<s>
C99	B-Language
(	O
previously	O
known	O
as	O
C9X	B-Language
)	O
is	O
an	O
informal	O
name	O
for	O
ISO/IEC	B-Language
9899:1999	I-Language
,	O
a	O
past	O
version	O
of	O
the	O
C	B-Language
programming	I-Language
language	I-Language
standard	O
.	O
</s>
<s>
It	O
extends	O
the	O
previous	O
version	O
(	O
C90	O
)	O
with	O
new	O
features	O
for	O
the	O
language	O
and	O
the	O
standard	B-Language
library	I-Language
,	O
and	O
helps	O
implementations	O
make	O
better	O
use	O
of	O
available	O
computer	O
hardware	O
,	O
such	O
as	O
IEEE	O
754-1985	O
floating-point	B-Algorithm
arithmetic	I-Algorithm
,	O
and	O
compiler	O
technology	O
.	O
</s>
<s>
The	O
C11	O
version	O
of	O
the	O
C	B-Language
programming	I-Language
language	I-Language
standard	O
,	O
published	O
in	O
2011	O
,	O
replaces	O
C99	B-Language
.	O
</s>
<s>
After	O
ANSI	O
produced	O
the	O
official	O
standard	O
for	O
the	O
C	B-Language
programming	I-Language
language	I-Language
in	O
1989	O
,	O
which	O
became	O
an	O
international	O
standard	O
in	O
1990	O
,	O
the	O
C	B-Language
language	I-Language
specification	O
remained	O
relatively	O
static	O
for	O
some	O
time	O
,	O
while	O
C++	B-Language
continued	O
to	O
evolve	O
,	O
largely	O
during	O
its	O
own	O
standardization	O
effort	O
.	O
</s>
<s>
Normative	O
Amendment	O
1	O
created	O
a	O
new	O
standard	O
for	O
C	B-Language
in	O
1995	O
,	O
but	O
only	O
to	O
correct	O
some	O
details	O
of	O
the	O
1989	O
standard	O
and	O
to	O
add	O
more	O
extensive	O
support	O
for	O
international	O
character	O
sets	O
.	O
</s>
<s>
The	O
standard	O
underwent	O
further	O
revision	O
in	O
the	O
late	O
1990s	O
,	O
leading	O
to	O
the	O
publication	O
of	O
ISO/IEC	B-Language
9899:1999	I-Language
in	O
1999	O
,	O
which	O
was	O
adopted	O
as	O
an	O
ANSI	O
standard	O
in	O
May	O
2000	O
.	O
</s>
<s>
The	O
language	O
defined	O
by	O
that	O
version	O
of	O
the	O
standard	O
is	O
commonly	O
referred	O
to	O
as	O
"	O
C99	B-Language
"	O
.	O
</s>
<s>
The	O
international	O
C	B-Language
standard	O
is	O
maintained	O
by	O
the	O
working	O
group	O
ISO/IEC	O
JTC1/SC22/WG14	O
.	O
</s>
<s>
C99	B-Language
is	O
,	O
for	O
the	O
most	O
part	O
,	O
backward	O
compatible	O
with	O
C89	O
,	O
but	O
it	O
is	O
stricter	O
in	O
some	O
ways	O
.	O
</s>
<s>
The	O
C	B-Language
standards	O
committee	O
decided	O
that	O
it	O
was	O
of	O
more	O
value	O
for	O
compilers	O
to	O
diagnose	O
inadvertent	O
omission	O
of	O
the	O
type	O
specifier	O
than	O
to	O
silently	O
process	O
legacy	O
code	O
that	O
relied	O
on	O
implicit	O
int	O
.	O
</s>
<s>
C99	B-Language
introduced	O
several	O
new	O
features	O
,	O
many	O
of	O
which	O
had	O
already	O
been	O
implemented	O
as	O
extensions	O
in	O
several	O
compilers	O
:	O
</s>
<s>
type-generic	O
math	O
(	O
macro	O
)	O
functions	O
,	O
in	O
<tgmath.h>	O
,	O
which	O
select	O
a	O
