<s>
In	O
computing	O
,	O
a	O
segmentation	B-General_Concept
fault	B-Application
(	O
often	O
shortened	O
to	O
segfault	O
)	O
or	O
access	O
violation	O
is	O
a	O
fault	B-Application
,	O
or	O
failure	O
condition	O
,	O
raised	O
by	O
hardware	O
with	O
memory	B-General_Concept
protection	I-General_Concept
,	O
notifying	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
the	O
software	O
has	O
attempted	O
to	O
access	O
a	O
restricted	O
area	O
of	O
memory	B-General_Concept
(	O
a	O
memory	B-General_Concept
access	O
violation	O
)	O
.	O
</s>
<s>
On	O
standard	O
x86	B-Operating_System
computers	O
,	O
this	O
is	O
a	O
form	O
of	O
general	B-Operating_System
protection	I-Operating_System
fault	I-Operating_System
.	O
</s>
<s>
The	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
will	O
,	O
in	O
response	O
,	O
usually	O
perform	O
some	O
corrective	O
action	O
,	O
generally	O
passing	O
the	O
fault	B-Application
on	O
to	O
the	O
offending	O
process	B-Operating_System
by	O
sending	O
the	O
process	B-Operating_System
a	O
signal	B-Operating_System
.	O
</s>
<s>
Processes	O
can	O
in	O
some	O
cases	O
install	O
a	O
custom	O
signal	B-Operating_System
handler	I-Operating_System
,	O
allowing	O
them	O
to	O
recover	O
on	O
their	O
own	O
,	O
but	O
otherwise	O
the	O
OS	O
default	O
signal	B-Operating_System
handler	I-Operating_System
is	O
used	O
,	O
generally	O
causing	O
abnormal	B-Application
termination	I-Application
of	O
the	O
process	B-Operating_System
(	O
a	O
program	B-General_Concept
crash	I-General_Concept
)	O
,	O
and	O
sometimes	O
a	O
core	B-Error_Name
dump	I-Error_Name
.	O
</s>
<s>
Segmentation	B-General_Concept
faults	O
are	O
a	O
common	O
class	O
of	O
error	O
in	O
programs	O
written	O
in	O
languages	O
like	O
C	B-Language
that	O
provide	O
low-level	O
memory	B-General_Concept
access	O
and	O
few	O
to	O
no	O
safety	O
checks	O
.	O
</s>
<s>
They	O
arise	O
primarily	O
due	O
to	O
errors	O
in	O
use	O
of	O
pointers	O
for	O
virtual	B-Architecture
memory	I-Architecture
addressing	O
,	O
particularly	O
illegal	O
access	O
.	O
</s>
<s>
Another	O
type	O
of	O
memory	B-General_Concept
access	O
error	O
is	O
a	O
bus	B-General_Concept
error	I-General_Concept
,	O
which	O
also	O
has	O
various	O
causes	O
,	O
but	O
is	O
today	O
much	O
rarer	O
;	O
these	O
occur	O
primarily	O
due	O
to	O
incorrect	O
physical	O
memory	B-General_Concept
addressing	I-General_Concept
,	O
or	O
due	O
to	O
misaligned	O
memory	B-General_Concept
access	O
–	O
these	O
are	O
memory	B-General_Concept
references	O
that	O
the	O
hardware	O
cannot	O
address	O
,	O
rather	O
than	O
references	O
that	O
a	O
process	B-Operating_System
is	O
not	O
allowed	O
to	O
address	O
.	O
</s>
<s>
Many	O
programming	O
languages	O
may	O
employ	O
mechanisms	O
designed	O
to	O
avoid	O
segmentation	B-General_Concept
faults	O
and	O
improve	O
memory	B-General_Concept
safety	O
.	O
</s>
<s>
For	O
example	O
,	O
Rust	B-Application
