<s>
In	O
computing	O
,	O
a	O
system	B-Operating_System
call	I-Operating_System
(	O
commonly	O
abbreviated	O
to	O
syscall	B-Operating_System
)	O
is	O
the	O
programmatic	O
way	O
in	O
which	O
a	O
computer	B-Application
program	I-Application
requests	O
a	O
service	O
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
on	O
which	O
it	O
is	O
executed	O
.	O
</s>
<s>
This	O
may	O
include	O
hardware-related	O
services	O
(	O
for	O
example	O
,	O
accessing	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
or	O
accessing	O
the	O
device	O
's	O
camera	O
)	O
,	O
creation	O
and	O
execution	O
of	O
new	O
processes	B-Operating_System
,	O
and	O
communication	O
with	O
integral	O
kernel	B-Operating_System
services	O
such	O
as	O
process	O
scheduling	O
.	O
</s>
<s>
System	B-Operating_System
calls	I-Operating_System
provide	O
an	O
essential	O
interface	O
between	O
a	O
process	O
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
In	O
most	O
systems	O
,	O
system	B-Operating_System
calls	I-Operating_System
can	O
only	O
be	O
made	O
from	O
userspace	B-Operating_System
processes	B-Operating_System
,	O
while	O
in	O
some	O
systems	O
,	O
OS/360	B-Application
and	I-Application
successors	I-Application
for	O
example	O
,	O
privileged	O
system	O
code	O
also	O
issues	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
The	O
architecture	B-General_Concept
of	O
most	O
modern	O
processors	O
,	O
with	O
the	O
exception	O
of	O
some	O
embedded	O
systems	O
,	O
involves	O
a	O
security	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
the	O
rings	B-Operating_System
model	O
specifies	O
multiple	O
privilege	B-Operating_System
levels	I-Operating_System
under	O
which	O
software	O
may	O
be	O
executed	O
:	O
a	O
program	O
is	O
usually	O
limited	O
to	O
its	O
own	O
address	B-General_Concept
space	I-General_Concept
so	O
that	O
it	O
cannot	O
access	O
or	O
modify	O
other	O
running	O
programs	O
or	O
the	O
operating	B-General_Concept
system	I-General_Concept
itself	O
,	O
and	O
is	O
usually	O
prevented	O
from	O
directly	O
manipulating	O
hardware	O
devices	O
(	O
e.g.	O
</s>
<s>
the	O
frame	B-Algorithm
buffer	I-Algorithm
or	O
network	B-Architecture
devices	O
)	O
.	O
</s>
<s>
However	O
,	O
many	O
applications	O
need	O
access	O
to	O
these	O
components	O
,	O
so	O
system	B-Operating_System
calls	I-Operating_System
are	O
made	O
available	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
provide	O
well-defined	O
,	O
safe	O
implementations	O
for	O
such	O
operations	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
executes	O
at	O
the	O
highest	O
level	O
of	O
privilege	O
,	O
and	O
allows	O
applications	O
to	O
request	O
services	O
via	O
system	B-Operating_System
calls	I-Operating_System
,	O
which	O
are	O
often	O
initiated	O
via	O
interrupts	B-Application
.	O
</s>
<s>
An	O
interrupt	B-Device
automatically	O
puts	O
the	O
CPU	O
into	O
some	O
elevated	O
privilege	O
level	O
and	O
then	O
passes	O
control	O
to	O
the	O
kernel	B-Operating_System
,	O
which	O
determines	O
whether	O
the	O
calling	O
program	O
should	O
be	O
granted	O
the	O
requested	O
service	O
.	O
</s>
<s>
If	O
the	O
service	O
is	O
granted	O
,	O
the	O
kernel	B-Operating_System
executes	O
a	O
specific	O
set	O
of	O
instructions	O
over	O
which	O
the	O
calling	O
program	O
has	O
no	O
direct	O
