Debug Assertion Failed error when accessing function in DLL - c++

Question

I'm currently learning how to create a C++ library to be referenced in other projects, and I am running into an issue with a "Debug Assertion Failed" error: is_block_type_valid(header-> _block_use). I followed the walkthrough shown here: Create and use your own Dynamic Link Library. Oddly, I am getting the expected answer if I just ignore the error.
My DLL currently only has one function:
cpp:
int calculate_crc(std::string msg)
{
std::vector<std::string> msg_vector = [](std::string& msg1) {
std::string next;
std::vector<std::string> result;
// for each char in string
for (std::string::const_iterator it = msg1.begin(); it != msg1.end(); it++)
{
// if we hit a terminal char
if (*it == ' ')
{
if (!next.empty())
{
// add them to the result vector
result.push_back(next);
next.clear();
}
}
else
{
next += *it;
}
}
if (!next.empty())
{
result.push_back(next);
}
return result;
} (msg);
int crcReg = 0xFFFF;
// iterate through each element in msgVector
for (auto&& element : msg_vector)
{
// step 2: xor operation performed on byte of msg and CRC register
crcReg ^= [](std::string hex) {
std::map<char, int> map;
map['0'] = 0;
map['1'] = 1;
map['2'] = 2;
map['3'] = 3;
map['4'] = 4;
map['5'] = 5;
map['6'] = 6;
map['7'] = 7;
map['8'] = 8;
map['9'] = 9;
map['a'] = 10;
map['b'] = 11;
map['c'] = 12;
map['d'] = 13;
map['e'] = 14;
map['f'] = 15;
return map[hex[1]] + (map[hex[0]] * 16);
} (element);
// step 3-5 are repeated until 8 bit shifts
for (int i = 0; i < 8; i++)
{
int crcCopy = crcReg;
crcReg >>= 1;
if ((crcCopy & 1) == 0)
continue;
else
crcReg ^= 0xA001;
}
}
return crcReg;
}
h:
#pragma once
#ifdef OMRONLIBRARY_EXPORTS
#define OMRONLIBRARY_API __declspec(dllexport)
#else
#define OMRONLIBRARY_API __declspec(dllimport)
#endif
#include <iostream>
extern "C" OMRONLIBRARY_API int calculate_crc(const std::string msg);

Answer 1

std::string is not a safe type to use in a DLL function parameter. Non-POD types should never be passed over a DLL boundary, unless they are type-erased (such as by using a void* pointer) and are only ever accessed directly by code on one side of the boundary and not the other side.
Assuming the caller is even using C++ at all (C-style DLLs can be used in non-C/C++ languages), it may be using a different std::string implementation. Or it may be using a different C++ compiler, or a different version of the same C++ compiler, or even just different settings for alignment, optimizations, etc. And even if all of that matches the DLL, it will likely be using a different instance of the memory manager that the DLL uses for its std::string implementation.
If you want to pass a string to a DLL function safely, use a C-style char* string instead. You can use std::string inside the DLL, if you want to, eg:
int calculate_crc(const char* msg)
{
use msg as-is ...
or
std::string s_msg = msg;
use s_msg as needed ...
}
extern "C" OMRONLIBRARY_API int calculate_crc(const char* msg);