what is different unistd.h used for?

miloody

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Aug 25, 2009, 7:39:41 AM8/25/09

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Dear all:
I am reading the book "advance programming in UNIX" and it say
unistd.h is posix symbolic constants.
I find all the unistd.h on my environment:
/usr/include$ find ./ -name 'unistd.h'
./asm/unistd.h
./sys/unistd.h
./unistd.h
./bits/unistd.h
./linux/unistd.h
/usr/include$

why there are so many unistd.h and under what circumstances should I
include each of them?
appreciate your help,
miloody

André Gillibert

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Aug 25, 2009, 8:43:42 AM8/25/09

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You should only use /usr/include/unistd.h which is included with:
#include <unistd.h>

Others are Linux-specific internal headers.
e.g. It looks like /usr/include/bits/unistd.h contains some inline functions and is conditionally included by /usr/include/unistd.h in cases where these functions should be inlined.
Never include it directly. It should make an error!
> #ifndef _UNISTD_H
> # error "Never include <bits/unistd.h> directly; use <unistd.h> instead."
> #endif
linux/unistd.h is a Linux specific header file containing symbolic constants for system call numbers and may be included by assembly source code.
asm/unistd.h, asm/unistd_32.h and asm/unistd_64.h are internally used by linux/unistd.h, with conditional directives, depending on target machine architecture. You should not mess with these 3 files.

sys/unistd.h simply includes unistd.h. It's probably there for compatibility with some old platform which used <sys/unistd.h> rather than <unistd.h>. I don't know the exact story behind this one.

Consequently, you should ONLY include <unistd.h> in normal C programs.
If you write low-level Linux-specific functions needing system call numbers, you may include <linux/unistd.h>.
Other headers should not be included directly.

--
André Gillibert

Daniel Molina Wegener

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Aug 25, 2009, 8:44:48 AM8/25/09

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-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

miloody <mil...@gmail.com>
on Tuesday 25 August 2009 07:39
wrote in comp.unix.programmer:

> Dear all:
> I am reading the book "advance programming in UNIX" and it say
> unistd.h is posix symbolic constants.

Not at all... look at the SUSv2 and SUSv3 specifications:

- -----8<----------8<----------8<----------8<-----
The <unistd.h> header defines miscellaneous symbolic constants and types,
and declares miscellaneous functions. The actual values of the constants
are unspecified except as shown. The contents of this header are shown
below.
- -----8<----------8<----------8<----------8<-----

As you see, the header must be included as <unistd.h>, this means that
it must be present in the system include path. In other words, it could
be located in /usr/include. The other unistd.h files are system specific
files. You could take a look on them ;)

> I find all the unistd.h on my environment:
> /usr/include$ find ./ -name 'unistd.h'
> ./asm/unistd.h
> ./sys/unistd.h
> ./unistd.h
> ./bits/unistd.h
> ./linux/unistd.h
> /usr/include$
>
> why there are so many unistd.h and under what circumstances should I
> include each of them?

Nop, just the <unistd.h> header file must be included. I think that
the <linux/unistd.h> file is related to the Linux kernel and must be
used in kernel development.

Some important constants in <unistd.h> are: _POSIX_VERSION,
_POSIX2_VERSION and _XOPEN_VERSION --- depending on your implementation.

> appreciate your help,
> miloody

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James Kanze

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Aug 25, 2009, 8:57:37 AM8/25/09

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On Aug 25, 1:39 pm, miloody <milo...@gmail.com> wrote:
> Dear all:
> I am reading the book "advance programming in UNIX" and it say
> unistd.h is posix symbolic constants. I find all the unistd.h
> on my environment:
> /usr/include$ find ./ -name 'unistd.h'
> ./asm/unistd.h
> ./sys/unistd.h
> ./unistd.h
> ./bits/unistd.h
> ./linux/unistd.h
> /usr/include$

> why there are so many unistd.h

Who knows. It's an implementation issue.

> and under what circumstances should I include each of them?

You include <unistd.h>, and that is all. The compiler will look
for it wherever it thinks appropriate (probably /usr/include).
Anything else is just implementation details.

--
James Kanze (GABI Software) email:james...@gmail.com
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