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problem defining a struct

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shew82

Joined: 25 Jul 2006
Posts: 3

problem defining a struct

Posted: Tue Oct 24, 2006 8:12 am

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I am attempting to define a stuct to make dealing with a data message easier...

I initially defined the struct as:

Code:

struct retrieved_messageB {
unsigned messageID: 5;
unsigned int dataA;
unsigned dataB:       4;
unsigned dataC:       4;
unsigned int dataD;
unsigned int dataE;
unsigned dataF:       4;
unsigned dataG:    4;
unsigned dataH:    6;
unsigned dataI:       2;
unsigned dataJ:       1;
unsigned dataK:       1;
unsigned spare:       2;
}

which compiled fine... however, I realised that I had put my stucture in the wrong order. I then carried out the, what I thought was, relatively simple task of re-ording the struct to produce:

Code:

struct retrieved_messageB {
unsigned spare:       2;
unsigned dataK:       1;
unsigned dataJ:       1;
unsigned dataI:       2;
unsigned dataH:    6;
unsigned dataG:       4;
unsigned dataF:       4;
unsigned int dataE;
unsigned int dataD;
unsigned dataC:       4;
unsigned dataB:       4;
unsigned int dataA;
unsigned messageID:    5;

};

which does NOT compile! giving the error "Error 35... Line 132(17,18): Number of bits is out of range" where line 132 is the line with "unsigned dataH".

Any ideas!?

Thanks,
Shaun

treitmey

Joined: 23 Jan 2004
Posts: 1094
Location: Appleton,WI USA

Posted: Tue Oct 24, 2006 8:31 am

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I think it is complaining because they need to line up on byte borders

Code:

unsigned spare: 2;
unsigned dataK: 1;
unsigned dataJ: 1;
unsigned dataI: 2;
unsigned dataH: 6;
unsigned dataG: 4;

Can you pad/waist some bits to get them on byte boundaries

Code:

unsigned spare: 2;
unsigned dataK: 1;
unsigned dataJ: 1;
unsigned waisted: 4; //8

unsigned dataI: 2;
unsigned dataH: 6;//8
unsigned dataG: 4;//last 4 can trail off i think

or you will have to read them in, then make a shift and mask to put them is the data area you want. Perhaps a union will work here?? will a union work with bit places??

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