employs	O
an	O
ownership-based	O
model	O
to	O
ensure	O
memory	B-General_Concept
safety	O
.	O
</s>
<s>
Other	O
languages	O
,	O
such	O
as	O
Lisp	B-Language
and	O
Java	B-Language
,	O
employ	O
garbage	B-General_Concept
collection	I-General_Concept
,	O
which	O
avoids	O
certain	O
classes	O
of	O
memory	B-General_Concept
errors	O
that	O
could	O
lead	O
to	O
segmentation	B-General_Concept
faults	O
.	O
</s>
<s>
A	O
segmentation	B-General_Concept
fault	B-Application
occurs	O
when	O
a	O
program	O
attempts	O
to	O
access	O
a	O
memory	B-General_Concept
location	I-General_Concept
that	O
it	O
is	O
not	O
allowed	O
to	O
access	O
,	O
or	O
attempts	O
to	O
access	O
a	O
memory	B-General_Concept
location	I-General_Concept
in	O
a	O
way	O
that	O
is	O
not	O
allowed	O
(	O
for	O
example	O
,	O
attempting	O
to	O
write	O
to	O
a	O
read-only	B-Device
location	O
,	O
or	O
to	O
overwrite	O
part	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
)	O
.	O
</s>
<s>
The	O
term	O
"	O
segmentation	B-General_Concept
"	O
has	O
various	O
uses	O
in	O
computing	O
;	O
in	O
the	O
context	O
of	O
"	O
segmentation	B-General_Concept
fault	B-Application
"	O
,	O
a	O
term	O
used	O
since	O
the	O
1950s	O
,	O
it	O
refers	O
to	O
the	O
address	O
space	O
of	O
a	O
program	O
.	O
</s>
<s>
With	O
memory	B-General_Concept
protection	I-General_Concept
,	O
only	O
the	O
program	O
's	O
own	O
address	O
space	O
is	O
readable	O
,	O
and	O
of	O
this	O
,	O
only	O
the	O
stack	B-General_Concept
and	O
the	O
read/write	O
portion	O
of	O
the	O
data	B-General_Concept
segment	I-General_Concept
of	O
a	O
program	O
are	O
writable	O
,	O
while	O
read-only	B-Device
data	O
and	O
the	O
code	B-General_Concept
segment	I-General_Concept
are	O
not	O
writable	O
.	O
</s>
<s>
Thus	O
attempting	O
to	O
read	O
outside	O
of	O
the	O
program	O
's	O
address	O
space	O
,	O
or	O
writing	O
to	O
a	O
read-only	B-Device
segment	O
of	O
the	O
address	O
space	O
,	O
results	O
in	O
a	O
segmentation	B-General_Concept
fault	B-Application
,	O
hence	O
the	O
name	O
.	O
</s>
<s>
On	O
systems	O
using	O
hardware	O
memory	B-General_Concept
segmentation	I-General_Concept
to	O
provide	O
virtual	B-Architecture
memory	I-Architecture
,	O
a	O
segmentation	B-General_Concept
fault	B-Application
occurs	O
when	O
the	O
hardware	O
detects	O
an	O
attempt	O
to	O
refer	O
to	O
a	O
non-existent	O
segment	O
,	O
or	O
to	O
refer	O
to	O
a	O
location	O
outside	O
the	O
bounds	O
of	O
a	O
segment	O
,	O
or	O
to	O
refer	O
to	O
a	O
location	O
in	O
a	O
fashion	O
not	O
allowed	O
by	O
the	O
permissions	O
granted	O
for	O
that	O
segment	O
.	O
</s>
<s>
On	O
systems	O
using	O
only	O
paging	B-Architecture
,	O
an	O
invalid	O
page	O
fault	B-Application
generally	O
leads	O
to	O
a	O
segmentation	B-General_Concept
fault	B-Application
,	O
and	O
segmentation	B-General_Concept
faults	O
and	O
page	O
faults	O
are	O
both	O
faults	O
raised	O
by	O
the	O
virtual	B-Architecture