control	O
,	O
returns	O
the	O
privilege	O
level	O
to	O
that	O
of	O
the	O
calling	O
program	O
,	O
and	O
then	O
returns	O
control	O
to	O
the	O
calling	O
program	O
.	O
</s>
<s>
Generally	O
,	O
systems	O
provide	O
a	O
library	B-Library
or	O
API	B-Application
that	O
sits	O
between	O
normal	O
programs	O
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
On	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
that	O
API	B-Application
is	O
usually	O
part	O
of	O
an	O
implementation	O
of	O
the	O
C	B-Language
library	I-Language
(	O
libc	B-Language
)	O
,	O
such	O
as	O
glibc	B-Language
,	O
that	O
provides	O
wrapper	O
functions	O
for	O
the	O
system	B-Operating_System
calls	I-Operating_System
,	O
often	O
named	O
the	O
same	O
as	O
the	O
system	B-Operating_System
calls	I-Operating_System
they	O
invoke	O
.	O
</s>
<s>
On	O
Windows	B-Device
NT	I-Device
,	O
that	O
API	B-Application
is	O
part	O
of	O
the	O
Native	B-Application
API	I-Application
,	O
in	O
the	O
library	B-Library
;	O
this	O
is	O
an	O
undocumented	O
API	B-Application
used	O
by	O
implementations	O
of	O
the	O
regular	O
Windows	B-Library
API	I-Library
and	O
directly	O
used	O
by	O
some	O
system	O
programs	O
on	O
Windows	B-Application
.	O
</s>
<s>
The	O
library	B-Library
's	O
wrapper	O
functions	O
expose	O
an	O
ordinary	O
function	O
calling	O
convention	O
(	O
a	O
subroutine	O
call	O
on	O
the	O
assembly	B-Language
level	O
)	O
for	O
using	O
the	O
system	B-Operating_System
call	I-Operating_System
,	O
as	O
well	O
as	O
making	O
the	O
system	B-Operating_System
call	I-Operating_System
more	O
modular	B-Architecture
.	O
</s>
<s>
Here	O
,	O
the	O
primary	O
function	O
of	O
the	O
wrapper	O
is	O
to	O
place	O
all	O
the	O
arguments	O
to	O
be	O
passed	O
to	O
the	O
system	B-Operating_System
call	I-Operating_System
in	O
the	O
appropriate	O
processor	B-General_Concept
registers	I-General_Concept
(	O
and	O
maybe	O
on	O
the	O
call	B-General_Concept
stack	I-General_Concept
as	O
well	O
)	O
,	O
and	O
also	O
setting	O
a	O
unique	O
system	B-Operating_System
call	I-Operating_System
number	O
for	O
the	O
kernel	B-Operating_System
to	O
call	O
.	O
</s>
<s>
In	O
this	O
way	O
the	O
library	B-Library
,	O
which	O
exists	O
between	O
the	O
OS	O
and	O
the	O
application	O
,	O
increases	O
portability	B-Architecture
.	O
</s>
<s>
The	O
call	O
to	O
the	O
library	B-Library
function	I-Library
itself	O
does	O
not	O
cause	O
a	O
switch	O
to	O
kernel	B-Operating_System
mode	O
and	O
is	O
usually	O
a	O
normal	O
subroutine	O
call	O
(	O
using	O
,	O
for	O
example	O
,	O
a	O
"	O
CALL	O
"	O
assembly	B-Language
instruction	O
in	O
some	O
Instruction	B-General_Concept
set	I-General_Concept
architectures	I-General_Concept
(	O
ISAs	O
)	O
)	O
.	O
</s>
<s>
The	O
actual	O
system	B-Operating_System
call	I-Operating_System
does	O
transfer	O
control	O
to	O
the	O
kernel	B-Operating_System
(	O
and	O
is	O
more	O
implementation-dependent	O
and	O
platform-dependent	O
than	O
the	O
library	B-Library
call	O
abstracting	O
it	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
fork	B-Language
and	O
execve	B-Operating_System
are	O
C	B-Language
library	I-Language
functions	O
that	O
in	O
turn	O
execute	B-Operating_System
instructions	O
that	O
invoke	O
the	O
fork	B-Language
and	O
exec	B-Operating_System
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
