<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
the	O
block	O
starting	O
symbol	O
(	O
abbreviated	O
to	O
.bss	B-General_Concept
or	O
bss	O
)	O
is	O
the	O
portion	O
of	O
an	O
object	B-Application
file	I-Application
,	O
executable	O
,	O
or	O
assembly	B-Language
language	I-Language
code	O
that	O
contains	O
statically	B-General_Concept
allocated	I-General_Concept
variables	I-General_Concept
that	O
are	O
declared	O
but	O
have	O
not	O
been	O
assigned	O
a	O
value	O
yet	O
.	O
</s>
<s>
It	O
is	O
often	O
referred	O
to	O
as	O
the	O
"	O
bss	O
section	O
"	O
or	O
"	O
bss	B-General_Concept
segment	I-General_Concept
"	O
.	O
</s>
<s>
Typically	O
only	O
the	O
length	O
of	O
the	O
bss	O
section	O
,	O
but	O
no	O
data	O
,	O
is	O
stored	O
in	O
the	O
object	B-Application
file	I-Application
.	O
</s>
<s>
The	O
program	B-Operating_System
loader	I-Operating_System
allocates	O
memory	O
for	O
the	O
bss	O
section	O
when	O
it	O
loads	O
the	O
program	O
.	O
</s>
<s>
By	O
placing	O
variables	O
with	O
no	O
value	O
in	O
the	O
.bss	B-General_Concept
section	O
,	O
instead	O
of	O
the	O
.data	B-General_Concept
or	O
.rodata	B-General_Concept
section	O
which	O
require	O
initial	O
value	O
data	O
,	O
the	O
size	O
of	O
the	O
object	B-Application
file	I-Application
is	O
reduced	O
.	O
</s>
<s>
Unix-like	B-Operating_System
systems	I-Operating_System
and	O
Windows	O
initialize	O
the	O
bss	O
section	O
to	O
zero	O
,	O
allowing	O
C	B-Language
and	O
C++	B-Language
statically	B-General_Concept
allocated	I-General_Concept
variables	I-General_Concept
initialized	O
to	O
values	O
represented	O
with	O
all	O
bits	O
zero	O
to	O
be	O
put	O
in	O
the	O
bss	B-General_Concept
segment	I-General_Concept
.	O
</s>
<s>
Operating	O
systems	O
may	O
use	O
a	O
technique	O
called	O
zero-fill-on-demand	O
to	O
efficiently	O
implement	O
the	O
bss	B-General_Concept
segment	I-General_Concept
.	O
</s>
<s>
In	O
embedded	O
software	O
,	O
the	O
bss	B-General_Concept
segment	I-General_Concept
is	O
mapped	O
into	O
memory	O
that	O
is	O
initialized	O
to	O
zero	O
by	O
the	O
C	B-Language
run-time	B-Device
system	I-Device
before	O
main( )	O
is	O
entered	O
.	O
</s>
<s>
Some	O
C	B-Language
run-time	B-Device
systems	I-Device
may	O
allow	O
part	O
of	O
the	O
bss	B-General_Concept
segment	I-General_Concept
not	O
to	O
be	O
initialized	O
;	O
C	B-Language
variables	O
must	O
explicitly	O
be	O
placed	O
into	O
that	O
portion	O
of	O
the	O
bss	B-General_Concept
segment	I-General_Concept
.	O
</s>
<s>
On	O
some	O
computer	B-General_Concept
architectures	I-General_Concept
,	O
the	O
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
also	O
supports	O
an	O
sbss	O
segment	O
for	O
"	O
small	O
data	O
"	O
.	O
</s>
<s>
Architectures	O
supporting	O
thread-local	O
storage	O
might	O
use	O
a	O
tbss	O
section	O
for	O
uninitialized	O
,	O
static	B-Language
data	O
marked	O
as	O
thread-local	O
.	O
</s>
<s>
Historically	O
,	O
BSS	O
(	O
from	O
Block	B-General_Concept
Started	I-General_Concept
by	I-General_Concept
Symbol	I-General_Concept
)	O
is	O
a	O
pseudo-operation	O
in	O
UA-SAP	B-Language
(	O
United	O
Aircraft	O
Symbolic	B-Language
Assembly	I-Language
Program	I-Language
)	O
,	O
the	O
assembler	B-Language
developed	O
in	O
the	O
mid-1950s	O
for	O
the	O
IBM	B-Device
704	I-Device
by	O
Roy	O
Nutt	O
,	O
Walter	O
Ramshaw	O
,	O
and	O
others	O
at	O
United	O
Aircraft	O
Corporation	O
.	O
</s>
<s>
The	O
BSS	O
keyword	O
was	O
later	O
incorporated	O
into	O
FORTRAN	B-Application
Assembly	O
Program	O
(	O
FAP	O
)	O
and	O
Macro	O
Assembly	O
Program	O
(	O
MAP	O
)	O
,	O
IBM	O
