<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
provides	O
several	O
interfaces	O
to	O
user-space	O
applications	O
that	O
are	O
used	O
for	O
different	O
purposes	O
and	O
that	O
have	O
different	O
properties	O
by	O
design	O
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
in	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
that	O
are	O
not	O
to	O
be	O
confused	O
:	O
the	O
"	O
kerneluser	O
space	O
"	O
API	B-Application
and	O
the	O
"	O
kernel	B-Operating_System
internal	O
"	O
API	B-Application
.	O
</s>
<s>
The	O
Linux	B-Operating_System
API	B-Application
is	O
the	O
kerneluser	O
space	O
API	B-Application
,	O
which	O
allows	O
programs	O
in	O
user	O
space	O
to	O
access	O
system	O
resources	O
and	O
services	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
It	O
is	O
composed	O
out	O
of	O
the	O
System	B-Operating_System
Call	I-Operating_System
Interface	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
and	O
the	O
subroutines	O
in	O
the	O
GNU	B-Language
C	I-Language
Library	I-Language
(	O
glibc	B-Language
)	O
.	O
</s>
<s>
The	O
focus	O
of	O
the	O
development	O
of	O
the	O
Linux	B-Operating_System
API	B-Application
has	O
been	O
to	O
provide	O
the	O
usable	O
features	O
of	O
the	O
specifications	O
defined	O
in	O
POSIX	O
in	O
a	O
way	O
which	O
is	O
reasonably	O
compatible	O
,	O
robust	O
and	O
performant	O
,	O
and	O
to	O
provide	O
additional	O
useful	O
features	O
not	O
defined	O
in	O
POSIX	O
,	O
just	O
as	O
the	O
kerneluser	O
space	O
APIs	B-Application
of	O
other	O
systems	O
implementing	O
the	O
POSIX	O
API	B-Application
also	O
provide	O
additional	O
features	O
not	O
defined	O
in	O
POSIX	O
.	O
</s>
<s>
The	O
Linux	B-Operating_System
API	B-Application
,	O
by	O
choice	O
,	O
has	O
been	O
kept	O
stable	O
over	O
the	O
decades	O
through	O
a	O
policy	O
of	O
not	O
introducing	O
breaking	O
changes	O
;	O
this	O
stability	O
guarantees	O
the	O
portability	O
of	O
source	O
code	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
Linux	B-Operating_System
kernel	I-Operating_System
developers	O
have	O
historically	O
been	O
conservative	O
and	O
meticulous	O
about	O
introducing	O
new	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
Much	O
available	O
free	B-License
and	I-License
open-source	I-License
software	I-License
is	O
written	O
for	O
the	O
POSIX	O
API	B-Application
.	O
</s>
<s>
Since	O
so	O
much	O
more	O
development	O
flows	O
into	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
as	O
compared	O
to	O
the	O
other	O
POSIX-compliant	O
combinations	O
of	O
kernel	B-Operating_System
and	O
C	B-Language
standard	I-Language
library	I-Language
,	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
and	O
its	O
API	B-Application
have	O
been	O
augmented	O
with	O
additional	O
features	O
.	O
</s>
<s>
As	O
far	O
as	O
these	O
additional	O
features	O
provide	O
a	O
technical	O
advantage	O
,	O
programming	O
for	O
the	O
Linux	B-Operating_System
API	B-Application
is	O
preferred	O
over	O
the	O
POSIX-API	O
.	O
</s>
<s>
Well-known	O
current	O
examples	O
are	O
udev	B-Application
,	O
systemd	B-Application
and	O
Weston	B-Application
.	O
</s>
<s>
People	O
such	O
as	O
Lennart	O
Poettering	O
openly	O
advocate	O
to	O
prefer	O
the	O
Linux	B-Operating_System
API	B-Application
over	O
the	O
POSIX	O
API	B-Application
,	O
where	O
this	O
offers	O
advantages	O
.	O
</s>
<s>
At	O
FOSDEM	O
2016	O
,	O
Michael	O
Kerrisk	O
explained	O
some	O
of	O
the	O
perceived	O
issues	O
with	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
's	O
user-space	O
API	B-Application
,	O
describing	O
that	O
it	O
contains	O
multiple	O
design	O
errors	O
by	O
being	O
non-extensible	O
,	O
unmaintainable	O
,	O
overly	O
complex	O
,	O
of	O
limited	O
purpose	O
,	O
in	O
violation	O
of	O
standards	O
