Writing Data to Bit Field

[Zero]

Hi everybody!

Is it possible to write a value into a bit field which initializes all
bits?

example:

struct Field
{
unsigned int Bit1 : 1;
unsigned int Bit2 : 2;
} MyFied;

Now, I would like to write the value 3 to the variable MyField,
so that Bit1 and Bit2 become TRUE ( == 1).

Any suggestion?

Thanks!

[Vladimir S. Oka]

Zero wrote:

Hi everybody!

Is it possible to write a value into a bit field which initializes all
bits?

example:

struct Field
{
unsigned int Bit1 : 1;
unsigned int Bit2 : 2;
I guess you mean `: 1` here as well (judging by the rest of the post).
As it is, Bit2 has width 2, not 1.
} MyFied;

Now, I would like to write the value 3 to the variable MyField,
so that Bit1 and Bit2 become TRUE ( == 1).

Any suggestion?

AFAIK, not in a portable way.

The closest you can portably get is to have a constant struct with Bit1
and Bit2 set to 1, and then assign that to variables of the same type.
E.g.:

struct Field all_bits_one = { 1, 1};

MyFied = all_bits_one;

If you don't want to be portable, you can find out how your
implementation represents integers (or similar handy objects) and try
constructing them appropriatelly and casting to your struct.

--
BR, Vladimir

[Vladimir S. Oka]

Vladimir S. Oka wrote:

Zero wrote:

Hi everybody!

Is it possible to write a value into a bit field which initializes all
bits?

example:

struct Field
{
unsigned int Bit1 : 1;
unsigned int Bit2 : 2;
I guess you mean `: 1` here as well (judging by the rest of the post).
As it is, Bit2 has width 2, not 1.

} MyFied;

Now, I would like to write the value 3 to the variable MyField,
so that Bit1 and Bit2 become TRUE ( == 1).

Any suggestion?

AFAIK, not in a portable way.

The closest you can portably get is to have a constant struct with Bit1
and Bit2 set to 1, and then assign that to variables of the same type.
E.g.:

struct Field all_bits_one = { 1, 1};

Obviously:

const struct Field all_bits_one = { 1, 1};

if you wanted it constant as I suggested.

MyFied = all_bits_one;

If you don't want to be portable, you can find out how your
implementation represents integers (or similar handy objects) and try
constructing them appropriatelly and casting to your struct.

--
BR, Vladimir

[Zero]

You were right, it must have been unsigned int Bit2 : 1;

[Zero]

Can you give me an example for the cast-operation?

[Vladimir S. Oka]

Zero wrote:

Can you give me an example for the cast-operation?

Please quote what, and who you're replying to. If you're using Google,
you need to click on Show Options, and then Reply that appears beneath
the header (and while you're at it, don't top-post either in case you
feel tempted).

I's also move to retract my suggestion that you can cast to a struct.
Casts only work on scalar types, and structs are not scalar types. I
blame insuficient coffee intake. Sorry.

Why would you want to do such a thing anyway?

--
BR, Vladimir

[Zero]

Vladimir S. Oka schrieb:

Zero wrote:

Can you give me an example for the cast-operation?
Please quote what, and who you're replying to. If you're using Google,
you need to click on Show Options, and then Reply that appears beneath
the header (and while you're at it, don't top-post either in case you
feel tempted).

Ok, I never have quoted before...
I's also move to retract my suggestion that you can cast to a struct.
Casts only work on scalar types, and structs are not scalar types. I
blame insuficient coffee intake. Sorry.
Yes, that is also what my compiler says...
Why would you want to do such a thing anyway?

It would be the fastest way to initialize the bits, wouldn't it?

Zero

[Vladimir S. Oka]

Zero wrote:

Vladimir S. Oka schrieb:

Zero wrote:

Can you give me an example for the cast-operation?

Please quote what, and who you're replying to. If you're using Google,
you need to click on Show Options, and then Reply that appears beneath
the header (and while you're at it, don't top-post either in case you
feel tempted).

Ok, I never have quoted before...

I's also move to retract my suggestion that you can cast to a struct.
Casts only work on scalar types, and structs are not scalar types. I
blame insuficient coffee intake. Sorry.

Yes, that is also what my compiler says...

Why would you want to do such a thing anyway?

It would be the fastest way to initialize the bits, wouldn't it?

It shouldn't be less efficient than what I suggested earlier (a good
compiler will probably behind the scenes effectively do what you were
trying to). It's also much more readable.

You should also know the first rule of optimisation:

1. Don't do it.

Compilers are generally better at it. Second rule is useful as well:

2. Don't do it yet.

--
BR, Vladimir

[Richard G. Riley]

On 2006-02-27, Zero <ch********@web .de> wrote:

snip Yet another tedious reminder about how to reply.
Doesn't worry me too much since if I'm interested in a thread then it
can easily be tagged or reconstructed at will despite being an online
newsreader. Personally I hate people constantly including the entire
previous post : the title and thread ID is usually enough for me to
know where I am. Still, horses for courses : and *some* context is
often nice especially in a "to and fro" type thread.

I's also move to retract my suggestion that you can cast to a struct.
Casts only work on scalar types, and structs are not scalar types. I
blame insuficient coffee intake. Sorry.

Yes, that is also what my compiler says...

Why would you want to do such a thing anyway?

It would be the fastest way to initialize the bits, wouldn't it?

Zero

Maybe. What About using a Union? BTW its horrible : address the fields
seperately and let the compiler take care of optimizations unless
there really is a need to write data to a word at exactly the same time.
--
Remove evomer to reply

[Zero]

Richard G. Riley schrieb:

Maybe. What About using a Union?

What do you mean with your hint?