Function behave differently with similar input (C++) - c++

Question

I am trying to work with the modbus protocol and right now i am calculating the LRC of the messages. I made a function which worked with no issue whatever i was putting into and then i noticed that id did not worked with one input and i can't find a logical explanation on why this don't work.
The function is :
void LRCstring(std::string example)
{
std::stringstream ss;
std::string hex =example.substr(1, example.length()-5);
std::vector<unsigned char> hexCh;
unsigned int buffer;
int offset = 0;
while (offset < hex.length()) {
ss.clear();
ss << std::hex << hex.substr(offset, 2);
ss >> buffer;
hexCh.push_back(static_cast<unsigned char>(buffer));
offset += 2;
}
unsigned char LRC=0x00;
int i;
for (i=0;i<hexCh.size();i++)
{
LRC=LRC+hexCh[i];
}
LRC = 0xFF-LRC; // 1 complement
LRC = LRC+1; // 2 complement
//std::string s = std::to_string(LRC);
//int deci = atoi(s.c_str());
int deci = LRC;
int reste=deci % 16;
std::string temp;
int partiehexa=(deci-reste)/16;
std::string temp2;
std::cout << "deci : " << deci << std::endl;
std::cout << "reste : " << reste << std::endl;
std::cout << "partiehexa : " << partiehexa << std::endl;
std::stringstream ss2;
ss2 << reste;
ss2 >> temp;
ss2 << partiehexa;
ss2 >> temp2;
if (partiehexa<10) {LRCascii+=temp2;}
if (partiehexa==10) {LRCascii+='A';}
if (partiehexa==11) {LRCascii+='B';}
if (partiehexa==12) {LRCascii+='C';}
if (partiehexa==13) {LRCascii+='D';}
if (partiehexa==14) {LRCascii+='E';}
if (partiehexa==15) {LRCascii+='F';}
if (reste<10) {LRCascii+=temp;}
if (reste==10) {LRCascii+='A';}
if (reste==11) {LRCascii+='B';}
if (reste==12) {LRCascii+='C';}
if (reste==13) {LRCascii+='D';}
if (reste==14) {LRCascii+='E';}
if (reste==15) {LRCascii+='F';}
std::cout << "LRC : " << LRCascii << std::endl;
return;
}
Examples on what is the input and the result when it is working :
input > ":040100130013??\r\n"
The cout display "LRC : D5"
input > ":0401CD6B05??\r\n"
The cout display "LRC : BE"
D5 and BE are the right results.
I tried with other inputs and had no problem until this :
input > ":0403006B0003??\r\n"
The cout display "LRC : B"
input > ":040306022B00000064??\r\n"
The cout display "LRC : 2"
It should be 8B and not simply B and it should be 62 and not simply 2.
We can see that the last part of the LRC is good but the other part is ignored. What is even stranger is that in this case the cout of "partiehexa" is showing "8" and "6", it is not like this int was empty. I fail to understand why this is happening in this case.

Answer 1

To me it seems to a C code. But I did quickly analyze your code. I Think the problem lies where your are calculating the
(partiehexa < 10)
The LRCascii only gets assigned while looping through the "reste" part of the code. In the "partiehexa" part of the code it do not get assigned to anything.
Suggestion: Change the code to the following and your code will work
if (partiehexa<10)
{
std::stringstream ss3;
ss3 << partiehexa;
ss3 >> temp2;
LRCascii += temp2;
}
Best solution would be to rewrite the code in proper optimized way.

Answer 2

To calculate the LRC
Add up all the data bytes in the message (before converting to ASCII and without the initial colon and final CR/LF).
Throw away any bits that carry over 8 bits.
Make the result negative (by twos compliment) to get the LRC byte.
In your example, checksum can be calculated mathematically as below:
String: 040306022B00000064
Checksum: 62
Byte# 1 Hex Value 04 Decimal Value 4
Byte# 2 Hex Value 03 Decimal Value 3
Byte# 3 Hex Value 06 Decimal Value 6
Byte# 4 Hex Value 02 Decimal Value 2
Byte# 5 Hex Value 2B Decimal Value 43
Byte# 6 Hex Value 00 Decimal Value 0
Byte# 7 Hex Value 00 Decimal Value 0
Byte# 8 Hex Value 00 Decimal Value 0
Byte# 9 Hex Value 64 Decimal Value 100
Total Hex Value 9E Decimal Value 158
LRC: Hex Value FFFFFFFF62 Decimal Value -158
So the single Hex LRC Byte in this example is 62.
Prashant..