Making	O
the	O
system	B-Operating_System
call	I-Operating_System
directly	O
in	O
the	O
application	O
code	O
is	O
more	O
complicated	O
and	O
may	O
require	O
embedded	O
assembly	B-Language
code	I-Language
to	O
be	O
used	O
(	O
in	O
C	B-Language
and	O
C++	B-Language
)	O
,	O
as	O
well	O
as	O
requiring	O
knowledge	O
of	O
the	O
low-level	O
binary	O
interface	O
for	O
the	O
system	B-Operating_System
call	I-Operating_System
operation	O
,	O
which	O
may	O
be	O
subject	O
to	O
change	O
over	O
time	O
and	O
thus	O
not	O
be	O
part	O
of	O
the	O
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
;	O
the	O
library	B-Library
functions	I-Library
are	O
meant	O
to	O
abstract	O
this	O
away	O
.	O
</s>
<s>
On	O
exokernel	B-Operating_System
based	O
systems	O
,	O
the	O
library	B-Library
is	O
especially	O
important	O
as	O
an	O
intermediary	O
.	O
</s>
<s>
On	O
exokernels	B-Operating_System
,	O
libraries	O
shield	O
user	O
applications	O
from	O
the	O
very	O
low	O
level	O
kernel	B-Operating_System
API	B-Application
,	O
and	O
provide	O
abstractions	B-Application
and	O
resource	B-General_Concept
management	O
.	O
</s>
<s>
IBM	O
's	O
OS/360	B-Application
,	O
DOS/360	B-Application
and	O
TSS/360	B-Application
implement	O
most	O
system	B-Operating_System
calls	I-Operating_System
through	O
a	O
library	B-Library
of	O
assembly	B-Language
language	I-Language
macros	O
,	O
although	O
there	O
are	O
a	O
few	O
services	O
with	O
a	O
call	O
linkage	O
.	O
</s>
<s>
This	O
reflects	O
their	O
origin	O
at	O
a	O
time	O
when	O
programming	O
in	O
assembly	B-Language
language	I-Language
was	O
more	O
common	O
than	O
high-level	B-Language
language	I-Language
usage	O
.	O
</s>
<s>
IBM	O
system	B-Operating_System
calls	I-Operating_System
were	O
therefore	O
not	O
directly	O
executable	O
by	O
high-level	B-Language
language	I-Language
programs	O
,	O
but	O
required	O
a	O
callable	O
assembly	B-Language
language	I-Language
wrapper	O
subroutine	O
.	O
</s>
<s>
Since	O
then	O
,	O
IBM	O
has	O
added	O
many	O
services	O
that	O
can	O
be	O
called	O
from	O
high	B-Language
level	I-Language
languages	I-Language
in	O
,	O
e.g.	O
,	O
z/OS	B-Application
and	O
z/VSE	B-Application
.	O
</s>
<s>
In	O
more	O
recent	O
release	O
of	O
MVS/SP	O
and	O
in	O
all	O
later	O
MVS	O
versions	O
,	O
some	O
system	B-Operating_System
call	I-Operating_System
macros	O
generate	O
Program	O
Call	O
(	O
PC	O
)	O
.	O
</s>
<s>
On	O
Unix	B-Application
,	O
Unix-like	B-Operating_System
and	O
other	O
POSIX-compliant	O
operating	B-General_Concept
systems	I-General_Concept
,	O
popular	O
system	B-Operating_System
calls	I-Operating_System
are	O
open	B-Language
,	O
read	B-Language
,	O
write	B-Language
,	O
close	B-Language
,	O
wait	B-Operating_System
,	O
exec	B-Operating_System
,	O
fork	B-Language
,	O
exit	B-Operating_System
,	O
and	O
kill	B-Operating_System
.	O
</s>
<s>
Many	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
have	O
hundreds	O
of	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
For	O
example	O
,	O
Linux	B-Operating_System
and	O
OpenBSD	B-Operating_System
each	O
have	O
over	O
300	O
different	O
calls	O
,	O
NetBSD	B-Device
has	O
close	B-Language
to	O
500	O
,	O
FreeBSD	B-Operating_System
has	O
over	O
500	O
,	O
Windows	B-Application
has	O
close	B-Language
to	O
2000	O
,	O
