The

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...

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 An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how t ...

(API) in the Linux kernel that are not to be confused: the "kernel–user space" API and the "kernel internal" API.

Linux API

The Linux API is the kernel–user space API, which 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 API, by choice, has been kept stable over the decades through a policy of not introducing breaking changes; this stability guarantees the portability of source code. At the same time, Linux kernel developers have historically been conservative and meticulous about introducing new system calls. Much available

free and open-source software Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...

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 systemd is a software suite that provides an array of system components for Linux operating systems. Its main aim is to unify service configuration and behavior across Linux distributions; Its primary component is a "system and service manager ...

and Weston. People such as Lennart Poettering openly advocate to prefer the Linux API over the POSIX API, where this offers advantages. At FOSDEM 2016,

Michael Kerrisk Michael Kerrisk is a technical author, programmer and, since 2004, maintainer of the Linux man-pages project, succeeding Andries Brouwer. He was born in 1961 in New Zealand and lives in Munich, Germany. Kerrisk has worked for Digital Equipment, ...

explained some of the perceived issues with the Linux kernel's user-space API, describing that it contains multiple design errors by being non-extensible, unmaintainable, overly complex, of limited purpose, in violation of standards, and inconsistent. Most of those mistakes cannot be fixed because doing so would break the ABI that the kernel presents to the user space.

System Call Interface of the Linux kernel

''System Call Interface'' is the denomination for the entirety of all implemented and available

system calls In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...

in a kernel. Various subsystems, such as e.g. the

Direct Rendering Manager The Direct Rendering Manager (DRM) is a subsystem of the Linux kernel responsible for interfacing with GPUs of modern video cards. DRM exposes an API that user-space programs can use to send commands and data to the GPU and perform operations su ...

(DRM), define their own system calls and the entirety is called System Call Interface. Various issues with the organization of the Linux kernel system calls are being publicly discussed. Issues have been pointed out by Andy Lutomirski, Michael Kerrisk and others.

The C standard library

Linux kernel System Call Interface and glibc

C standard library The C standard library or libc is the standard library for the C programming language, as specified in the ISO C standard.ISO/IEC (2018). '' ISO/IEC 9899:2018(E): Programming Languages - C §7'' Starting from the original ANSI C standard, it wa ...

is a wrapper around the system calls of the Linux kernel; the combination of the Linux kernel System Call Interface and a C standard library is what builds the Linux API. * glibc * uClibc * klibc * Newlib * musl * dietlibc * libbionic and libhybris

Additions to POSIX

As in other Unix-like systems, additional capabilities of the Linux kernel exist that are not part of POSIX: * cgroups subsystem, the system calls it introduces and libcgroup * The system calls of the

, especially the driver-private ioctls for 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), epoll,

splice 
Splice may refer to:
 Connections
* Rope splicing, joining two pieces of rope or cable by weaving the strands of each into the other
** Eye splice, a method of creating a permanent loop in the end of multi stranded rope by means of rope splicing
*  ...

dnotify 
dnotify is a file system event monitor for the Linux kernel, one of the subfeatures of the fcntl call. It was introduced in the 2.4 kernel series. It has been obsoleted by inotify, but will be retained for compatibility reasons.

Its function is es ...

, fanotify, and inotify have been exclusive to the Linux kernel so far. * The system call

getrandom 
Entropy-supplying system calls are  system calls in Unix-like operating system  kernels through which processes can obtain entropic or random data. The first of these was getentropy, introduced to the OpenBSD operating system in release 5.6 (Novemb ...

was introduced in version 3.17 of the Linux kernel mainline * memfd was proposed by the kdbus developers ** memfd_create was merged into the Linux kernel mainline in kernel version 3.17 * readahead initiates a file "read-ahead" into page cache 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.

Further libraries

* libdrm (for

) * libnl (The libnl suite is a collection of libraries providing APIs to netlink protocol based Linux kernel interfaces.) * libevdev (for evdev) * libasound ( Advanced Linux Sound Architecture) * ...

Linux ABI

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 In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ' ...

. 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 distribution A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading one ...

s and independent software vendors (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. An ABI has to be defined for every instruction set, such as x86, x86-64, MIPS, ARMv7-A (32-Bit), ARMv8-A (64-Bit), etc. with the endianness, if both are supported. 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

are e.g. GNU Compiler Collection, LLVM/ Clang. End-users are in fact not all interested in the Linux API (or the Windows API), but in the ABIs.

In-kernel APIs

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.

In-kernel ABIs

Since there are no stable in-kernel APIs, there cannot be stable in-kernel ABIs.

Abstraction APIs

For several use cases, the Linux API is considered too low-level and higher abstraction APIs are used. Such of course still need to work on top of the low-level Linux APIs. Examples: * implementation of the

OpenGL OpenGL (Open Graphics Library) is a cross-language, cross-platform application programming interface (API) for rendering 2D and 3D vector graphics. The API is typically used to interact with a graphics processing unit (GPU), to achieve hardwa ...

and Vulkan specifications in proprietary Linux graphics drivers and the free and open-source implementation in Mesa * implementation of the OpenAL specification * Simple DirectMedia Layer: abstraction API for input/sound/etc. available for many operating systems * Simple and Fast Multimedia Library: like above