<s>
In	O
the	O
C	B-Language
and	O
C++	B-Language
programming	I-Language
languages	I-Language
,	O
unistd.h	B-Language
is	O
the	O
name	O
of	O
the	O
header	B-Language
file	I-Language
that	O
provides	O
access	O
to	O
the	O
POSIX	O
operating	B-General_Concept
system	I-General_Concept
API	B-Application
.	O
</s>
<s>
It	O
is	O
defined	O
by	O
the	O
POSIX.1	O
standard	O
,	O
the	O
base	O
of	O
the	O
Single	O
Unix	B-Application
Specification	O
,	O
and	O
should	O
therefore	O
be	O
available	O
in	O
any	O
POSIX-compliant	O
operating	B-General_Concept
system	I-General_Concept
and	O
compiler	B-Language
.	O
</s>
<s>
For	O
instance	O
,	O
this	O
includes	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
such	O
as	O
GNU	B-Application
variants	I-Application
,	O
distributions	O
of	O
Linux	B-Application
and	O
BSD	B-Operating_System
,	O
and	O
macOS	B-Application
,	O
and	O
compilers	B-Language
such	O
as	O
GCC	B-Application
and	O
LLVM	B-Application
.	O
</s>
<s>
On	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
the	O
interface	O
defined	O
by	O
unistd.h	B-Language
is	O
typically	O
made	O
up	O
largely	O
of	O
system	O
call	O
wrapper	O
functions	O
such	O
as	O
fork	B-Language
,	O
pipe	B-Operating_System
and	O
I/O	B-General_Concept
primitives	O
(	O
read	O
,	O
write	O
,	O
close	O
,	O
etc	O
.	O
</s>
<s>
Unix	B-Application
compatibility	O
layers	O
such	O
as	O
Cygwin	B-Language
and	O
MinGW	B-Application
also	O
provide	O
their	O
own	O
versions	O
of	O
unistd.h.	O
</s>
<s>
In	O
Cygwin	B-Language
,	O
a	O
header	B-Language
file	I-Language
can	O
be	O
found	O
in	O
/usr/include	O
that	O
sub-includes	O
a	O
file	O
of	O
the	O
same	O
name	O
in	O
/usr/include/sys	O
.	O
</s>
<s>
Not	O
everything	O
is	O
defined	O
in	O
there	O
but	O
some	O
definitions	O
are	O
done	O
by	O
references	O
to	O
the	O
GNU	B-Application
C	I-Application
standard	O
library	O
headers	O
(	O
like	O
stddef.h	O
)	O
which	O
provide	O
the	O
type	O
size_t	O
and	O
many	O
more	O
.	O
</s>
<s>
Thus	O
,	O
unistd.h	B-Language
is	O
only	O
a	O
generically	O
defined	O
adaptive	O
layer	O
that	O
might	O
be	O
based	O
upon	O
already	O
existing	O
system	O
and	O
compiler	B-Language
specific	O
definitions	O
.	O
</s>
<s>
This	O
has	O
the	O
general	O
advantage	O
of	O
not	O
having	O
a	O
possibly	O
concurrent	O
set	O
of	O
header	B-Language
file	I-Language
defined	O
,	O
but	O
one	O
that	O
is	O
built	O
upon	O
the	O
same	O
root	O
which	O
,	O
for	O
this	O
reason	O
,	O
will	O
raise	O
much	O
fewer	O
concerns	O
in	O
combined	O
usage	O
cases	O
.	O
</s>