math	B-Library
library	I-Library
function	B-Library
based	O
upon	O
float	B-Algorithm
,	O
double	O
,	O
or	O
long	B-Language
double	I-Language
arguments	O
,	O
etc	O
.	O
</s>
<s>
Parts	O
of	O
the	O
C99	B-Language
standard	I-Language
are	O
included	O
in	O
the	O
current	O
version	O
of	O
the	O
C++	B-Language
standard	O
,	O
including	O
integer	O
types	O
,	O
headers	B-Language
,	O
and	O
library	B-Library
functions	I-Library
.	O
</s>
<s>
Variable-length	B-Data_Structure
arrays	I-Data_Structure
are	O
not	O
among	O
these	O
included	O
parts	O
because	O
C++'s	O
Standard	B-Application
Template	I-Application
Library	I-Application
already	O
includes	O
similar	O
functionality	O
.	O
</s>
<s>
A	O
major	O
feature	O
of	O
C99	B-Language
is	O
its	O
numerics	B-Algorithm
support	O
,	O
and	O
in	O
particular	O
its	O
support	O
for	O
access	O
to	O
the	O
features	O
of	O
IEEE	O
754-1985	O
(	O
also	O
known	O
as	O
IEC60559	O
)	O
floating-point	B-Algorithm
hardware	O
present	O
in	O
the	O
vast	O
majority	O
of	O
modern	O
processors	O
(	O
defined	O
in	O
"	O
Annex	O
F	O
IEC	O
60559	O
floating-point	B-Algorithm
arithmetic	I-Algorithm
"	O
)	O
.	O
</s>
<s>
On	O
platforms	O
with	O
IEEE754	O
floating	B-Algorithm
point	I-Algorithm
:	O
</s>
<s>
FLT_EVAL_METHOD	O
==	O
2	O
tends	O
to	O
limit	O
the	O
risk	O
of	O
rounding	B-Algorithm
errors	I-Algorithm
affecting	O
numerically	O
unstable	O
expressions	O
(	O
see	O
IEEE	O
754	O
design	O
rationale	O
)	O
and	O
is	O
the	O
designed	O
default	O
method	O
for	O
x87	B-Application
hardware	O
,	O
but	O
yields	O
unintuitive	O
behavior	O
for	O
the	O
unwary	O
user	O
;	O
FLT_EVAL_METHOD	O
==	O
1	O
was	O
the	O
default	O
evaluation	O
method	O
originally	O
used	O
in	O
K&R	O
C	B-Language
,	O
which	O
promoted	O
all	O
floats	B-Algorithm
to	O
double	O
in	O
expressions	O
;	O
and	O
FLT_EVAL_METHOD	O
==	O
0	O
is	O
also	O
commonly	O
used	O
and	O
specifies	O
a	O
strict	O
"	O
evaluate	O
to	O
type	O
"	O
of	O
the	O
operands	O
.	O
</s>
<s>
(	O
For	O
gcc	B-Application
,	O
FLT_EVAL_METHOD	O
==	O
2	O
is	O
the	O
default	O
on	O
32bit	O
x86	B-Operating_System
,	O
and	O
FLT_EVAL_METHOD	O
==	O
0	O
is	O
the	O
default	O
on	O
64bit	O
x86-64	B-Device
,	O
but	O
FLT_EVAL_METHOD	O
==	O
2	O
can	O
be	O
specified	O
on	O
x86-64	B-Device
with	O
option	O
-mfpmath	O
=	O
387	O
.	O
)	O
</s>
<s>
Before	O
C99	B-Language
,	O
compilers	O
could	O
round	O
intermediate	O
results	O
inconsistently	O
,	O
especially	O
when	O
using	O
x87	B-Application
floating-point	B-Algorithm
hardware	O
,	O
leading	O
to	O
compiler-specific	O
behaviour	O
;	O
such	O
inconsistencies	O
are	O
not	O
permitted	O
in	O
compilers	O
conforming	O
to	O
C99	B-Language
(	O
annex	O
F	O
)	O
.	O
</s>
<s>
The	O
following	O
annotated	O
example	O
C99	B-Language
code	O
for	O
computing	O
a	O
continued	O
fraction	O