's	O
standard	O
assemblers	B-Language
for	O
its	O
709	O
and	O
7090/94	O
computers	O
.	O
</s>
<s>
Some	O
assemblers	B-Language
support	O
a	O
complementary	O
or	O
alternative	O
directive	O
BES	O
,	O
for	O
Block	O
Ended	O
by	O
Symbol	O
,	O
where	O
the	O
specified	O
symbol	O
corresponds	O
to	O
the	O
end	O
of	O
the	O
reserved	O
block	O
.	O
</s>
<s>
In	O
C	B-Language
,	O
statically	O
allocated	O
objects	O
without	O
an	O
explicit	O
initializer	O
are	O
initialized	O
to	O
zero	O
(	O
for	O
arithmetic	O
types	O
)	O
or	O
a	O
null	O
pointer	O
(	O
for	O
pointer	O
types	O
)	O
.	O
</s>
<s>
Implementations	O
of	O
C	B-Language
typically	O
represent	O
zero	O
values	O
and	O
null	O
pointer	O
values	O
using	O
a	O
bit	O
pattern	O
consisting	O
solely	O
of	O
zero-valued	O
bits	O
(	O
despite	O
filling	O
bss	O
with	O
zero	O
is	O
not	O
required	O
by	O
the	O
C	B-Language
standard	O
,	O
all	O
variables	O
in	O
.bss	B-General_Concept
are	O
required	O
to	O
be	O
individually	O
initialized	O
to	O
some	O
sort	O
of	O
zeroes	O
according	O
to	O
Section	O
6.7.8	O
of	O
C	B-Language
ISO	O
Standard	O
9899:1999	O
or	O
section	O
6.7.9	O
for	O
newer	O
standards	O
)	O
.	O
</s>
<s>
Hence	O
,	O
the	O
BSS	B-General_Concept
segment	I-General_Concept
typically	O
includes	O
all	O
uninitialized	O
objects	O
(	O
both	O
variables	O
and	O
constants	O
)	O
declared	O
at	O
file	O
scope	O
(	O
i.e.	O
,	O
outside	O
any	O
function	O
)	O
as	O
well	O
as	O
uninitialized	O
static	B-Language
local	O
variables	O
(	O
local	O
variables	O
declared	O
with	O
the	O
static	B-Language
keyword	O
)	O
;	O
static	B-Language
local	O
constants	O
must	O
be	O
initialized	O
at	O
declaration	O
,	O
however	O
,	O
as	O
they	O
do	O
not	O
have	O
a	O
separate	O
declaration	O
,	O
and	O
thus	O
are	O
typically	O
not	O
in	O
the	O
BSS	O
section	O
,	O
though	O
they	O
may	O
be	O
implicitly	O
or	O
explicitly	O
initialized	O
to	O
zero	O
.	O
</s>
<s>
Peter	O
van	O
der	O
Linden	O
,	O
a	O
C	B-Language
programmer	O
and	O
author	O
,	O
says	O
,	O
"	O
Some	O
people	O
like	O
to	O
remember	O
it	O
as	O
'	O
Better	O
Save	O
Space.	O
'	O
</s>
<s>
Since	O
the	O
BSS	B-General_Concept
segment	I-General_Concept
only	O
holds	O
variables	O
that	O
do	O
n't	O
have	O
any	O
value	O
yet	O
,	O
it	O
does	O
n't	O
actually	O
need	O
to	O
store	O
the	O
image	O
of	O
these	O
variables	O
.	O
</s>
<s>
The	O
size	O
that	O
BSS	O
will	O
require	O
at	O
runtime	O
is	O
recorded	O
in	O
the	O
object	B-Application
file	I-Application
,	O
but	O
BSS	O
(	O
unlike	O
the	O
data	B-General_Concept
segment	I-General_Concept
)	O
does	O
n't	O
take	O
up	O
any	O
actual	O
space	O
in	O
the	O
object	O
file.	O
"	O
</s>
<s>
In	O
Fortran	B-Application
,	O
common	B-Application
block	I-Application
variables	O
are	O
allocated	O
in	O
this	O
segment	O
.	O
</s>
<s>
Some	O
compilers	O
may	O
,	O
for	O
64-bit	B-Device
instruction	O
sets	O
,	O
limit	O
offsets	O
,	O
in	O
instructions	O
that	O
access	O
this	O
segment	O
,	O
to	O
32bits	O
,	O
limiting	O
its	O
size	O
to	O
2GB	O
or	O
4GB	O
.	O
</s>
<s>
Also	O
,	O
note	O
that	O
Fortran	B-Application
does	O
not	O
require	O
static	B-Language
data	O
to	O
be	O
initialized	O
to	O
zero	O
.	O
</s>
<s>
On	O
those	O
systems	O
where	O
the	O
bss	B-General_Concept
segment	I-General_Concept
is	O
initialized	O
to	O
zero	O
,	O
putting	O
common	B-Application
block	I-Application
variables	O
and	O
other	O
static	B-Language
data	O
into	O
that	O
segment	O
guarantees	O
that	O
it	O
will	O
be	O
zero	O
,	O
but	O
for	O
portability	O
,	O
programmers	O
should	O
not	O
depend	O
on	O
that	O
.	O
</s>
