I can appreciate the difficulties, although the example in this post was relatively simple.

When you say that there is a good chance that I’m not adding the standard C++ library flag for linking, what would that be?

If you're attempting to use functions in the C standard library that are mentioned in C header files, then you don't need to explicitly link to the library.  The Fortran compiler will link to the C standard library regardless.

You only need to add the standard C++ library flag for linking if you're linking with the standard C++ library.  There's a good chance you're not, though.

If you have the accompanying C source files for your header file, you can just add them to your project in Simply Fortran, and they should also be compiled.

To be clear, though, interfacing C routines isn't particularly simple.  I'm providing a very high level overview of what you need.  It could get substantially more complicated quickly if you need to link multiple libraries (or you have to build a C library yourself first...).

What I mean by the interoperability procedures is (1) to use iso_c_binding, (2) including the interface, and (3) adding -lstdc++ in the linker for compiler flags in the project options.

Thank you for any assistance you can provide.

    printf("aaa \r\n");

I'm wondering, does the code work as expected in an External Console without the carriage return?

float timestwo(float x)
{
    printf("aaa \n");
    printf("%f \n", x);
    printf("bbb\n");
   
    return x * 2.0;
}

The output ignored the newlines and gave this

aaa 3.000000 bbb

Any thoughts?
K

program main
use iso_c_binding
implicit none

    real::timesval

    interface
        function timestwo(x) bind(c)
        use iso_c_binding
        real(kind=c_float)::timestwo
        real(kind=c_float), value::x
        end function timestwo
    end interface

    print *, "Times two via C"

    write(*, '(1X, A21)', advance='no') "Enter a real number: "
    read(*, *) timesval

    write(*, '(1X, A43, F8.3, A4, F8.3)') "The C language tells me that two times ", &
                                          real(timesval, kind=c_float), &
                                          " is ", &
                                          timestwo(timesval)

end program main

You'll notice I've written a Fortran interface defining a function, timestwo that is a C function (hence the bind(c) appearing in the function's definition.  Also, I've marked the passed argument x as value because Fortran always passes by reference, while C always passes by value.  I can then create a C file that contains:

float timestwo(float x)
{
    return x * 2.0;
}

If I add both files to a Simply Fortran project, it will just work, and the output is what you'd expect:

 Times two via C
 Enter a real number: 6.0
     The C language tells me that two times    6.000 is   12.000

I'll have to find some more complicated examples to display.  But the key is using the iso_c_binding module.  Simply Fortran will just accept a C file added to a project, though, so there is no complication from a development environment perspective.

A project containing C++ files does need an explicit library linking command, though, because of our default linking step.  In Project Options under Compiler flags, one would need to add:

-lstdc++

to the Linker box to ensure that the C++ standard library is also linked. 

There's really no additional steps.  You can mix C and Fortran source in the same project (I do this regularly), and Simply Fortran will compile it all.