<s>
(	O
also	O
known	O
as	O
)	O
is	O
a	O
set	O
of	O
execution	O
startup	O
routines	O
linked	B-Application
into	O
a	O
C	B-Language
program	I-Language
that	O
performs	O
any	O
initialization	O
work	O
required	O
before	O
calling	O
the	O
program	O
's	O
main	B-Language
function	I-Language
.	O
</s>
<s>
Crt0	B-Language
generally	O
takes	O
the	O
form	O
of	O
an	O
object	B-Application
file	I-Application
called	O
,	O
often	O
written	O
in	O
assembly	B-Language
language	I-Language
,	O
which	O
is	O
automatically	O
included	O
by	O
the	O
linker	B-Application
into	O
every	O
executable	B-Application
file	I-Application
it	O
builds	O
.	O
</s>
<s>
contains	O
the	O
most	O
basic	O
parts	O
of	O
the	O
runtime	B-Library
library	I-Library
.	O
</s>
<s>
As	O
such	O
,	O
the	O
exact	O
work	O
it	O
performs	O
depends	O
on	O
the	O
program	O
's	O
compiler	O
,	O
operating	O
system	O
and	O
C	B-Language
standard	I-Language
library	I-Language
implementation	O
.	O
</s>
<s>
Beside	O
the	O
initialization	O
work	O
required	O
by	O
the	O
environment	O
and	O
toolchain	B-General_Concept
,	O
can	O
perform	O
additional	O
operations	O
defined	O
by	O
the	O
programmer	O
,	O
such	O
as	O
executing	O
C++	B-Language
global	B-Language
constructors	I-Language
and	O
C	B-Language
functions	I-Language
carrying	O
GCC	B-Application
's	O
attribute	O
.	O
</s>
<s>
"	O
crt	O
"	O
stands	O
for	O
"	O
C	B-Language
runtime	I-Language
"	O
,	O
and	O
the	O
zero	O
stands	O
for	O
"	O
the	O
very	O
beginning	O
"	O
.	O
</s>
<s>
However	O
,	O
when	O
programs	O
are	O
compiled	O
using	O
GCC	B-Application
,	O
it	O
is	O
also	O
used	O
for	O
languages	O
other	O
than	O
C	B-Language
.	O
Alternative	O
versions	O
of	O
are	O
available	O
for	O
special	O
usage	O
scenarios	O
;	O
for	O
example	O
,	O
to	O
enable	O
profiling	O
with	O
gprof	O
,	O
programs	O
must	O
be	O
compiled	O
with	O
instead	O
.	O
</s>
<s>
This	O
example	O
is	O
for	O
Linux	B-Operating_System
x86-64	B-Device
with	O
AT&T	O
syntax	O
,	O
without	O
an	O
actual	O
C	B-Language
runtime	I-Language
.	O
</s>
