Array of bitfields

[Mehta Shailendrakumar]

Hi,

I would like to know why array of bitfields is not possible.
Is there any relation with processor architecture for this?
Thank you for your time.

Regards,
Shailendra

[junky_fellow]

Mehta Shailendrakumar wrote:

Hi,

I would like to know why array of bitfields is not possible.
Is there any relation with processor architecture for this?
Thank you for your time.

Regards,
Shailendra

If the bit is an addressable unit on your implementation,
you may have an array of bitfields.

[SM Ryan]

"Mehta Shailendrakumar " <sh************ *******@de.bosc h.com> wrote:
# Hi,
#
# I would like to know why array of bitfields is not possible.
# Is there any relation with processor architecture for this?
# Thank you for your time.

Because C concentrates on things that are easy to implement in the hardware
and generally leaves harder stuff to libraries rather than implemented
in the syntax. Not many CPUs provide extracting or storing or moving
bit vectors of arbitrary length or orientation. You can do this in your
own code. For example if your CPU allows unaligned access to integers,
32 bit integers and 8 bit bytes,

int getSignedBits(c har *base,int bitoffset,int bitlength) {
assert(bitlengt h<=32);
int *intaddress = base+(bitoffset >>3); // First byte with value.
int value = *intaddress; // Get all of the bits.
value <<= offset & 7; // field sign bit -> int sign.
value >>= 32-bitlength; // field lsb -> int lsb.
return value;
}

--
SM Ryan http://www.rawbw.com/~wyrmwif/
I think that's kinda of personal; I don't think I should answer that.

[Eric Sosman]

ju**********@ya hoo.co.in wrote:

Mehta Shailendrakumar wrote:

Hi,

I would like to know why array of bitfields is not possible.
Is there any relation with processor architecture for this?
Thank you for your time.

Regards,
Shailendra

If the bit is an addressable unit on your implementation,
you may have an array of bitfields.

Not in C, you can't.

The problem is that C's array operations are defined
in terms of pointers to the array elements -- a[i] is
defined to be *(a+i), for example. However, a C pointer
cannot point to a bit-field; the "smallest" object a C
pointer can point to is a char. Since it's not possible
to create a pointer to a bit-field, it's not possible to
carry out operations like array indexing that are defined
in terms of pointers.

The underlying hardware may be capable of addressing
individual bits, but C has no way to use that capability.
(The underlying hardware usually has a lot of things C
cannot use: circular shifts, a "carry" flag, memory
protection, vector instructions, hyperthreading, and
so on. C's vocabulary is not rich enough to talk about
such things, so C programs cannot use them.)

--
Eric Sosman
es*****@acm-dot-org.invalid

[Lawrence Kirby]

On Fri, 17 Jun 2005 13:39:21 +0200, Mehta Shailendrakumar wrote:

Hi,

I would like to know why array of bitfields is not possible.
Because the designers of C decided not to add support for these to the
language. We can try to guess why that is the case, but that is the
fundamental reason.
Is there any relation with processor architecture for this?

Nor really. They would be easier/more efficient to implement on some
architectures than others but they are doable on any architecture that
supports C. That's easy to show because you can write portable C code that
implements them using bit operations on integer types. That is one
possible reason why the designers of C decided that the language didn't
need to support them directly.

Lawrence

[Jean-Claude Arbaut]

Le 17/06/2005 19:09, dans pa************* *************** @netactive.co.u k,
«*Lawrence Kirby*» <lk****@netacti ve.co.uk> a écrit*:

On Fri, 17 Jun 2005 13:39:21 +0200, Mehta Shailendrakumar wrote:

Hi,

I would like to know why array of bitfields is not possible.

Because the designers of C decided not to add support for these to the
language. We can try to guess why that is the case, but that is the
fundamental reason.

That reminds me of MATLAB's "why" command :-)

Is there any relation with processor architecture for this?

Nor really. They would be easier/more efficient to implement on some
architectures than others but they are doable on any architecture that
supports C. That's easy to show because you can write portable C code that
implements them using bit operations on integer types. That is one
possible reason why the designers of C decided that the language didn't
need to support them directly.

I'm glad they decided that "goto" and "if" were not enough for
flow control :-) F66 designers were not as bright :-D

[Walter Roberson]

In article <BE************ *************** ***@laposte.net >,
Jean-Claude Arbaut <je************ ****@laposte.ne t> wrote:

I'm glad they decided that "goto" and "if" were not enough for
flow control :-) F66 designers were not as bright :-D

Even Fortran I had a DO loop.

[I posted the reference a couple of days ago.]
--
Entropy is the logarithm of probability -- Boltzmann

[Jean-Claude Arbaut]

Le 17/06/2005 21:49, dans d8*********@can opus.cc.umanito ba.ca, «*Walter
Roberson*» <ro******@ibd.n rc-cnrc.gc.ca> a écrit*:

In article <BE************ *************** ***@laposte.net >,
Jean-Claude Arbaut <je************ ****@laposte.ne t> wrote:

I'm glad they decided that "goto" and "if" were not enough for
flow control :-) F66 designers were not as bright :-D

Even Fortran I had a DO loop.

But no WHILE, and if I remember well, it had a strange IF.
I exaggerated, the Fortran language was and still is
a great language. And in 1954, it was an extraordinary
breakthrough. It's just bad luck the language has
evolved to slowly.

[Martijn]

> I would like to know why array of bitfields is not possible.

Is there any relation with processor architecture for this?
Thank you for your time.

Well, on with the shameless plug: if you check my website (the libraries
section) you will find an implementation of a bitmatrix (2 x 2 array of
bits) - it might help you get what you want.

Good luck,

--
Martijn
http://www.sereneconcepts.nl

[yatindran]

I agree with you.But,if you have programmed in the Keil 8051 C
compiler, you have a datatype called "bit", which can store a value of
0 or 1.It is used to store certain bit-addressable pin values and is
very commonplace in 8051 C.
for Ex,
bit PIN1 = 0;
if(PIN1)
{
.....
}
else
{
....
}
etc.

Eric Sosman wrote:

Not in C, you can't.

The problem is that C's array operations are defined
in terms of pointers to the array elements -- a[i] is
defined to be *(a+i), for example. However, a C pointer
cannot point to a bit-field; the "smallest" object a C
pointer can point to is a char. Since it's not possible
to create a pointer to a bit-field, it's not possible to
carry out operations like array indexing that are defined
in terms of pointers.

The underlying hardware may be capable of addressing
individual bits, but C has no way to use that capability.
(The underlying hardware usually has a lot of things C
cannot use: circular shifts, a "carry" flag, memory
protection, vector instructions, hyperthreading, and
so on. C's vocabulary is not rich enough to talk about
such things, so C programs cannot use them.)

--
Eric Sosman
es*****@acm-dot-org.invalid