drivers". Phoronix. 2014-03-19.
- "The Linux Kernel Driver Interface".
- The API of Linux kernel 2.6.20 – sadly no current version available
- The Linux Programming Interface
- Interactive Linux kernel map with main API functions and structures
- Linux Device Drivers by Jonathan Corbet, Greg Kroah-Hartman and Alessandro Rubini, 3rd edition
- Linux Kernel Linked List Explained
- TLPI: API changes, Linux API changes since The Linux Programming Interface was released in 2010