function	B-Library
demonstrates	O
the	O
main	O
features	O
:	O
</s>
<s>
As	O
the	O
IEEE754	O
status	O
flags	O
are	O
manipulated	O
in	O
this	O
function	B-Library
,	O
this	O
#pragma	O
is	O
needed	O
to	O
avoid	O
the	O
compiler	O
incorrectly	O
rearranging	O
such	O
tests	O
when	O
optimising	O
.	O
</s>
<s>
(	O
Pragmas	O
are	O
usually	O
implementation-defined	O
,	O
but	O
those	O
prefixed	O
with	O
STDC	O
are	O
defined	O
in	O
the	O
C	B-Language
standard	O
.	O
)	O
</s>
<s>
C99	B-Language
defines	O
a	O
limited	O
number	O
of	O
expression	O
evaluation	O
methods	O
:	O
the	O
current	O
compilation	O
mode	O
can	O
be	O
checked	O
to	O
ensure	O
it	O
meets	O
the	O
assumptions	O
the	O
code	O
was	O
written	O
under	O
.	O
</s>
<s>
The	O
special	O
values	O
such	O
as	O
NaN	O
and	O
positive	O
or	O
negative	O
infinity	O
can	O
be	O
tested	O
and	O
set	O
.	O
</s>
<s>
long	B-Language
double	I-Language
is	O
defined	O
as	O
IEEE	O
754	O
double	B-Algorithm
extended	I-Algorithm
or	O
quad	O
precision	O
if	O
available	O
.	O
</s>
<s>
Using	O
higher	O
precision	O
than	O
required	O
for	O
intermediate	O
computations	O
can	O
minimize	O
round-off	B-Algorithm
error	I-Algorithm
(	O
the	O
typedef	B-Language
double_t	O
can	O
be	O
used	O
for	O
code	O
that	O
is	O
portable	O
under	O
all	O
FLT_EVAL_METHODs	O
)	O
.	O
</s>
<s>
The	O
main	O
function	B-Library
to	O
be	O
evaluated	O
.	O
</s>
<s>
Although	O
it	O
appears	O
that	O
some	O
arguments	O
to	O
this	O
continued	O
fraction	O
,	O
e.g.	O
,	O
3.0	O
,	O
would	O
lead	O
to	O
a	O
divide-by-zero	O
error	O
,	O
in	O
fact	O
the	O
function	B-Library
is	O
well-defined	O
at	O
3.0	O
and	O
division	O
by	O
0	O
will	O
simply	O
return	O
a	O
+infinity	O
that	O
will	O
then	O
correctly	O
lead	O
to	O
a	O
finite	O
result	O
:	O
IEEE	O
754	O
is	O
defined	O
not	O
to	O
trap	O
on	O
such	O
exceptions	O
by	O
default	O
and	O
is	O
designed	O
so	O
that	O
they	O
can	O
very	O
often	O
be	O
ignored	O
,	O
as	O
in	O
this	O
case	O
.	O
</s>
<s>
(	O
If	O
FLT_EVAL_METHOD	O
is	O
defined	O
as	O
2	O
then	O
all	O
internal	O
computations	O
including	O
constants	O
will	O
be	O
performed	O
in	O
long	B-Language
double	I-Language
precision	O
;	O
if	O
FLT_EVAL_METHOD	O
is	O
defined	O
as	O
0	O
then	O
additional	O
care	O
is	O
need	O
to	O
ensure	O
this	O
,	O
including	O
possibly	O
additional	O
casts	O
and	O
explicit	O
specification	O
of	O
constants	O
as	O
long	B-Language
double	I-Language
.	O
)	O
</s>
<s>
__STDC_IEC_559__	O
is	O
to	O
be	O
defined	O
only	O
if	O
"	O
Annex	O
F	O
IEC	O
60559	O
floating-point	B-Algorithm
arithmetic	I-Algorithm
"	O
is	O
fully	O
implemented	O
by	O
the	O
compiler	O
and	O
the	O
C	B-Language
library	I-Language
(	O
users	O
should	O
be	O
aware	O
that	O
this	O
macro	O
is	O
sometimes	O
defined	O
while	O
it	O
should	O
not	O
be	O
)	O
.	O
</s>
<s>
But	O
depending	O
