Bit Field Question

[Marc Ueberall]

Hi there!

I've got the following problem. I have to read an integer value out of a
2 byte data block. The structure is like following:

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
x y |---- numeric data ----|

Bit 15 and 14 are just some flags. I've got to get the numeric data
encoded in the bits 13-0.

I tried this:

struct BitField
{
unsigned short x: 1;
unsigned short y: 1;
unsigned short value: 14;
};

struct BitField *testField= (struct BitField*)aValue;

aValue->value...

But this didn't work.

Does someone knows how to get the numeric data???

Thanks a lot for your help!

[shez]

Marc Ueberall wrote:

Hi there!

I've got the following problem. I have to read an integer value out of a 2 byte data block. The structure is like following:

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
x y |---- numeric data ----|

Bit 15 and 14 are just some flags. I've got to get the numeric data
encoded in the bits 13-0.

I tried this:

struct BitField
{
unsigned short x: 1;
unsigned short y: 1;
unsigned short value: 14;
};

struct BitField *testField= (struct BitField*)aValue;

aValue->value...

But this didn't work.

Does someone knows how to get the numeric data???

Thanks a lot for your help!

What error are you getting? It might be an alignment problem (i
think). Instead of:

struct BitField *testField= (struct BitField*)aValue;

Try:

BitField testField;
std::memcpy(&testField, aValue, sizeof(testField));

I'm using C++ syntax here instead of C syntax (this is, after all, a
C++ group).

-shez-

[Marc Ueberall / DuskEngine]

shez wrote:

Marc Ueberall wrote:

Hi there!

I've got the following problem. I have to read an integer value out

of a

2 byte data block. The structure is like following:

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
x y |---- numeric data ----|

Bit 15 and 14 are just some flags. I've got to get the numeric data
encoded in the bits 13-0.

I tried this:

struct BitField
{
unsigned short x: 1;
unsigned short y: 1;
unsigned short value: 14;
};

struct BitField *testField= (struct BitField*)aValue;

aValue->value...

But this didn't work.

Does someone knows how to get the numeric data???

Thanks a lot for your help!

What error are you getting? It might be an alignment problem (i
think). Instead of:

struct BitField *testField= (struct BitField*)aValue;

Try:

BitField testField;
std::memcpy(&testField, aValue, sizeof(testField));

I'm using C++ syntax here instead of C syntax (this is, after all, a
C++ group).

-shez-

Thanks a lot! That works great! :D

[Patrick Leslie Polzer]

shez wrote:

Try:

BitField testField;
std::memcpy(&testField, aValue, sizeof(testField));

Umm... is that portable? I'd say it would be a cleaner way to mask out
the unwanted bits.

Leslie

[msalters]

Patrick Leslie Polzer wrote:

shez wrote:

Try:

BitField testField;
std::memcpy(&testField, aValue, sizeof(testField)); Umm... is that portable? I'd say it would be a cleaner way to mask

out the unwanted bits.

50/50 in the real world; it won't work on the compilers which
allocate bitfield from the other end i.e. end up with 14:1:1

Bitmasking of course is possible and in this case a lot easier.
We really need something like std::bits<from,to>(int&) so we
can say
int x=1;
bits<0,13>(x)=0;
bits<14,14>(x) = 3; // sets precisely one bit
int a = bits(x); // a == 1<<14

It's simple and yet will encourage proper programming.
Regards,
Michiel Salters

[Thomas Matthews]

Marc Ueberall wrote:

Hi there!

I've got the following problem. I have to read an integer value out of a
2 byte data block. The structure is like following:

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
x y |---- numeric data ----|

Bit 15 and 14 are just some flags. I've got to get the numeric data
encoded in the bits 13-0.

I tried this:

struct BitField
{
unsigned short x: 1;
unsigned short y: 1;
unsigned short value: 14;
};

struct BitField *testField= (struct BitField*)aValue;

aValue->value...

But this didn't work.

Does someone knows how to get the numeric data???

Thanks a lot for your help!

What about this:
value = high_byte & 0x3F + low_byte;
or
value = two_byte_value & 0x3FFF;
--
Thomas Matthews

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