bitfield array

bitfield array

[Massimiliano Cialdi]

Is it possible to declare a bitfield array?
I tried with

typedef struct
{
  u8_t busy[10]:1;
}dummy;

but I obtain an error:
error: bit-field `busy' has invalid type

I also tried with

typedef struct
{
  u8_t busy:1;
}dummy;
dummy a[10];

but then a is as large as 10 bytes.

thanks

-- 
Massimiliano Cialdi
cialdi@firenze.net
m.cialdi@oksys.it

Re: bitfield array

[wwp]

Hi Massimiliano Cialdi,


On Mon, 12 Jan 2004 11:10:16 +0100 Massimiliano Cialdi <cialdi@firenze.net> wrote:

> Is it possible to declare a bitfield array?
> I tried with
> 
> typedef struct
> {
>   u8_t busy[10]:1;
> }dummy;
> 
> but I obtain an error:
> error: bit-field `busy' has invalid type
> 
> I also tried with
> 
> typedef struct
> {
>   u8_t busy:1;
> }dummy;
> dummy a[10];
> 
> but then a is as large as 10 bytes.

AFAIK, bitfields are stored in memory block which can only be allocated with
1-byte minimum.. Try to allocate 1 bit in memory, sounds not possible :-).

IOW, if you use a 1-bit bitfield, it will still be require an amount of memory
counted in bytes. That's partly why bitfields are not so used for
optimization, 'cause following the compiler you don't gaim memory space not
cpu time.

My 2cts.


Regards,

-- 
wwp

Re: bitfield array

[Massimiliano Cialdi]

On Mon, 12 Jan 2004 11:29:26 +0100
wwp <subscript@free.fr> wrote:

> AFAIK, bitfields are stored in memory block which can only be
> allocated with 1-byte minimum.. Try to allocate 1 bit in memory,
> sounds not possible :-).
I know but I don't want to allocate a single bit. Since I need an array
of binary flag, I thought that the compiler could help me. 10 bits could
be placed in 2 bytes. Else I need 10 bytes (80% of space lost).

> IOW, if you use a 1-bit bitfield, it will still be require an amount
> of memory counted in bytes. That's partly why bitfields are not so
> used for optimization, 'cause following the compiler you don't gaim
> memory space not cpu time.
I work with a (very) limited memory embedded system. I need to save
memory, no matter in cpu time.

thanks

-- 
Massimiliano Cialdi
cialdi@firenze.net
m.cialdi@oksys.it

RE: bitfield array

[Mariano Moreyra]

Maybe you should use a 2-byte variable, and access each bit with a mask.
In this case, if you only need 10 one bit flags, there will be 6 unused
bits, but you may use those bits with future flags...
Anyway...you will be using only 2 bytes...and not 10 bytes... you still gain
some memory.


-----Mensaje original-----
De: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org]En nombre de
Massimiliano Cialdi
Enviado el: Lunes, 12 de Enero de 2004 07:57
Para: linux-c-programming
CC: wwp
Asunto: Re: bitfield array


On Mon, 12 Jan 2004 11:29:26 +0100
wwp <subscript@free.fr> wrote:

> AFAIK, bitfields are stored in memory block which can only be
> allocated with 1-byte minimum.. Try to allocate 1 bit in memory,
> sounds not possible :-).
I know but I don't want to allocate a single bit. Since I need an array
of binary flag, I thought that the compiler could help me. 10 bits could
be placed in 2 bytes. Else I need 10 bytes (80% of space lost).

> IOW, if you use a 1-bit bitfield, it will still be require an amount
> of memory counted in bytes. That's partly why bitfields are not so
> used for optimization, 'cause following the compiler you don't gaim
> memory space not cpu time.
I work with a (very) limited memory embedded system. I need to save
memory, no matter in cpu time.

thanks

--
Massimiliano Cialdi
cialdi@firenze.net
m.cialdi@oksys.it
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Re: bitfield array

[James Stevenson]

> > IOW, if you use a 1-bit bitfield, it will still be require an amount
> > of memory counted in bytes. That's partly why bitfields are not so
> > used for optimization, 'cause following the compiler you don't gaim
> > memory space not cpu time.
> I work with a (very) limited memory embedded system. I need to save
> memory, no matter in cpu time.


if this is the case you could create a macro / function to access 
the individtual bits.

It is slow but you dont have a problem with that
something like

int grab_bit(u8 *bits, int bit) {
	return ((bits[bit >> 3] & (1 << (bit >> 3) & 255)) >> bit);
}

void set_bit(u8 *bits, int bit) {
	bits[bit >> 3] |= 1 << ((bit >> 3) & 255);
}

I am pritty sure the above is wrong but you should get the rought idea
you have an array of 8bits and you access by dividing by 8 then using the
reminder value as the mask to access the individtual bits.

It should be possible todo it with shifts so you dont loose soo much 
speed. Then just use the functions with a declared array of bits.


	James

Re: bitfield array

[Glynn Clements]

Massimiliano Cialdi wrote:

> Is it possible to declare a bitfield array?

No.

> I also tried with
> 
> typedef struct
> {
>   u8_t busy:1;
> }dummy;
> dummy a[10];
> 
> but then a is as large as 10 bytes.

Structures which contain bitfields will be padded to an integral
number of bytes.

> > AFAIK, bitfields are stored in memory block which can only be
> > allocated with 1-byte minimum.. Try to allocate 1 bit in memory,
> > sounds not possible :-).
> 
> I know but I don't want to allocate a single bit. Since I need an array
> of binary flag, I thought that the compiler could help me. 10 bits could
> be placed in 2 bytes. Else I need 10 bytes (80% of space lost).

Declare a 16-bit field and set of macros to access the individual
bits, e.g.

	typedef struct
	{
		uint16_t bits;
	} dummy;
	
	#define get_flag(s, i) (((s).bits >> (i)) & 1)
	#define set_flag(s, i) ((s).bits |= 1 << (i))
	#define clear_flag(s, i) ((s).bits &= ~(1 << (i)))

-- 
Glynn Clements <glynn.clements@virgin.net>