on	O
the	O
function	B-Library
,	O
numerical	O
instabilities	O
cannot	O
always	O
be	O
detected	O
.	O
</s>
<s>
A	O
standard	O
macro	O
__STDC_VERSION__	O
is	O
defined	O
with	O
value	O
199901L	O
to	O
indicate	O
that	O
C99	B-Language
support	O
is	O
available	O
.	O
</s>
<s>
As	O
with	O
the	O
__STDC__	O
macro	O
for	O
C90	O
,	O
__STDC_VERSION__	O
can	O
be	O
used	O
to	O
write	O
code	O
that	O
will	O
compile	O
differently	O
for	O
C90	O
and	O
C99	B-Language
compilers	O
,	O
as	O
in	O
this	O
example	O
that	O
ensures	O
that	O
inline	O
is	O
available	O
in	O
either	O
case	O
(	O
by	O
replacing	O
it	O
with	O
static	O
in	O
C90	O
to	O
avoid	O
linker	O
errors	O
)	O
.	O
</s>
<s>
Most	O
C	B-Language
compilers	O
provide	O
support	O
for	O
at	O
least	O
some	O
of	O
the	O
features	O
introduced	O
in	O
C99	B-Language
.	O
</s>
<s>
Historically	O
,	O
Microsoft	O
has	O
been	O
slow	O
to	O
implement	O
new	O
C	B-Language
features	O
in	O
their	O
Visual	B-Application
C++	I-Application
tools	O
,	O
instead	O
focusing	O
mainly	O
on	O
supporting	O
developments	O
in	O
the	O
C++	B-Language
standards	O
.	O
</s>
<s>
However	O
,	O
with	O
the	O
introduction	O
of	O
Visual	B-Application
C++	I-Application
2013	O
Microsoft	O
implemented	O
a	O
limited	O
subset	O
of	O
C99	B-Language
,	O
which	O
was	O
expanded	O
in	O
Visual	B-Application
C++	I-Application
2015	O
.	O
</s>
<s>
Compiler	O
Level	O
of	O
support	O
C99	B-Language
compatibility	O
details	O
Acorn	B-Device
C/C	I-Device
++	I-Device
The	O
official	O
documentation	O
states	O
that	O
"	O
most	O
"	O
compiler	O
features	O
are	O
supported	O
,	O
along	O
with	O
"	O
some	O
"	O
of	O
the	O
library	B-Library
functions	I-Library
.	O
</s>
<s>
AMD	O
x86	B-Operating_System
Open64	B-Application
Compiler	I-Application
Suite	I-Application
Has	O
C99	B-Language
support	O
equal	O
to	O
that	O
of	O
GCC	B-Application
.	O
</s>
<s>
cc65	B-Application
Full	O
C89	O
and	O
C99	B-Language
support	O
is	O
not	O
implemented	O
,	O
partly	O
due	O
to	O
platform	O
limitations	O
(	O
MOS	B-General_Concept
Technology	I-General_Concept
6502	I-General_Concept
)	O
.	O
</s>
<s>
There	O
is	O
no	O
support	O
planned	O
for	O
some	O
C99	B-Language
types	O
like	O
_Complex	O
and	O
64-bit	O
integers	O
(	O
long	O
long	O
)	O
.	O
</s>
<s>
Ch	B-Language
Supports	O
major	O
C99	B-Language
features	O
.	O
</s>
<s>
Clang	B-Application
Supports	O
all	O
features	O
except	O
C99	B-Language
floating-point	B-Algorithm
pragmas	O
.	O
</s>
<s>
CompCert	B-Application
A	O
certified	O
compiler	O
,	O
formally	O
proved	O
correct	O
.	O
</s>
<s>
Supports	O
all	O
features	O
except	O
C99	B-Language
complex	O
numbers	O
and	O
VLA	O
,	O
and	O
minor	O
restrictions	O
on	O
switch	O
statements	O
(	O
no	O
Duff	O
's	O
device	O
)	O
.	O
</s>
<s>
cparser	O
Supports	O
C99	B-Language
features	O
.	O
</s>