memory	I-Architecture
management	O
system	O
.	O
</s>
<s>
Segmentation	B-General_Concept
faults	O
can	O
also	O
occur	O
independently	O
of	O
page	O
faults	O
:	O
illegal	O
access	O
to	O
a	O
valid	O
page	O
is	O
a	O
segmentation	B-General_Concept
fault	B-Application
,	O
but	O
not	O
an	O
invalid	O
page	O
fault	B-Application
,	O
and	O
segmentation	B-General_Concept
faults	O
can	O
occur	O
in	O
the	O
middle	O
of	O
a	O
page	O
(	O
hence	O
no	O
page	O
fault	B-Application
)	O
,	O
for	O
example	O
in	O
a	O
buffer	B-General_Concept
overflow	I-General_Concept
that	O
stays	O
within	O
a	O
page	O
but	O
illegally	O
overwrites	O
memory	B-General_Concept
.	O
</s>
<s>
At	O
the	O
hardware	O
level	O
,	O
the	O
fault	B-Application
is	O
initially	O
raised	O
by	O
the	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
on	O
illegal	O
access	O
(	O
if	O
the	O
referenced	O
memory	B-General_Concept
exists	O
)	O
,	O
as	O
part	O
of	O
its	O
memory	B-General_Concept
protection	I-General_Concept
feature	O
,	O
or	O
an	O
invalid	O
page	O
fault	B-Application
(	O
if	O
the	O
referenced	O
memory	B-General_Concept
does	O
not	O
exist	O
)	O
.	O
</s>
<s>
If	O
the	O
problem	O
is	O
not	O
an	O
invalid	O
logical	O
address	O
but	O
instead	O
an	O
invalid	O
physical	O
address	O
,	O
a	O
bus	B-General_Concept
error	I-General_Concept
is	O
raised	O
instead	O
,	O
though	O
these	O
are	O
not	O
always	O
distinguished	O
.	O
</s>
<s>
At	O
the	O
operating	B-General_Concept
system	I-General_Concept
level	O
,	O
this	O
fault	B-Application
is	O
caught	O
and	O
a	O
signal	B-Operating_System
is	O
passed	O
on	O
to	O
the	O
offending	O
process	B-Operating_System
,	O
activating	O
the	O
process	B-Operating_System
's	O
handler	O
for	O
that	O
signal	B-Operating_System
.	O
</s>
<s>
Different	O
operating	B-General_Concept
systems	I-General_Concept
have	O
different	O
signal	B-Operating_System
names	O
to	O
indicate	O
that	O
a	O
segmentation	B-General_Concept
fault	B-Application
has	O
occurred	O
.	O
</s>
<s>
On	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
a	O
signal	B-Operating_System
called	O
SIGSEGV	B-General_Concept
(	O
abbreviated	O
from	O
segmentation	B-General_Concept
violation	O
)	O
is	O
sent	O
to	O
the	O
offending	O
process	B-Operating_System
.	O
</s>
<s>
On	O
Microsoft	B-Application
Windows	I-Application
,	O
the	O
offending	O
process	B-Operating_System
receives	O
a	O
STATUS_ACCESS_VIOLATION	O
exception	B-General_Concept
.	O
</s>
<s>
The	O
conditions	O
under	O
which	O
segmentation	B-General_Concept
violations	O
occur	O
and	O
how	O
they	O
manifest	O
themselves	O
are	O
specific	O
to	O
hardware	O
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
:	O
different	O
hardware	O
raises	O
different	O
faults	O
for	O
given	O
conditions	O
,	O
and	O
different	O
operating	B-General_Concept
systems	I-General_Concept
convert	O
these	O
to	O
different	O
signals	O
that	O
are	O
passed	O
on	O
to	O
processes	O
