Fortran symbol not in load table (unable to call loaded symbol in R) - r

Question

I am trying to build a Fortran DLL with Absoft Pro Fortran 13.0.3, 64 bits, for use within R, on Windows 7 64 bits.
Here is my file mycalc.f (it's a dumb example, just to test functionality):
subroutine mycalcf(a,b,c)
real*8 a,b,c
dll_export mycalcf
c=a+b*b
end
The statement dll_export is not standard, but is found in some Fortran compilers (AFAIK it's also found in Lahey and CVF and Intel Fortran has a compiler directive instead). It just tells the compiler which symbols are to be exported.
I compile successfuly with:
af90 -m64 -dll -YDLL_NAMES=LCS mycalc.f -o mycalc.dll
The option -YDLL_NAMES=LCS tells the compiler to build a library with lowercase symbols, which seems better for R.
If I run dumpbin /exports mycalc.dll, I can find mycalcf in exported symbols, in lowercase, without any underscore before or after.
Now, from R (64 bit version), the following works:
dyn.load("mycalc.dll")
is.loaded("mycalcf")
.Fortran("mycalcf", a=4, b=5, c=0)
I get c=29 on return, as expected.
BUT, if I restart R, the following does not work (notice I only removed the is.loaded test):
dyn.load("mycalc.dll")
.Fortran("mycalcf", a=4, b=5, c=0)
I get the error: Fortran symbol name "mycalcf" not in load table.
Now my question is: why is this test so important?
For comparison, when I try the same with gfortran instead of Absoft, I have no problem at all. I compile with: gfortran -m64 -shared -o mycalc2.dll mycalc.f (after commenting out the dll_export statement, which is not needed, nor even recognized, by gfortran).
Then in R:
dyn.load("mycalc2.dll")
.Fortran("mycalcf", a=4, b=5, c=0)
And I get c=29, no error.
Now, I suspect there is something the gcc linker does that is not done automatically by the Absoft linker (actually it's Microsoft's link.exe). But I have no hint what it can be.
Any idea is welcome!
Ok, solution after a good question from Vladimir F (see comments).
Actually, one must append underscores to symbol names. Since there is no way to do it by compiler option, one needs CDEC$ directives (see HP or Intel documentation).
Here it's simply:
subroutine mycalcf(a,b,c)
CDEC$ attributes alias:'mycalcf_' :: mycalcf
real*8 a,b,c
dll_export mycalcf
c=a+b*b
end
Second solution, from Absoft forum: actually I was wrong from the very beginning. Contrary to what I thought, one need not use the dll_export statement, and it even introduced the problem: without it, the compiler appends underscores. All symbols are exported by default, as in gfortran. So the correct code is simply:
subroutine mycalcf(a,b,c)
real*8 a,b,c
c=a+b*b
end
There is even no need for any option to get lowercase symbols, it's also the defaults.
However, one question remains: does the R function .Fortran always add underscore (is there a way to tell it not to?), and if it's always added, why does the call work when is.loaded is called beforehand? R seems to be doing something weird here.
I tried to track is.loaded in R source code (up to do_isloaded in src\main\dotcode.c), to no avail.