<s>
C++	B-Language
Builder	I-Language
Digital	B-Language
Mars	I-Language
C/C	I-Language
++	I-Language
Compiler	I-Language
Lacks	O
support	O
for	O
some	O
features	O
,	O
such	O
as	O
<tgmath.h>	O
and	O
_Pragma	O
.	O
</s>
<s>
GCC	B-Application
,	O
standard	O
pragmas	O
and	O
IEEE	O
754/IEC	O
60559	O
floating-point	B-Algorithm
support	O
are	O
missing	O
in	O
mainline	O
GCC	B-Application
.	O
</s>
<s>
Additionally	O
,	O
some	O
features	O
(	O
such	O
as	O
extended	O
integer	O
types	O
and	O
new	O
library	B-Library
functions	I-Library
)	O
must	O
be	O
provided	O
by	O
the	B-Language
C	I-Language
standard	I-Language
library	I-Language
and	O
are	O
out	O
of	O
scope	O
for	O
GCC	B-Application
.	O
</s>
<s>
GCC	B-Application
's	O
4.6	O
and	O
4.7	O
releases	O
also	O
provide	O
the	O
same	O
level	O
of	O
compliance	O
.	O
</s>
<s>
Partial	O
IEEE	O
754	O
support	O
,	O
even	O
when	O
the	O
hardware	O
is	O
compliant	O
:	O
some	O
compiler	O
options	O
may	O
be	O
needed	O
to	O
avoid	O
incorrect	O
optimizations	O
(	O
e.g.	O
,	O
-std	O
=	O
c99	B-Language
and	O
-fsignaling-nans	O
)	O
,	O
but	O
full	O
support	O
of	O
directed	O
rounding	O
modes	O
is	O
missing	O
even	O
when	O
-frounding-math	O
is	O
used	O
.	O
</s>
<s>
Green	O
Hills	O
Software	O
IBM	O
C	B-Language
for	O
AIX	O
,	O
V6	O
and	O
XL	B-Language
C/C	I-Language
++	I-Language
V11.1	O
for	O
AIX	O
IBM	O
Rational	O
logiscopeUntil	O
Logiscope	O
6.3	O
,	O
only	O
basic	O
constructs	O
of	O
C99	B-Language
were	O
supported	O
.	O
</s>
<s>
C99	B-Language
is	O
officially	O
supported	O
in	O
Logiscope	O
6.4	O
and	O
later	O
versions	O
.	O
</s>
<s>
The	B-Application
Portland	I-Application
Group	I-Application
PGI	O
C/C	B-Language
++	I-Language
IAR	O
Systems	O
Embedded	O
Workbench	O
Does	O
not	O
support	O
UCN	O
(	O
universal	O
character	O
names	O
)	O
.	O
</s>
<s>
Compiler	O
for	O
embedded	O
targets	O
,	O
such	O
as	O
ARM	O
,	O
Coldfire	O
,	O
MSP430	O
,	O
AVR	O
,	O
AVR32	O
,	O
8051	O
,	O
...	O
No	O
x86	B-Operating_System
targets	O
.	O
</s>
<s>
Intel	B-Language
C++	I-Language
compiler	I-Language
Microsoft	B-Application
Visual	I-Application
C++	I-Application
Visual	B-Application
C++	I-Application
2012	O
and	O
earlier	O
did	O
not	O
support	O
C99	B-Language
.	O
</s>
<s>
Visual	B-Application
C++	I-Application
2013	O
implements	O
a	O
limited	O
subset	O
of	O
C99	B-Language
required	O
to	O
compile	O
popular	O
open-source	O
projects.Visual	O
C++	O
2015	O
implements	O
the	O
C99	B-Language
standard	I-Language
library	I-Language
,	O
with	O
the	O
exception	O
of	O
any	O
library	O
features	O
that	O
depend	O
on	O
compiler	O
features	O
not	O
yet	O
supported	O
by	O
the	O
compiler	O
(	O
for	O
example	O
,	O
<tgmath.h>	O
is	O
not	O
implemented	O
)	O
.Visual	O
C++	O
2019	O
(	O
16.6	O
)	O
adds	O
opt-in	O
support	O
for	O
a	O
C99	B-Language