,	O
and	O
inconsistent	O
.	O
</s>
<s>
Most	O
of	O
those	O
mistakes	O
cannot	O
be	O
fixed	O
because	O
doing	O
so	O
would	O
break	O
the	O
ABI	B-Operating_System
that	O
the	O
kernel	B-Operating_System
presents	O
to	O
the	O
user	O
space	O
.	O
</s>
<s>
System	B-Operating_System
Call	I-Operating_System
Interface	O
is	O
the	O
denomination	O
for	O
the	O
entirety	O
of	O
all	O
implemented	O
and	O
available	O
system	B-Operating_System
calls	I-Operating_System
in	O
a	O
kernel	B-Operating_System
.	O
</s>
<s>
the	O
Direct	B-Application
Rendering	I-Application
Manager	I-Application
(	O
DRM	B-Application
)	O
,	O
define	O
their	O
own	O
system	B-Operating_System
calls	I-Operating_System
and	O
the	O
entirety	O
is	O
called	O
System	B-Operating_System
Call	I-Operating_System
Interface	O
.	O
</s>
<s>
Various	O
issues	O
with	O
the	O
organization	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
system	B-Operating_System
calls	I-Operating_System
are	O
being	O
publicly	O
discussed	O
.	O
</s>
<s>
A	O
C	B-Language
standard	I-Language
library	I-Language
is	O
a	O
wrapper	O
around	O
the	O
system	B-Operating_System
calls	I-Operating_System
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
;	O
the	O
combination	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
System	B-Operating_System
Call	I-Operating_System
Interface	O
and	O
a	O
C	B-Language
standard	I-Language
library	I-Language
is	O
what	O
builds	O
the	O
Linux	B-Operating_System
API	B-Application
.	O
</s>
<s>
As	O
in	O
other	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
additional	O
capabilities	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
exist	O
that	O
are	O
not	O
part	O
of	O
POSIX	O
:	O
</s>
<s>
The	O
system	B-Operating_System
calls	I-Operating_System
of	O
the	O
Direct	B-Application
Rendering	I-Application
Manager	I-Application
,	O
especially	O
the	O
driver-private	O
ioctls	B-Operating_System
for	O
the	O
command	O
submission	O
,	O
are	O
not	O
part	O
of	O
the	O
POSIX	O
specifications	O
.	O
</s>
<s>
The	O
system	B-Operating_System
calls	I-Operating_System
futex	B-Application
(	O
fast	O
userspace	O
mutex	O
)	O
,	O
epoll	B-Application
,	O
splice	B-Application
,	O
dnotify	B-Application
,	O
fanotify	O
,	O
and	O
inotify	B-Application
have	O
been	O
exclusive	O
to	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
so	O
far	O
.	O
</s>
<s>
DRM	B-Application
has	O
been	O
paramount	O
for	O
the	O
development	O
and	O
implementations	O
of	O
well-defined	O
and	O
performant	O
free	B-License
and	I-License
open-source	I-License
graphics	O
device	O
drivers	B-Device
without	O
which	O
no	O
rendering	O
acceleration	O
would	O
be	O
available	O
at	O
all	O
,	O
or	O
even	O
worse	O
,	O
only	O
the	O
2D	O
drivers	B-Device
would	O
be	O
available	O
in	O
the	O
X.Org	B-Operating_System
Server	I-Operating_System
.	O
</s>
<s>
DRM	B-Application
was	O
developed	O
for	O
Linux	B-Operating_System
,	O
and	O
since	O
has	O
been	O
ported	O
to	O
other	O
operating	O
systems	O
as	O
well	O
.	O
</s>
<s>
libnl	O
(	O
The	O
libnl	O
suite	O
is	O
a	O
collection	O
of	O
libraries	O
providing	O
APIs	B-Application
to	O
netlink	B-Application
protocol	O
based	O
Linux	B-Application
kernel	I-Application
interfaces	I-Application
.	O
)	O
</s>
<s>
The	O
term	O
Linux	B-Operating_System
ABI	B-Operating_System
refers	O
to	O
a	O
kernel	B-Operating_System
–	O
user	O
space	O
ABI	B-Operating_System
.	O
</s>
<s>
The	O
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
refers	O
to	O
the	O
compiled	O
binaries	O
,	O
in	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
Any	O
such	O
ABI	B-Operating_System
is	O
therefore	O
bound	O
to	O
the	O
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
Defining	O
a	O
useful	O
ABI	B-Operating_System
and	O