divided	O
between	O
win32k	O
(	O
graphical	O
)	O
and	O
ntdll	B-Application
(	O
core	O
)	O
system	B-Operating_System
calls	I-Operating_System
while	O
Plan	B-Operating_System
9	I-Operating_System
has	O
51	O
.	O
</s>
<s>
Tools	O
such	O
as	O
strace	B-Language
,	O
ftrace	B-Application
and	O
truss	B-Device
allow	O
a	O
process	O
to	O
execute	B-Operating_System
from	O
start	O
and	O
report	O
all	O
system	B-Operating_System
calls	I-Operating_System
the	O
process	O
invokes	O
,	O
or	O
can	O
attach	O
to	O
an	O
already	O
running	O
process	O
and	O
intercept	O
any	O
system	B-Operating_System
call	I-Operating_System
made	O
by	O
the	O
said	O
process	O
if	O
the	O
operation	O
does	O
not	O
violate	O
the	O
permissions	O
of	O
the	O
user	O
.	O
</s>
<s>
This	O
special	O
ability	O
of	O
the	O
program	O
is	O
usually	O
also	O
implemented	O
with	O
system	B-Operating_System
calls	I-Operating_System
such	O
as	O
ptrace	B-Operating_System
or	O
system	B-Operating_System
calls	I-Operating_System
on	O
files	O
in	O
procfs	B-Application
.	O
</s>
<s>
Implementing	O
system	B-Operating_System
calls	I-Operating_System
requires	O
a	O
transfer	O
of	O
control	O
from	O
user	B-Operating_System
space	I-Operating_System
to	O
kernel	B-Operating_System
space	I-Operating_System
,	O
which	O
involves	O
some	O
sort	O
of	O
architecture-specific	O
feature	O
.	O
</s>
<s>
A	O
typical	O
way	O
to	O
implement	O
this	O
is	O
to	O
use	O
a	O
software	O
interrupt	B-Device
or	O
trap	B-Application
.	O
</s>
<s>
Interrupts	B-Application
transfer	O
control	O
to	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
,	O
so	O
software	O
simply	O
needs	O
to	O
set	O
up	O
some	O
register	B-General_Concept
with	O
the	O
system	B-Operating_System
call	I-Operating_System
number	O
needed	O
,	O
and	O
execute	B-Operating_System
the	O
software	O
interrupt	B-Device
.	O
</s>
<s>
This	O
is	O
the	O
only	O
technique	O
provided	O
for	O
many	O
RISC	B-Architecture
processors	I-Architecture
,	O
but	O
CISC	B-Architecture
architectures	I-Architecture
such	O
as	O
x86	B-Operating_System
support	O
additional	O
techniques	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
x86	B-Operating_System
instruction	B-General_Concept
set	I-General_Concept
contains	O
the	O
instructions	O
SYSCALL/SYSRET	O
and	O
SYSENTER/SYSEXIT	O
(	O
these	O
two	O
mechanisms	O
were	O
independently	O
created	O
by	O
AMD	O
and	O
Intel	O
,	O
respectively	O
,	O
but	O
in	O
essence	O
they	O
do	O
the	O
same	O
thing	O
)	O
.	O
</s>
<s>
These	O
are	O
"	O
fast	O
"	O
control	O
transfer	O
instructions	O
that	O
are	O
designed	O
to	O
quickly	O
transfer	O
control	O
to	O
the	O
kernel	B-Operating_System
for	O
a	O
system	B-Operating_System
call	I-Operating_System
without	O
the	O
overhead	O
of	O
an	O
interrupt	B-Device
.	O
</s>
<s>
Linux	B-Operating_System
2.5	O
began	O
using	O
this	O
on	O
the	O
x86	B-Operating_System
,	O
where	O
available	O
;	O
formerly	O
it	O
used	O
the	O
INT	B-Device
instruction	O
,	O
where	O
the	O
system	B-Operating_System
call	I-Operating_System
number	O
was	O
placed	O
in	O
the	O
EAX	O
register	B-General_Concept
before	O
interrupt	B-Device
0x80	O
was	O
executed	O
.	O
</s>
<s>
An	O
older	O
mechanism	O
is	O
the	O
call	B-Device
gate	I-Device
;	O
originally	O
used	O
in	O
Multics	B-Application
and	O
later	O
,	O
for	O
example	O
