Question

i2c_receiveData(sDevice *psDevice, byte_t *pbBuffer, uint16_t *puiLen)
{
.
.    
//extract the packet data length
unFrameLen = (*(pbBuffer+1) << 8) | *(pbBuffer + 2);
if(unFrameLen > *puiLen)
unFrameLen=*puiLen;
.
.
}

Here how this statement is finding Frame Length?

unFrameLen = (*(pbBuffer+1) << 8) | *(pbBuffer + 2);

Here pbBuffer is a pointer to unsigned char array.

calling function was,

i2c_receiveData(psDevice, prgDataRecv, &unRegLen);

Answer 1

In this case, it appears the “frame length” is stored at offset 1 into the passed buffer.

It also appears that it is a 16-bit integer.

In order to get a usable 16-bit integer you must unpack from the buffer. It would be better to cast and use htons/ntohs instead, but I presume the architecture is well-known and portability isn’t a concern.

For an input of, say, pbBuffer = {0, 1, 2}, this ends up being:

(1 << 8) | 2;

… which gives:

(00000001b << 8) | 00000010b

… shifting 1b left 8 bits gives:

100000000b

… and OR-ing with 10b:

100000010b

Now you have a 16-bit integer from the two 8-bit integers in pbBuffer[1..2]:

100000010b = 0x102