c and c++ project

Simply Fortran has reasonably good support for the C language (in terms of the File Outline and searching for procedure definitions) and somewhat basic support for C++. It also includes the C and C++ compiler on Windows and macOS.  You can just add the C or C++ files to a project just like any Fortran file.  When you build the project, it should have no problem with those files. 

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.

I'll have to work up some more complex examples, but you'll want to look into using the iso_c_binding module, which allows interfacing Fortran with C (and, hence, a whole collection of C-compatible languages).  For example, I can write a very simple Fortran program that calls a C floating-point routine:

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.

In your FORTRAN with c code, I replaced timestwo with this

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