BitField Struct Help

[beefeater267]

Hi,

I'm new to C++ and am trying to use BITFIELDS. Here's the code i have thus far:

typedef unsigned int BITFIELD;

typedef struct SelfRegistrationMsg_T
{
//byte 5
BITFIELD rackID : 8;

//byte 6
BITFIELD controllerID : 8;

//byte 7 / 8
BITFIELD controllerNetmask : 8;


} SelfRegistrationMsg;


union JOTRONCmd_T
{
byte *payload;
SelfRegistrationMsg_T selfRegMsg;
};


class CJOTRON
{

public:
CJOTRON::CJOTRON();
virtual CJOTRON::~CJOTRON();

JOTRONCmd_T cmd;

bool BuildCmd(const BYTE payload[]);

private:
};


All above is in a .h file..... and in in the cooresponding .cpp file i have:


bool CJOTRON::BuildCmd(const BYTE payload[])
{
bool bReturn = false;
int payloadSize = ((sizeof(payload) / sizeof(byte)) - 1);
this->cmd.payload = new byte[payloadSize];
memcpy(&this->cmd.payload[0],&payload[0],payloadSize);
bReturn = true;
return bReturn;
}


So, i'm trying to use the BITFIELD in a test.cpp class, with code such as:

CJOTRON* jotCmd = new CJOTRON();
byte* payload;
payload = new byte[3];
memset(payload, 0, (sizeof(byte) * 3));
payload[0] = 0x02;
payload[1] = 0x03;
payload[2] = 0x04;
jotCmd->BuildCmd(payload);

unsigned int a = jotCmd->cmd.selfRegMsg.rackID;
unsigned int b = jotCmd->cmd.selfRegMsg.controllerID;
unsigned int c = jotCmd->cmd.selfRegMsg.controllerNetmask;

//**************************************
So, in my test, all i'm doing is just setting 3 values of an array and copying it into my object with BuildCmd, and just testing to read them back with my bitField struct.

however, instead of getting what i expect:
(rackID = 0x02, controllerID = 0x03, controllerNetmask = 0x04), i get back other values.

Any idea what i may be doing wrong?

 

[ArkM]

You can't compile this code: too many syntax errors. For example, byte and BYTE and so on.

union JOTRONCmd_T        
{
        byte *payload;
        SelfRegistrationMsg_T selfRegMsg;
}

A pointer to an array (not array himself) and a structure are overlapped. So if you modify selfRegMsg then you modify a pointer too. Obviously it's not expected behaviour...

int payloadSize = ((sizeof(payload) / sizeof(byte))- 1);

1. A size of an array is sizeof(array)/sizeof(element), why -1?
2. But it's true only if sizeof operation argument is a true array, not a parameter. In your case sizeof(paiload[])is sizeof of a pointer to array!

In C and C++ array declaration w/o number of elements (parameter list only) is a pointer to the 1st element.

Alas, you present an example of a very dangerous (too intricate and complicated and none-portable code. For example, C and C++ languages standards do not guarantee good adjustment for your bit field structure and byte (char) array in unions...

Avoid using bit fields in C and C++: it's old fashion badly defined surrogate of bit strings...

 

[beefeater267]

ArkM,

This code compiles for me no problem. But regardless, if you think Bit fields are too old fashioned, what objects can i use to do my code?

Bottom line is that i'm writing an interface to an ICD, and i have the bit command structures, and the positions for what each byte means.

Should i use a CByteArray and just use indicies to pull out the data i want to look at?

for ex: (my syntax may be wrong, but here's some pseudocode)

CByteArray myCommand = new CByteArray();
myCommand.SetSize(3);

byte radioID = myCommand[0];
byte controllerID = myCommand[1];
byte netmaskCtrl = myCommand[2];

THanks!

 

[ArkM]

Use unsigned char array as is (it's BYTE type alias from windef.h). The unsigned char is an integral type in C and C++; you may assign (or get) values 0xHH directly via indicies.
You may add syntax shugar to access these bytes, of course: it depends on your problem (sorry, I can't see any problem to set or get three bytes in memory).