number of bits in a byte (char) 1

[peterworth]

does the code

BYTE m_byte:5 ;

set the number of bits in m_byte to 5? if so, would this line:

BYTE ret:nCodeSize = *lpBufComp;

read nCodeSize number of bits from the buffer lpBufComp? the last question is this: after having done this, how would i move the pointer lpBufComp along to the next unread bit?

 

[Zyrenthian]

You still need to deal with padding issues. Most likely it will still have 8. Look into defining your own type within a struct using pramas

#pragma pack(push:1)

struct my_struct
{
unsigned fiveBits:5;
};

#pragma pack(pop)

I am not sure if this will work for you because (i think) it will still be represented with 8 bits. sizeof will most likely still return 1.

However, making an array of these and using pointer arithmetic MAY increment by 5 bits... not sure.

Matt

 

[ArkM]

Is it C++ ???

BYTE m_byte:5 ; // outside of a structure declaration???
and especially:
BYTE ret:nCodeSize = *lpBufComp; // colon ???

May be this forum is dedicated to some other language now?...

 

[Salem]

> BYTE m_byte:5 ;
> set the number of bits in m_byte to 5?
No - it sets the range of the data, but doesn't really say much about how much storage it takes.
For example, if BYTE is an unsigned type, then from

m_byte = 31;
m_byte++;  // wraps around to zero

Oh, and unless you're using some non-standard compiler, this notation is only allowed within structures.

> BYTE ret:nCodeSize = *lpBufComp;
> read nCodeSize number of bits from the buffer lpBufComp?
No it won't.
You have to write a function to extract the next 'n' bits from a buffer, using bitwise operators.

Something like this prototype say.

// starting at bit bitnum of buff[bytenum], extract numbits
// updates bytenum and bitnum to index the next unused bit
int readbits ( unsigned char *buff, int *bytenum, int *bitnum, int numbits );

> However, making an array of these and using pointer arithmetic MAY increment by 5 bits
It won't. Pointer arithmetic only works down to the level of a byte.

--

 

[peterworth]

ok, thanks everyone. i didnt know what the colon notation meant basically, and didnt know it can only be used within a struct.

 

[peterworth]

yep, it looks like i'm going to have to read this in bit by bit because the data is stored sometimes in 8 bit chunks and sometimes in 9,10,11 or 12 bits. has anyone got any links with tutorials on bitwise stuff?

 

[peterworth]

more specifically, how could i make an 11(for example) bit struct? i assume i'll have to use structs.

i then want to use this type as a key to a CMap instance, is this possible?

 

[cpjust]

Could an STL bitset object help?