<s>
PhotoRec	B-Language
is	O
a	O
free	B-License
and	I-License
open-source	I-License
utility	B-Application
software	I-Application
for	O
data	B-Application
recovery	I-Application
with	O
text-based	B-Application
user	I-Application
interface	I-Application
using	O
data	O
carving	O
techniques	O
,	O
designed	O
to	O
recover	O
lost	O
files	O
from	O
various	O
digital	O
camera	O
memory	O
,	O
hard	B-Device
disk	I-Device
and	O
CD-ROM	B-Device
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
add	O
custom	O
file	B-General_Concept
signature	I-General_Concept
to	O
detect	O
less	O
known	O
files	O
.	O
</s>
<s>
PhotoRec	B-Language
does	O
not	O
attempt	O
to	O
write	O
to	O
the	O
damaged	O
media	O
the	O
user	O
is	O
about	O
to	O
recover	O
from	O
.	O
</s>
<s>
Recovered	O
files	O
are	O
instead	O
written	O
to	O
the	O
directory	O
from	O
which	O
PhotoRec	B-Language
is	O
run	O
,	O
any	O
other	O
directory	O
may	O
be	O
chosen	O
.	O
</s>
<s>
It	O
can	O
be	O
used	O
for	O
data	B-Application
recovery	I-Application
or	O
in	O
a	O
digital	O
forensics	O
context	O
.	O
</s>
<s>
PhotoRec	B-Language
is	O
shipped	O
with	O
TestDisk	B-Application
.	O
</s>
<s>
FAT	B-Application
,	O
NTFS	B-Application
,	O
ext2/ext3/ext4	O
file	B-Application
systems	I-Application
store	O
files	O
in	O
data	O
blocks	O
(	O
also	O
called	O
data	B-Application
clusters	I-Application
under	O
Windows	O
)	O
.	O
</s>
<s>
The	O
cluster	O
or	O
block	O
size	O
remains	O
at	O
a	O
constant	O
number	O
of	O
sectors	O
after	O
being	O
initialized	O
during	O
the	O
formatting	O
of	O
the	O
filesystem	B-Application
.	O
</s>
<s>
In	O
general	O
,	O
most	O
operating	O
systems	O
try	O
to	O
store	O
the	O
data	O
in	O
a	O
contiguous	O
way	O
so	O
as	O
to	O
minimize	O
data	B-Architecture
fragmentation	I-Architecture
.	O
</s>
<s>
The	O
seek	O
time	O
of	O
mechanical	O
drives	O
is	O
significant	O
for	O
writing	O
and	O
reading	O
data	O
to/from	O
a	O
hard	B-Device
disk	I-Device
,	O
so	O
that	O
is	O
why	O
it	O
is	O
important	O
to	O
keep	O
the	O
fragmentation	B-Architecture
to	O
a	O
minimum	O
level	O
.	O
</s>
<s>
is	O
lost	O
;	O
e.g.	O
,	O
in	O
an	O
ext3/ext4	O
filesystem	O
,	O
the	O
names	O
of	O
deleted	O
files	O
are	O
still	O
present	O
,	O
but	O
the	O
location	O
of	O
the	O
first	O
data	O
block	O
is	O
removed	O
.	O
</s>
<s>
This	O
means	O
the	O
data	O
is	O
still	O
present	O
on	O
the	O
filesystem	B-Application
,	O
but	O
only	O
until	O
some	O
or	O
all	O
of	O
it	O
is	O
overwritten	O
by	O
new	O
file	O
data	O
.	O
</s>
<s>
To	O
recover	O
these	O
"	O
lost	O
"	O
files	O
,	O
PhotoRec	B-Language
first	O
tries	O
to	O
find	O
the	O
data	O
block	O
(	O
or	O
cluster	O
)	O
size	O
.	O
</s>
<s>
If	O
the	O
filesystem	B-Application
is	O
not	O
corrupted	O
,	O
this	O
value	O
can	O
be	O
read	O
from	O
the	O
superblock	O
(	O
ext2/ext3/ext4	O
)	O
or	O
volume	O
boot	O
record	O
(	O
FAT	B-Application
,	O
NTFS	B-Application
)	O
.	O
</s>
<s>
Otherwise	O
,	O
PhotoRec	B-Language
reads	O
the	O
media	O
,	O
sector	O
by	O
sector	O
,	O
searching	O
for	O
the	O
first	O
ten	O
files	O
,	O
from	O
which	O
it	O
calculates	O
the	O
block/cluster	O
size	O
from	O
their	O
locations	O
.	O
</s>
<s>
Once	O
this	O
block	O
size	O
is	O
known	O
,	O
PhotoRec	B-Language
reads	O
the	O
media	O
block	O
by	O
block	O
(	O
or	O
cluster	O
by	O
cluster	O
)	O
.	O
</s>
<s>
Each	O
block	O
is	O
checked	O
against	O
a	O
signature	O
database	O
;	O
which	O
comes	O
with	O
the	O
program	O
and	O
has	O
been	O
growing	O
in	O
the	O
type	O
of	O
files	O
it	O
can	O
recover	O
ever	O
since	O
PhotoRec	B-Language
's	O
first	O
version	O
came	O
out	O
.	O
</s>
<s>
It	O
is	O
a	O
common	O
data	B-Application
recovery	I-Application
method	O
called	O
file	O
carving	O
.	O
</s>
<s>
For	O
example	O
,	O
PhotoRec	B-Language
identifies	O
a	O
JPEG	O
file	O
when	O
a	O
block	O
begins	O
with	O
:	O
</s>
<s>
If	O
PhotoRec	B-Language
has	O
already	O
started	O
to	O
recover	O
a	O
file	O
,	O
it	O
stops	O
its	O
recovery	O
,	O
checks	O
the	O
consistency	O
of	O
the	O
file	O
when	O
possible	O
and	O
starts	O
to	O
save	O
the	O
new	O
file	O
(	O
which	O
it	O
determined	O
from	O
the	O
signature	O
it	O
found	O
)	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
PhotoRec	B-Language
can	O
learn	O
the	O
original	O
file	O
size	O
from	O
the	O
file	O
header	O
,	O
so	O
the	O
recovered	O
file	O
is	O
truncated	O
to	O
the	O
correct	O
size	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
PhotoRec	B-Language
parses	O
the	O
recovered	O
data	O
,	O
then	O
stops	O
the	O
recovery	O
when	O
the	O
stream	O
ends	O
.	O
</s>
<s>
When	O
a	O
file	O
is	O
recovered	O
successfully	O
,	O
PhotoRec	B-Language
checks	O
the	O
previous	O
data	O
blocks	O
to	O
see	O
whether	O
a	O
file	B-General_Concept
signature	I-General_Concept
was	O
found	O
but	O
the	O
file	O
was	O
not	O
able	O
to	O
be	O
successfully	O
recovered	O
(	O
i.e.	O
,	O
the	O
file	O
was	O
too	O
small	O
)	O
,	O
and	O
it	O
tries	O
again	O
.	O
</s>
<s>
PhotoRec	B-Language
is	O
compatible	O
with	O
:	O
</s>
<s>
PhotoRec	B-Language
and	O
TestDisk	B-Application
are	O
shipped	O
together	O
.	O
</s>
<s>
These	O
utilities	O
can	O
be	O
found	O
on	O
various	O
Linux	B-Application
Live	O
CDs	O
:	O
</s>
<s>
They	O
are	O
also	O
packaged	O
for	O
numerous	O
*	B-Operating_System
nix	I-Operating_System
(	O
mostly	O
Linux	B-Application
based	O
)	O
distributions	O
:	O
</s>
