how to pass LPBYTE value?

Hi all,

I'm trying to perform CRC Calculation for a set of byte.
My Transmit Buffer is LPBYTE.

here is my code

Code:

---

        CRCCalculator objCRCCalculator;
        LPBYTE TransBuf = new BYTE[12];
        TransBuf[0]=0xE1;
        TransBuf[1]=0x80;
        TransBuf[2]=0x01;
        TransBuf[3]=0xE1;
        TransBuf[4]=0xD2;
        TransBuf[5]=0xC3;
        TransBuf[6]=0x00;
        TransBuf[7]=0x00;
        TransBuf[8]=0x00;
        TransBuf[9]=0x00;
        TransBuf[10]=0x00;
        TransBuf[11]=0x00;
        objCRCCalculator.CalculateCRC(TransBuf);




//here is my CRC calculation


unsigned short CRCCalculator::update_crc_16( unsigned short crc, unsigned char  c )
{
    unsigned short tmp, short_c;       
    short_c = 0x00ff & (unsigned short)c;
    if ( ! crc_tab16_init )
                init_crc16_tab();
    tmp =  crc      ^ short_c;
    crc = (crc >> 8) ^ crc_tab16[ tmp & 0xff ]; 
    return crc;

} 

static void init_crc16_tab( void )
{
    int i, j;
    unsigned short crc, c;
    for (i=0; i<256; i++)
        {
                crc = 0;
        c  = (unsigned short) i;

        for (j=0; j<8; j++)
                {

            if ( (crc ^ c) & 0x0001 )
                                crc = ( crc >> 1 ) ^ P_16;
            else                     
                                crc =  crc >> 1;
            c = c >> 1;
        }

        crc_tab16[i] = crc;
    }
    crc_tab16_init = TRUE;
}

void CRCCalculator::CalculateCRC(LPBYTE& TxBuf)
{       
        DWORD CRCValue;
        WORD crc_16_modbus  = 0xffff;
        for(int i=0;i<12;i++)
                crc_16_modbus = update_crc_16(crc_16_modbus,TxBuf[i]);       
        CRCValue=crc_16_modbus;
        LPBYTE CRCByte = new BYTE[2];
        CRCByte[0]=HIBYTE(CRCValue);
        CRCByte[1]=LOBYTE(CRCValue);
}

---

I'm trying to get the CRC value for transmit buffer. but some constant value is overwrite the current values of transmit buffer (TxBuf). I think it is because of passing the LPBYTE by reference. Can anyone tell me how to pass this value? or suggest some way to implement it.

With Thanks & regards,
Saravana

@ Saravana.

Your function is expecting an Adress to a pointer to buffer... LPBYTE is BYTE* and your function parameter is expecting adress of a BYTE*.


And, you are passing plainly a buffer into the function. Are you sure you had given that for the corect intent?

yes I'm sure that i'm passing an LPBYTE only

see the declaration which is shown in the 2nd line of code

Code:

---

LPBYTE TransBuf = new BYTE[12];

---

And I have got it.. It is working well. I have made one bleddy mistake in my code. becoz of that only the problem araised.....

whats tat is in my CRC calculation

Code:

---

void CRCCalculator::CalculateCRC(LPBYTE& TxBuf)
{       
        DWORD CRCValue;
        WORD crc_16_modbus  = 0xffff;
        for(int i=0;i<=12;i++)                        //here the problem is, range i have given is not correct
                crc_16_modbus = update_crc_16(crc_16_modbus,TxBuf[i]);       
        CRCValue=crc_16_modbus;
        LPBYTE CRCByte = new BYTE[2];
        CRCByte[0]=HIBYTE(CRCValue);
        CRCByte[1]=LOBYTE(CRCValue);
}

---

Thanks for your response. Its now working well

with regards,
Saravana

Why are you passing a reference to the buffer (LPBYTE& TxBuf)? You only need that if you want to modify the pointer that points to the start of the buffer. Your function should look like this:

Code:

---

void CRCCalculator::CalculateCRC(const LPBYTE TxBuf)

---

And in general, when you pass buffers, also pass the size of the buffers and don't assume.

Code:

---

void CRCCalculator::CalculateCRC(const LPBYTE TxBuf, size_t size)
{
}

---

yeah. Thanks for your update Cilu.. I'll take the second one which is more suitable for any implementation..

thanks once again.

With regards,
Saravana