Question

I am using CRC for the first time (boost::crc_32_type), and I noticed that calling the process_bytes() method twice with the same parameters I get different results. Is it normal?

#include <boost/crc.hpp>
#include <ios>              // for std::ios_base, etc.
#include <iostream>
#include <string>

using namespace std;

int main()
{
    string              data1, data2;
    boost::crc_32_type  result1, result2;

    data1 = "This is a test string";
    data2 = data1;

    result1.process_bytes(data1.c_str(), data1.length());

    cout << "result1: " << hex << uppercase << result1.checksum() << endl;

    result1.process_bytes(data1.c_str(), data1.length());

    cout << "result1: " << hex << uppercase << result1.checksum() << endl;

    result2.process_bytes(data1.c_str(), data1.length());

    cout << "result2: " << hex << uppercase << result2.checksum() << endl;

    return 0;
}

This is the output:

result1: 2DB69898
result1: E29C91
result2: 2DB69898

Answer 1

According to the manual, checksum() returns “the CRC checksum of the data passed in so far”. So, the second checksum is the checksum of the concatenation of data1 with itself and thus naturally different from the checksum of data1.