,	O
see	O
call	B-Device
gate	I-Device
on	O
the	O
Intel	B-Operating_System
x86	I-Operating_System
.	O
</s>
<s>
It	O
allows	O
a	O
program	O
to	O
call	O
a	O
kernel	B-Operating_System
function	O
directly	O
using	O
a	O
safe	O
control	O
transfer	O
mechanism	O
,	O
which	O
the	O
operating	B-General_Concept
system	I-General_Concept
sets	O
up	O
in	O
advance	O
.	O
</s>
<s>
This	O
approach	O
has	O
been	O
unpopular	O
on	O
x86	B-Operating_System
,	O
presumably	O
due	O
to	O
the	O
requirement	O
of	O
a	O
far	O
call	O
(	O
a	O
call	O
to	O
a	O
procedure	O
located	O
in	O
a	O
different	O
segment	O
than	O
the	O
current	O
code	O
segment	O
)	O
which	O
uses	O
x86	B-Device
memory	I-Device
segmentation	I-Device
and	O
the	O
resulting	O
lack	O
of	O
portability	B-Architecture
it	O
causes	O
,	O
and	O
the	O
existence	O
of	O
the	O
faster	O
instructions	O
mentioned	O
above	O
.	O
</s>
<s>
For	O
IA-64	B-General_Concept
architecture	B-General_Concept
,	O
EPC	O
(	O
Enter	O
Privileged	O
Code	O
)	O
instruction	O
is	O
used	O
.	O
</s>
<s>
The	O
first	O
eight	O
system	B-Operating_System
call	I-Operating_System
arguments	O
are	O
passed	O
in	O
registers	O
,	O
and	O
the	O
rest	O
are	O
passed	O
on	O
the	O
stack	O
.	O
</s>
<s>
In	O
the	O
IBM	B-Application
System/360	I-Application
mainframe	O
family	O
,	O
and	O
its	O
successors	O
,	O
a	O
Supervisor	B-Operating_System
Call	I-Operating_System
instruction	I-Operating_System
(	O
)	O
,	O
with	O
the	O
number	O
in	O
the	O
instruction	O
rather	O
than	O
in	O
a	O
register	B-General_Concept
,	O
implements	O
a	O
system	B-Operating_System
call	I-Operating_System
for	O
legacy	O
facilities	O
in	O
most	O
of	O
IBM	O
's	O
own	O
operating	B-General_Concept
systems	I-General_Concept
,	O
and	O
for	O
all	O
system	B-Operating_System
calls	I-Operating_System
in	O
Linux	B-Operating_System
.	O
</s>
<s>
In	O
particular	O
,	O
PC	O
is	O
used	O
when	O
the	O
caller	O
might	O
be	O
in	O
Service	B-Application
Request	I-Application
Block	I-Application
(	O
SRB	O
)	O
mode	O
.	O
</s>
<s>
The	O
PDP-11	B-Device
minicomputer	B-Architecture
used	O
the	O
and	O
instructions	O
,	O
which	O
,	O
similar	O
to	O
the	O
IBM	B-Application
System/360	I-Application
and	O
x86	B-Operating_System
,	O
put	O
the	O
code	O
in	O
the	O
instruction	O
;	O
they	O
generate	O
interrupts	B-Application
to	O
specific	O
addresses	O
,	O
transferring	O
control	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
VAX	B-Device
32-bit	O
successor	O
to	O
the	O
PDP-11	B-Device
series	O
used	O
the	O
,	O
,	O
and	O
instructions	O
to	O
make	O
system	B-Operating_System
calls	I-Operating_System
to	O
privileged	O
code	O
at	O
various	O
levels	O
;	O
the	O
code	O
is	O
an	O
argument	O
to	O
the	O
instruction	O
.	O
</s>
<s>
System	B-Operating_System
calls	I-Operating_System
can	O
be	O
grouped	O
roughly	O
into	O
six	O
major	O
categories	O
:	O
</s>
<s>
System	B-Operating_System
calls	I-Operating_System
in	O
most	O
Unix-like	B-Operating_System
systems	I-Operating_System
are	O
processed	O
in	O
kernel	B-Operating_System
mode	O
,	O
which	O
is	O
accomplished	O
by	O
changing	O
the	O
processor	O
execution	O
mode	O
to	O
a	O
more	O
privileged	O
one	O
,	O
but	O
no	O
process	O
context	B-Operating_System
switch	I-Operating_System
is	O
necessary	O
although	O
a	O
privilege	O
context	B-Operating_System