.	O
</s>
<s>
The	O
proximate	O
cause	O
is	O
a	O
memory	B-General_Concept
access	O
violation	O
,	O
while	O
the	O
underlying	O
cause	O
is	O
generally	O
a	O
software	B-Error_Name
bug	I-Error_Name
of	O
some	O
sort	O
.	O
</s>
<s>
Determining	O
the	O
root	O
cause	O
–	O
debugging	O
the	O
bug	B-Error_Name
–	O
can	O
be	O
simple	O
in	O
some	O
cases	O
,	O
where	O
the	O
program	O
will	O
consistently	O
cause	O
a	O
segmentation	B-General_Concept
fault	B-Application
(	O
e.g.	O
,	O
dereferencing	O
a	O
null	O
pointer	O
)	O
,	O
while	O
in	O
other	O
cases	O
the	O
bug	B-Error_Name
can	O
be	O
difficult	O
to	O
reproduce	O
and	O
depend	O
on	O
memory	B-General_Concept
allocation	O
on	O
each	O
run	O
(	O
e.g.	O
,	O
dereferencing	O
a	O
dangling	B-Error_Name
pointer	I-Error_Name
)	O
.	O
</s>
<s>
The	O
following	O
are	O
some	O
typical	O
causes	O
of	O
a	O
segmentation	B-General_Concept
fault	B-Application
:	O
</s>
<s>
These	O
in	O
turn	O
are	O
often	O
caused	O
by	O
programming	B-Error_Name
errors	I-Error_Name
that	O
result	O
in	O
invalid	O
memory	B-General_Concept
access	O
:	O
</s>
<s>
Attempting	O
to	O
execute	O
a	O
program	O
that	O
does	O
not	O
compile	B-Language
correctly	O
.	O
</s>
<s>
(	O
Some	O
compilers	B-Language
will	O
output	O
an	O
executable	B-Application
file	I-Application
despite	O
the	O
presence	O
of	O
compile-time	O
errors	O
.	O
)	O
</s>
<s>
In	O
C	B-Language
code	O
,	O
segmentation	B-General_Concept
faults	O
most	O
often	O
occur	O
because	O
of	O
errors	O
in	O
pointer	O
use	O
,	O
particularly	O
in	O
C	B-Language
dynamic	I-Language
memory	I-Language
allocation	I-Language
.	O
</s>
<s>
Dereferencing	O
a	O
null	O
pointer	O
,	O
which	O
results	O
in	O
undefined	B-Language
behavior	I-Language
,	O
will	O
usually	O
cause	O
a	O
segmentation	B-General_Concept
fault	B-Application
.	O
</s>
<s>
This	O
is	O
because	O
a	O
null	O
pointer	O
cannot	O
be	O
a	O
valid	O
memory	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
wild	B-Error_Name
pointers	I-Error_Name
and	O
dangling	B-Error_Name
pointers	I-Error_Name
point	O
to	O
memory	B-General_Concept
that	O
may	O
or	O
may	O
not	O
exist	O
,	O
and	O
may	O
or	O
may	O
not	O
be	O
readable	O
or	O
writable	O
,	O
and	O
thus	O
can	O
result	O
in	O
transient	O
bugs	B-Error_Name
.	O
</s>
<s>
Dereferencing	O
any	O
of	O
these	O
variables	O
could	O
cause	O
a	O
segmentation	B-General_Concept
fault	B-Application
:	O
dereferencing	O
the	O
null	O
pointer	O
generally	O
will	O
cause	O
a	O
segfault	O
,	O
while	O
reading	O
from	O
the	O
wild	B-Error_Name
pointer	I-Error_Name
may	O
instead	O
result	O
in	O
random	O
data	O
but	O
no	O
segfault	O
,	O
and	O
reading	O
from	O
the	O
dangling	B-Error_Name
pointer	I-Error_Name
may	O
result	O
in	O
valid	O
data	O
for	O
a	O
while	O
,	O
and	O
then	O
random	O
data	O
as	O
it	O
is	O
overwritten	O
.	O
</s>
<s>
The	O
default	O
action	O
for	O
a	O
segmentation	B-General_Concept
