Error in reading value from SV in C++ function using DPI - c++

Question

I am trying to pass a string from SV to C++ function but the value is not getting passed properly to C++ function
Code in SV side:
import "DPI" function string mainsha(string str);
class scoreboard ;
string text_i_cplus;
string text_o_cplus;
text_i_cplus="abc";
text_o_cplus=mainsha(text_i_cplus);
That's how I am sending value to C++ . At the C++ side I am taking value as:
extern "C" string mainsha(string input)
{
string output1 = sha256(input);
cout << "sha256('"<< input << "'):" << output1 << endl;
return output1;
}
I am getting correct output when I running the C++ prog alone. But at the console I am getting the following output:
sha256(''):e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
Can someone please suggest where I am going wrong or am I missing something?

Answer 1

The DPI in current SystemVerilog standard only supports the C language, not C++. In fact you should be using import "DPI-C". in C++, string is a class and C only has arrays of char
extern "C" const char * mainsha(const char * input)
{
string output1 = sha256(input); // not sure if you need a cast here
cout << "sha256('"<< input << "'):" << output1 << endl;
return output1.c_str();
}
If you are using Modelsim/Questa, there is a -dpiheader switch that you can use that automatically generates the DPI prototypes for you and you can include that file when you compile your C++ code. That way you will get a compiler error if there is a mismatch and not have to debug run-time behaviors.