switch	I-Operating_System
does	O
occur	O
.	O
</s>
<s>
The	O
hardware	O
sees	O
the	O
world	O
in	O
terms	O
of	O
the	O
execution	O
mode	O
according	O
to	O
the	O
processor	O
status	B-General_Concept
register	I-General_Concept
,	O
and	O
processes	B-Operating_System
are	O
an	O
abstraction	B-Application
provided	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
A	O
system	B-Operating_System
call	I-Operating_System
does	O
not	O
generally	O
require	O
a	O
context	B-Operating_System
switch	I-Operating_System
to	O
another	O
process	O
;	O
instead	O
,	O
it	O
is	O
processed	O
in	O
the	O
context	O
of	O
whichever	O
process	O
invoked	O
it	O
.	O
</s>
<s>
In	O
a	O
multithreaded	B-General_Concept
process	O
,	O
system	B-Operating_System
calls	I-Operating_System
can	O
be	O
made	O
from	O
multiple	O
threads	B-Operating_System
.	O
</s>
<s>
The	O
handling	O
of	O
such	O
calls	O
is	O
dependent	O
on	O
the	O
design	O
of	O
the	O
specific	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
and	O
the	O
application	O
runtime	O
environment	O
.	O
</s>
<s>
The	O
following	O
list	O
shows	O
typical	O
models	O
followed	O
by	O
operating	B-General_Concept
systems	I-General_Concept
:	O
</s>
<s>
Many-to-one	O
model	O
:	O
All	O
system	B-Operating_System
calls	I-Operating_System
from	O
any	O
user	O
thread	B-Operating_System
in	O
a	O
process	O
are	O
handled	O
by	O
a	O
single	O
kernel-level	O
thread	B-Operating_System
.	O
</s>
<s>
This	O
model	O
has	O
a	O
serious	O
drawback	O
any	O
blocking	O
system	B-Operating_System
call	I-Operating_System
(	O
like	O
awaiting	O
input	O
from	O
the	O
user	O
)	O
can	O
freeze	O
all	O
the	O
other	O
threads	B-Operating_System
.	O
</s>
<s>
Also	O
,	O
since	O
only	O
one	O
thread	B-Operating_System
can	O
access	O
the	O
kernel	B-Operating_System
at	O
a	O
time	O
,	O
this	O
model	O
cannot	O
utilize	O
multiple	O
cores	O
of	O
processors	O
.	O
</s>
<s>
One-to-one	O
model	O
:	O
Every	O
user	O
thread	B-Operating_System
gets	O
attached	O
to	O
a	O
distinct	O
kernel-level	O
thread	B-Operating_System
during	O
a	O
system	B-Operating_System
call	I-Operating_System
.	O
</s>
<s>
This	O
model	O
solves	O
the	O
above	O
problem	O
of	O
blocking	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
It	O
is	O
found	O
in	O
all	O
major	O
Linux	B-Application
distributions	I-Application
,	O
macOS	B-Application
,	O
iOS	B-Application
,	O
recent	O
Windows	B-Application
and	O
Solaris	B-Application
versions	O
.	O
</s>
<s>
Many-to-many	O
model	O
:	O
In	O
this	O
model	O
,	O
a	O
pool	O
of	O
user	O
threads	B-Operating_System
is	O
mapped	O
to	O
a	O
pool	O
of	O
kernel	B-Operating_System
threads	B-Operating_System
.	O
</s>
<s>
All	O
system	B-Operating_System
calls	I-Operating_System
from	O
a	O
user	O
thread	B-Operating_System
pool	I-Operating_System
are	O
handled	O
by	O
the	O
threads	B-Operating_System
in	O
their	O
corresponding	O
kernel	B-Operating_System
thread	B-Operating_System
pool	I-Operating_System
.	O
</s>
<s>
Hybrid	O
model	O
:	O
This	O
model	O
implements	O
both	O
many	O
to	O
many	O
and	O
one	O
to	O
one	O
models	O
depending	O
upon	O
the	O
choice	O
made	O
by	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
is	O
found	O
in	O
old	O
versions	O
of	O
IRIX	B-Operating_System
,	O
HP-UX	B-Application
and	O
Solaris	B-Application
.	O
</s>