fault	B-Application
or	O
bus	B-General_Concept
error	I-General_Concept
is	O
abnormal	B-Application
termination	I-Application
of	O
the	O
process	B-Operating_System
that	O
triggered	O
it	O
.	O
</s>
<s>
A	O
core	B-Error_Name
file	I-Error_Name
may	O
be	O
generated	O
to	O
aid	O
debugging	O
,	O
and	O
other	O
platform-dependent	O
actions	O
may	O
also	O
be	O
performed	O
.	O
</s>
<s>
For	O
example	O
,	O
Linux	B-Application
systems	O
using	O
the	O
grsecurity	O
patch	O
may	O
log	O
SIGSEGV	B-General_Concept
signals	O
in	O
order	O
to	O
monitor	O
for	O
possible	O
intrusion	O
attempts	O
using	O
buffer	B-General_Concept
overflows	I-General_Concept
.	O
</s>
<s>
On	O
some	O
systems	O
,	O
like	O
Linux	B-Application
and	O
Windows	O
,	O
it	O
is	O
possible	O
for	O
the	O
program	O
itself	O
to	O
handle	O
a	O
segmentation	B-General_Concept
fault	B-Application
.	O
</s>
<s>
Depending	O
on	O
the	O
architecture	O
and	O
operating	B-General_Concept
system	I-General_Concept
,	O
the	O
running	O
program	O
can	O
not	O
only	O
handle	O
the	O
event	O
but	O
may	O
extract	O
some	O
information	O
about	O
its	O
state	O
like	O
getting	O
a	O
stack	B-Language
trace	I-Language
,	O
processor	B-General_Concept
register	I-General_Concept
values	O
,	O
the	O
line	O
of	O
the	O
source	O
code	O
when	O
it	O
was	O
triggered	O
,	O
memory	B-General_Concept
address	I-General_Concept
that	O
was	O
invalidly	O
accessed	O
and	O
whether	O
the	O
action	O
was	O
a	O
read	O
or	O
a	O
write	O
.	O
</s>
<s>
Although	O
a	O
segmentation	B-General_Concept
fault	B-Application
generally	O
means	O
that	O
the	O
program	O
has	O
a	O
bug	B-Error_Name
that	O
needs	O
fixing	O
,	O
it	O
is	O
also	O
possible	O
to	O
intentionally	O
cause	O
such	O
failure	O
for	O
the	O
purposes	O
of	O
testing	O
,	O
debugging	O
and	O
also	O
to	O
emulate	O
platforms	O
where	O
direct	O
access	O
to	O
memory	B-General_Concept
is	O
needed	O
.	O
</s>
<s>
On	O
the	O
latter	O
case	O
,	O
the	O
system	O
must	O
be	O
able	O
to	O
allow	O
the	O
program	O
to	O
run	O
even	O
after	O
the	O
fault	B-Application
occurs	O
.	O
</s>
<s>
Writing	O
to	O
read-only	B-Device
memory	I-Device
raises	O
a	O
segmentation	B-General_Concept
fault	B-Application
.	O
</s>
<s>
At	O
the	O
level	O
of	O
code	O
errors	O
,	O
this	O
occurs	O
when	O
the	O
program	O
writes	O
to	O
part	O
of	O
its	O
own	O
code	B-General_Concept
segment	I-General_Concept
or	O
the	O
read-only	B-Device
portion	O
of	O
the	O
data	B-General_Concept
segment	I-General_Concept
,	O
as	O
these	O
are	O
loaded	O
by	O
the	O
OS	O
into	O
read-only	B-Device
memory	I-Device
.	O
</s>
<s>
Here	O
is	O
an	O
example	O
of	O
ANSI	O
C	B-Language
code	O
that	O
will	O
generally	O
cause	O
a	O
segmentation	B-General_Concept
fault	B-Application
on	O
platforms	O
with	O
memory	B-General_Concept
protection	I-General_Concept
.	O
</s>
<s>
It	O
attempts	O
to	O
modify	O
a	O
string	B-Language
literal	I-Language
,	O
which	O
is	O
