using both c/c++ codes, float returning C++ function doesn't work in C code

Question

I'm using both c/c++ codes in VS2019
And float returning C++ function doesn't work in C code.

//C++ code
extern "C" float cppfunc()
{
return 1.0f;
}

//C code
void cfunc()
{
float val = cppfunc();
PRINT(val); // actually I look up the variable with debugger, And It seems like 21210656.0
}

I need some help.

Answer 1

Try the following and see if it does what you are attempting.

(1) Create a Win32 Console project.

(2) Add a C source code file to the project, with this code:

/* C_Code.c */
#include <stdio.h>

extern float cppfunc();

void cfunc()
{
    float val = cppfunc();
    printf("%f\n", val);
}

(3) Add a C++ source code file to the project, with this code:

// CPP_Code.cpp
#include <iostream>

extern "C" void cfunc();

extern "C" float cppfunc()
{
    return 1.0f;
}

int main()
{
    std::cout << cppfunc() << "\n";
    cfunc();
}

(4) Build and Run.

Expected output:

1
1.000000

• Wayne

Answer 2

IgorTandetnik-1300 avatar image IgorTandetnik-1300 · 10 hours ago
It seems that you haven't provided a declaration for cppfunc in your C program. Absent a declaration, C compiler assumes int return value (whereas C++ compiler complains; C++ doesn't allow calling undeclared functions). So basically the caller takes whatever random garbage happens to be sitting in EAX register after cppfunc returns, and assumes it to be the function's return value (the actual return value is sitting on the FPU register).

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It works. I appreciate you.