conformant	O
preprocessor	O
.	O
</s>
<s>
Open	B-Application
Watcom	I-Application
Implements	O
the	O
most	O
commonly	O
used	O
parts	O
of	O
the	O
standard	O
.	O
</s>
<s>
Three	O
C99	B-Language
features	O
have	O
been	O
bundled	O
as	O
C90	O
extensions	O
since	O
pre-v1.0	O
:	O
C++	B-Language
style	O
comments	O
(//	O
)	O
,	O
flexible	B-Language
array	I-Language
members	I-Language
,	O
trailing	O
comma	O
allowed	O
in	O
enum	O
declaration	O
.	O
</s>
<s>
Pelles	O
C	B-Language
Supports	O
all	O
C99	B-Language
features	O
.	O
</s>
<s>
Portable	B-Application
C	I-Application
compiler	I-Application
Working	O
towards	O
becoming	O
C99-compliant	O
.	O
</s>
<s>
Sun	B-Application
Studio	I-Application
The	B-Device
Amsterdam	I-Device
Compiler	I-Device
Kit	I-Device
A	O
C99	B-Language
frontend	O
is	O
currently	O
under	O
investigation	O
.	O
</s>
<s>
Tiny	B-Language
C	I-Language
Compiler	I-Language
Does	O
not	O
support	O
complex	O
numbers.According	O
to	O
the	O
project	O
's	O
TODO	O
list	O
complex	O
types	O
are	O
the	O
only	O
missing	O
C99	B-Language
feature	O
.	O
</s>
<s>
Variable	B-Data_Structure
Length	I-Data_Structure
Arrays	I-Data_Structure
have	O
been	O
added	O
in	O
TCC	O
0.9.26	O
Variable	B-Data_Structure
Length	I-Data_Structure
Arrays	I-Data_Structure
are	O
supported	O
but	O
not	O
as	O
arguments	O
in	O
functions	O
.	O
</s>
<s>
Since	O
ratification	O
of	O
the	O
1999	O
C	B-Language
standard	O
,	O
the	O
standards	O
working	O
group	O
prepared	O
technical	O
reports	O
specifying	O
improved	O
support	O
for	O
embedded	O
processing	O
,	O
additional	O
character	O
data	O
types	O
(	O
Unicode	O
support	O
)	O
,	O
and	O
library	B-Library
functions	I-Library
with	O
improved	O
bounds	B-Data_Structure
checking	I-Data_Structure
.	O
</s>
<s>
Work	O
continues	O
on	O
technical	O
reports	O
addressing	O
decimal	O
floating	B-Algorithm
point	I-Algorithm
,	O
additional	O
mathematical	O
special	O
functions	O
,	O
and	O
additional	O
dynamic	B-General_Concept
memory	I-General_Concept
allocation	I-General_Concept
functions	O
.	O
</s>
<s>
The	O
C	B-Language
and	O
C++	B-Language
standards	O
committees	O
have	O
been	O
collaborating	O
on	O
specifications	O
for	O
threaded	B-Operating_System
programming	O
.	O
</s>
<s>
The	O
next	O
revision	O
of	O
the	O
C	B-Language
standard	O
,	O
C11	O
,	O
was	O
ratified	O
in	O
2011	O
.	O
</s>
<s>
The	O
C	B-Language
standards	O
committee	O
adopted	O
guidelines	O
that	O
limited	O
the	O
adoption	O
of	O
new	O
features	O
that	O
have	O
not	O
been	O
tested	O
by	O
existing	O
implementations	O
.	O
</s>
<s>
Much	O
effort	O
went	O
into	O
developing	O
a	O
memory	B-General_Concept
model	I-General_Concept
,	O
in	O
order	O
to	O
clarify	O
sequence	B-Application
points	I-Application
and	O
to	O
support	O
threaded	B-Operating_System
programming	O
.	O
</s>