undefined	B-Language
behavior	I-Language
according	O
to	O
the	O
ANSI	O
C	B-Language
standard	O
.	O
</s>
<s>
Most	O
compilers	B-Language
will	O
not	O
catch	O
this	O
at	O
compile	B-Language
time	O
,	O
and	O
instead	O
compile	B-Language
this	O
to	O
executable	B-Application
code	I-Application
that	O
will	O
crash	B-General_Concept
:	O
</s>
<s>
When	O
the	O
program	O
containing	O
this	O
code	O
is	O
compiled	B-Language
,	O
the	O
string	O
"	O
hello	O
world	O
"	O
is	O
placed	O
in	O
the	O
rodata	O
section	O
of	O
the	O
program	O
executable	B-Application
file	I-Application
:	O
the	O
read-only	B-Device
section	O
of	O
the	O
data	B-General_Concept
segment	I-General_Concept
.	O
</s>
<s>
When	O
loaded	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
places	O
it	O
with	O
other	O
strings	O
and	O
constant	O
data	O
in	O
a	O
read-only	B-Device
segment	O
of	O
memory	B-General_Concept
.	O
</s>
<s>
When	O
executed	O
,	O
a	O
variable	O
,	O
s	O
,	O
is	O
set	O
to	O
point	O
to	O
the	O
string	O
's	O
location	O
,	O
and	O
an	O
attempt	O
is	O
made	O
to	O
write	O
an	O
H	O
character	O
through	O
the	O
variable	O
into	O
the	O
memory	B-General_Concept
,	O
causing	O
a	O
segmentation	B-General_Concept
fault	B-Application
.	O
</s>
<s>
Compiling	B-Language
such	O
a	O
program	O
with	O
a	O
compiler	B-Language
that	O
does	O
not	O
check	O
for	O
the	O
assignment	O
of	O
read-only	B-Device
locations	O
at	O
compile	B-Language
time	O
,	O
and	O
running	O
it	O
on	O
a	O
Unix-like	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
produces	O
the	O
following	O
runtime	B-Library
error	I-Library
:	O
</s>
<s>
Backtrace	B-Language
of	O
the	O
core	B-Error_Name
file	I-Error_Name
from	O
GDB	B-Language
:	O
</s>
<s>
This	O
code	O
can	O
be	O
corrected	O
by	O
using	O
an	O
array	O
instead	O
of	O
a	O
character	O
pointer	O
,	O
as	O
this	O
allocates	O
memory	B-General_Concept
on	O
stack	B-General_Concept
and	O
initializes	O
it	O
to	O
the	O
value	O
of	O
the	O
string	B-Language
literal	I-Language
:	O
</s>
<s>
Even	O
though	O
string	B-Language
literals	I-Language
should	O
not	O
be	O
modified	O
(	O
this	O
has	O
undefined	B-Language
behavior	I-Language
in	O
the	O
C	B-Language
standard	O
)	O
,	O
in	O
C	B-Language
they	O
are	O
of	O
static	O
char	O
 [  ] 	O
type	O
,	O
so	O
there	O
is	O
no	O
implicit	O
conversion	O
in	O
the	O
original	O
code	O
(	O
which	O
points	O
a	O
char	O
*	O
at	O
that	O
array	O
)	O
,	O
while	O
in	O
C++	O
they	O
are	O
of	O
static	O
const	O
char	O
 [  ] 	O
type	O
,	O
and	O
thus	O
there	O
is	O
an	O
implicit	O
conversion	O
,	O
so	O
compilers	B-Language
will	O
generally	O
catch	O
this	O
particular	O
error	O
.	O
</s>
<s>
In	O
C	B-Language
and	O
C-like	O
languages	O
,	O
null	O
pointers	O
are	O
used	O
to	O
mean	O
"	O
pointer	O
to	O
no	O
object	O
"	O
and	O
as	O
an	O
error	O
indicator	O
,	O
and	O
dereferencing	O
a	O
null	O
pointer	O
(	O
a	O
read	O
or	O
write	O
through	O
a	O
