<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
conditional	B-Application
compilation	I-Application
is	O
a	O
compilation	B-Language
technique	O
which	O
results	O
in	O
an	O
executable	B-Application
program	I-Application
that	O
is	O
able	O
to	O
be	O
altered	O
by	O
changing	O
specified	O
parameters	O
.	O
</s>
<s>
This	O
technique	O
is	O
commonly	O
used	O
when	O
these	O
alterations	O
to	O
the	O
program	O
are	O
needed	O
to	O
run	O
it	O
on	O
different	B-Operating_System
platforms	I-Operating_System
,	O
or	O
with	O
different	O
versions	O
of	O
required	O
libraries	B-Library
or	O
hardware	B-Architecture
.	O
</s>
<s>
Many	O
programming	O
languages	O
support	O
conditional	B-Application
compilation	I-Application
.	O
</s>
<s>
Typically	O
compiler	B-Language
directives	O
define	O
or	O
"	O
undefine	O
"	O
certain	O
variables	O
;	O
other	O
directives	O
test	O
these	O
variables	O
and	O
modify	O
compilation	B-Language
accordingly	O
.	O
</s>
<s>
For	O
example	O
,	O
not	O
using	O
an	O
actual	O
language	O
,	O
the	O
compiler	B-Language
may	O
be	O
set	O
to	O
define	O
"	O
Macintosh	O
"	O
and	O
undefine	O
"	O
PC	O
"	O
,	O
and	O
the	O
code	O
may	O
contain	O
:	O
</s>
<s>
In	O
C	B-Language
and	O
some	O
languages	O
with	O
a	O
similar	O
syntax	O
,	O
this	O
is	O
done	O
using	O
an	O
'	O
#ifdef	O
'	O
directive	O
.	O
</s>
<s>
A	O
similar	O
procedure	O
,	O
using	O
the	O
name	O
"	O
conditional	B-Application
comment	I-Application
"	O
,	O
is	O
used	O
by	O
Microsoft	B-Application
Internet	I-Application
Explorer	I-Application
from	O
version	O
5	O
to	O
9	O
to	O
interpret	O
HTML	B-Language
code	I-Language
.	O
</s>
<s>
There	O
is	O
also	O
a	O
similar	O
proprietary	O
mechanism	O
for	O
adding	O
conditional	B-Application
comments	I-Application
within	O
JScript	B-Language
,	O
known	O
as	O
conditional	B-Application
compilation	I-Application
.	O
</s>
<s>
When	O
conditional	B-Application
compilation	I-Application
depends	O
on	O
too	O
many	O
variables	O
,	O
it	O
can	O
make	O
the	O
code	O
harder	O
to	O
reason	O
about	O
as	O
the	O
number	O
of	O
possible	O
combinations	O
of	O
configuration	O
increases	O
exponentially	O
.	O
</s>
<s>
When	O
conditional	B-Application
compilation	I-Application
is	O
done	O
via	O
a	O
preprocessor	B-General_Concept
that	O
does	O
not	O
guarantee	O
syntactically	O
correct	O
output	O
in	O
the	O
source	O
language	O
,	O
such	O
as	O
the	O
C	B-Language
preprocessor	I-Language
,	O
this	O
may	O
lead	O
to	O
hard-to-debug	O
compilation	B-Language
errors	O
,	O
which	O
is	O
sometimes	O
called	O
"	O
#ifdef	O
hell.	O
"	O
</s>