keeping	O
it	O
stable	O
is	O
less	O
the	O
responsibility	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
developers	O
or	O
of	O
the	O
developers	O
of	O
the	O
GNU	B-Language
C	I-Language
Library	I-Language
,	O
and	O
more	O
the	O
task	O
for	O
Linux	B-Application
distributions	I-Application
and	O
independent	B-Application
software	I-Application
vendors	I-Application
(	O
ISVs	O
)	O
who	O
wish	O
to	O
sell	O
and	O
provide	O
support	O
for	O
their	O
proprietary	O
software	O
as	O
binaries	O
only	O
for	O
such	O
a	O
single	O
Linux	B-Operating_System
ABI	B-Operating_System
,	O
as	O
opposed	O
to	O
supporting	O
multiple	O
Linux	B-Operating_System
ABIs	O
.	O
</s>
<s>
An	O
ABI	B-Operating_System
has	O
to	O
be	O
defined	O
for	O
every	O
instruction	B-General_Concept
set	I-General_Concept
,	O
such	O
as	O
x86	B-Operating_System
,	O
x86-64	B-Device
,	O
MIPS	B-Device
,	O
ARMv7-A	O
(	O
32-Bit	O
)	O
,	O
ARMv8-A	O
(	O
64-Bit	O
)	O
,	O
etc	O
.	O
</s>
<s>
It	O
should	O
be	O
able	O
to	O
compile	O
the	O
software	O
with	O
different	O
compilers	O
against	O
the	O
definitions	O
specified	O
in	O
the	O
ABI	B-Operating_System
and	O
achieve	O
full	O
binary	O
compatibility	O
.	O
</s>
<s>
Compilers	O
that	O
are	O
free	B-License
and	I-License
open-source	I-License
software	I-License
are	O
e.g.	O
</s>
<s>
GNU	B-Application
Compiler	I-Application
Collection	I-Application
,	O
LLVM/Clang	O
.	O
</s>
<s>
End-users	O
are	O
in	O
fact	O
not	O
all	O
interested	O
in	O
the	O
Linux	B-Operating_System
API	B-Application
(	O
or	O
the	O
Windows	B-Library
API	I-Library
)	O
,	O
but	O
in	O
the	O
ABIs	O
.	O
</s>
<s>
There	O
are	O
a	O
lot	O
of	O
kernel-internal	O
APIs	B-Application
for	O
all	O
the	O
subsystems	O
to	O
interface	O
with	O
one	O
another	O
.	O
</s>
<s>
In	O
case	O
new	O
research	O
or	O
insights	O
make	O
a	O
change	O
seem	O
favorable	O
,	O
an	O
API	B-Application
is	O
changed	O
,	O
all	O
necessary	O
rewrite	O
and	O
testing	O
have	O
to	O
be	O
done	O
by	O
the	O
author	O
.	O
</s>
<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
is	O
a	O
monolithic	O
kernel	B-Operating_System
,	O
hence	O
device	O
drivers	B-Device
are	O
kernel	B-Operating_System
components	O
.	O
</s>
<s>
To	O
ease	O
the	O
burden	O
of	O
companies	O
maintaining	O
their	O
(	O
proprietary	O
)	O
device	O
drivers	B-Device
out-of-tree	O
,	O
stable	O
APIs	B-Application
for	O
the	O
device	O
drivers	B-Device
have	O
been	O
repeatedly	O
requested	O
.	O
</s>
<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
developers	O
have	O
repeatedly	O
denied	O
guaranteeing	O
stable	O
in-kernel	O
APIs	B-Application
for	O
device	O
drivers	B-Device
.	O
</s>
<s>
Guaranteeing	O
such	O
would	O
have	O
faltered	O
the	O
development	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
in	O
the	O
past	O
and	O
would	O
still	O
in	O
the	O
future	O
and	O
,	O
due	O
to	O
the	O
nature	O
of	O
free	B-License
and	I-License
open-source	I-License
software	I-License
,	O
are	O
not	O
necessary	O
.	O
</s>
<s>
Ergo	O
,	O
by	O
choice	O
,	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
has	O
no	O
stable	O
in-kernel	O
API	B-Application
.	O
</s>
<s>
Since	O
there	O
are	O
no	O
stable	O
in-kernel	O
APIs	B-Application
,	O
there	O
cannot	O
be	O
stable	O
in-kernel	O
ABIs	O
.	O
</s>
<s>
For	O
several	O
use	O
cases	O
,	O
the	O
Linux	B-Operating_System
API	B-Application
is	O
considered	O
too	O
low-level	O
and	O
higher	O
abstraction	O
APIs	B-Application
are	O
used	O
.	O
</s>
<s>
Such	O
of	O
course	O
still	O
need	O
to	O
work	O
on	O
top	O
of	O
the	O
low-level	O
Linux	B-Operating_System
APIs	B-Application
.	O
</s>
<s>
Simple	B-Operating_System
DirectMedia	I-Operating_System
Layer	I-Operating_System
:	O
abstraction	O
API	B-Application
for	O
input/sound/etc	O
.	O
</s>