null	O
pointer	O
)	O
is	O
a	O
very	O
common	O
program	O
error	O
.	O
</s>
<s>
The	O
C	B-Language
standard	O
does	O
not	O
say	O
that	O
the	O
null	O
pointer	O
is	O
the	O
same	O
as	O
the	O
pointer	O
to	O
memory	B-General_Concept
address0	O
,	O
though	O
that	O
may	O
be	O
the	O
case	O
in	O
practice	O
.	O
</s>
<s>
Most	O
operating	B-General_Concept
systems	I-General_Concept
map	O
the	O
null	O
pointer	O
's	O
address	O
such	O
that	O
accessing	O
it	O
causes	O
a	O
segmentation	B-General_Concept
fault	B-Application
.	O
</s>
<s>
This	O
behavior	O
is	O
not	O
guaranteed	O
by	O
the	O
C	B-Language
standard	O
.	O
</s>
<s>
Dereferencing	O
a	O
null	O
pointer	O
is	O
undefined	B-Language
behavior	I-Language
in	O
C	B-Language
,	O
and	O
a	O
conforming	O
implementation	O
is	O
allowed	O
to	O
assume	O
that	O
any	O
pointer	O
that	O
is	O
dereferenced	O
is	O
not	O
null	O
.	O
</s>
<s>
Doing	O
so	O
causes	O
a	O
segmentation	B-General_Concept
fault	B-Application
at	B-Library
runtime	I-Library
on	O
many	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Dereferencing	O
a	O
null	O
pointer	O
and	O
then	O
assigning	O
to	O
it	O
(	O
writing	O
a	O
value	O
to	O
a	O
non-existent	O
target	O
)	O
also	O
usually	O
causes	O
a	O
segmentation	B-General_Concept
fault	B-Application
:	O
</s>
<s>
The	O
following	O
code	O
includes	O
a	O
null	O
pointer	O
dereference	O
,	O
but	O
when	O
compiled	B-Language
will	O
often	O
not	O
result	O
in	O
a	O
segmentation	B-General_Concept
fault	B-Application
,	O
as	O
the	O
value	O
is	O
unused	O
and	O
thus	O
the	O
dereference	O
will	O
often	O
be	O
optimized	O
away	O
by	O
dead	O
code	O
elimination	O
:	O
</s>
<s>
Depending	O
on	O
the	O
compiler	B-Language
and	O
the	O
processor	O
,	O
this	O
may	O
result	O
in	O
a	O
segmentation	B-General_Concept
fault	B-Application
.	O
</s>
<s>
which	O
causes	O
the	O
stack	B-Error_Name
to	I-Error_Name
overflow	I-Error_Name
which	O
results	O
in	O
a	O
segmentation	B-General_Concept
fault	B-Application
.	O
</s>
<s>
Infinite	O
recursion	O
may	O
not	O
necessarily	O
result	O
in	O
a	O
stack	B-Error_Name
overflow	I-Error_Name
depending	O
on	O
the	O
language	O
,	O
optimizations	O
performed	O
by	O
the	O
compiler	B-Language
and	O
the	O
exact	O
structure	O
of	O
a	O
code	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
behavior	O
of	O
unreachable	O
code	O
(	O
the	O
return	O
statement	O
)	O
is	O
undefined	O
,	O
so	O
the	O
compiler	B-Language
can	O
eliminate	O
it	O
and	O
use	O
a	O
tail	B-Language
call	I-Language
optimization	I-Language
that	O
might	O
result	O
in	O
no	O
stack	B-General_Concept
usage	O
.	O
</s>
<s>
Other	O
optimizations	O
could	O
include	O
translating	O
the	O
recursion	O
into	O
iteration	O
,	O
which	O
given	O
the	O
structure	O
of	O
the	O
example	O
function	O
would	O
result	O
in	O
the	O
program	O
running	O
forever	O
,	O
while	O
probably	O
not	O
overflowing	O
its	O
stack	B-General_Concept
.	O
</s>
