boost crc yields different output each time

Question

Background

I am trying to calculate CRC-16/CRC2 for a given byte array using boost crc lib.

Note: I am a beginner at best in C++ development

#include <iostream>
#include <vector>
#include <boost/crc.hpp>

namespace APP{
    class CrcUtil{
        public:
            static uint16_t crc16(const std::vector<uint8_t> input) {
                boost::crc_16_type result;
                result.process_bytes(&input, input.size());
                return result.checksum();
            }

            CrcUtil()=delete;
    };
};

I am using catch2 as my test framework. Here is the code for test:

#include "catch.hpp"

#include "../include/crcUtil.h"

TEST_CASE("is crc calculation correct", "[crcUtil.h TESTS]"){
    std::vector<uint8_t> bytes = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08};
    auto expectedCRC2 = 0x3c9d;
    auto actualCRC2 = APP::CrcUtil::crc16(bytes);

    REQUIRE(expectedCRC2 == actualCRC2);
}

Issue

Each time I ran my test calculated CRC is different.

First run:

/.../test/crcUtilTests.cpp:10: FAILED:
    REQUIRE( expectedCRC2 == actualCRC2 )
with expansion:
    15517 (0x3c9d) == 63180

Second run:

/.../test/crcUtilTests.cpp:10: FAILED:
    REQUIRE( expectedCRC2 == actualCRC2 )
with expansion:
    15517 (0x3c9d) == 33478

Nth run:

/.../test/crcUtilTests.cpp:10: FAILED:
    REQUIRE( expectedCRC2 == actualCRC2 )
with expansion:
    15517 (0x3c9d) == 47016

Question

Is there something wrong with my code?

Why CRC16 is different for same input?

How can reliably calculate CRC16 for a given byte array?

Answer 1

&input

This does not give you a pointer to the data buffer! It gives you a pointer to the vector object itself, so you are interpreting the internals of that object as the data buffer. It will be different every time because it contains things like dynamically allocated pointers to the real data buffer.

Also, the object representation of the vector probably isn't the same size as input.size(), and may have some padding bytes too. So most likely you are also invoking Undefined Behaviour by reading uninitialised memory, meaning your program is completely invalid and anything could happen (including appearing to work normally).

Use input.data() to get a pointer to the contained data, like this:

result.process_bytes(input.data(), input.size());