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Re: Problem with calling C++ from Fortran

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Michael S <already5chosen@yahoo.com> writes:

> On Tuesday, September 12, 2023 at 2:26:49 AM UTC+3, Ben Bacarisse wrote:
>> db <dieterh...@gmail.com> writes: 
>> 
>> > On 10.09.2023 15.22, Ben Bacarisse wrote: 
>> >> db <dieterh...@gmail.com> writes: 
>> >> 
>> >>> On 09.09.2023 11.02, Mut...@dastardlyhq.com wrote: 
>> >>>> On Sat, 9 Sep 2023 10:54:33 +0200 
>> >>>> db <dieterh...@gmail.com> wrote: 
>> >>>>> but now it says that C-double doesn't exist. 
>> >>>>> 
>> >>>>> Any ideas? 
>> >>>> C++ mangles function names so that the name you give the function isn't 
>> >>>> the 
>> >>>> name the dynamic loader sees in the binary. You need to put this around whatever 
>> >>>> function you want to call from Fortran so it has C style linkage which means 
>> >>>> the name remains the same: 
>> >>>> extern "C" { 
>> >>>> void calledFromFortran() 
>> >>>> { 
>> >>>> : 
>> >>>> } 
>> >>>> } 
>> >>>> 
>> >>> Where should this go in the code? I added it at the end as is, 
>> >>> and now I get this: 
>> >>> 
>> >>> ~/ownlib90/tests> ./jbandtest 
>> >>> jband.cpp:88:1: error: expected initializer before 'extern' 
>> >>> 88 | extern "C" { 
>> >>> | ^~~~~~ 
>> >>> jband.cpp:96:1: error: expected unqualified-id before '{' token 
>> >>> 96 | { 
>> >>> | ^ 
>> >>> jband.cpp: In function 'void calledFromFortran()': 
>> >>> jband.cpp:350:9: error: expected primary-expression before ':' token 
>> >>> 350 | : 
>> >>> ... 
>> >> You are giving people very little to go on. There's an unknown command 
>> >> (./jbandtest) the gives some errors where only one line of code can be 
>> >> seen. 
>> >> Can you produce a cut-down version to the program that shows the same 
>> >> error -- mimimal example of the C++ and of the Fortran that should 
>> >> calls it but which fails? That would mean you could post the code. 
>> >> If that is not possible, can you link to the source so those who know 
>> >> both C++ and Fortran can take a proper look? 
>> > 
>> > I put the C++ function here: www.dieterbritz.dk/jband.cpp 
>> > After getting errors about the extern lines, whch I had 
>> > added at the end, I shifted it to near the start: 
>> > 
>> > #include <iostream> 
>> > #include <fstream> 
>> > #include <iomanip> 
>> > #include "math.h" 
>> > 
>> > using namespace std; 
>> > 
>> > extern "C" { 
>> > 
>> > void calledFromFortran() 
>> > { 
>> > : 
>> > } 
>> > 
>> > }
>> Remove them all. You have two options. One: correct the C++. To do 
>> this put this, and only this, 
>> 
>> extern "C" { 
>> const long double iband(const long double tbar); 
>> } 
>> 
>> just after the #include lines. This is perfect (the first const is a 
>> meaningless word here and the compiler may warn you about that) but it 
>> will do. 
>> 
>> Next replace 
>> 
>> #include "math.h" 
>> 
>> with 
>> 
>> #include <math.h> 
>> 
>> I'd also remove the two unnecessary #include lines (for iomanip and 
>> fstream). All of these are "tidying up". Only the extern "C" part I 
>> gave corrects an actual mistake. 
>> 
>> The second option is the turn the code in C which links directly with 
>> Fortran. The only C++ used by the code is one "cout << " line (not a 
>> good way to report an error, but that's not important right now) and a 
>> setting for pi that is not a compile-time constant (4*atanl(1)) written 
>> in a more fussy way. 
>> 
>> To convert to C, copy the file to jband.c (rather than .cpp) and replce 
>> the declaration for pi. Given the code is littered with explicit 21 
>> digits decimals (a little more than the precision of C's usualy long 
>> double type) I would just write 
>> 
>> static const long double pi = 3.14159265358979323846L; 
>> 
>> Now replace the cout line with 
>> 
>> fputs("\nError in stredges::iband(): tbar must be > 0", stderr); 
>> 
>> This writes the message to the error stream and put only these two 
>> include lines at the top: 
>> 
>> #include <math.h> 
>> #include <stdio.h> 
>> 
>> Now the file is C code. 
>> 
>> Frankly, this is the way I'd go, since linking C is just that bit 
>> simpler than linking C++. But if the code is going to become fully 
>> fledged (and complex) C++, obviously use my first option.
>> > I still get errors pertaining to these lines. But after adding 
>> > the -c to the two compilation lines in the script, these are 
>> > now the only errors. I think I am getting closer. The errors 
>> > are now
>> Yes, you must compile it like this: 
>> 
>> g++ -c jband.cpp (for C++ code) 
>> gcc -c jband.c (for C code) 
>> 
>> You don't need any -o but you could add in lots of warnings like 
>> 
>> gcc -Wall -Wextra -c jband.c
>> > jband.cpp: In function 'void calledFromFortran()': 
>> > jband.cpp:12:9: error: expected primary-expression before ':' token 
>> > 12 | : 
>> > | ^ 
>> > jband.cpp: At global scope: 
>> > jband.cpp:96:1: error: expected initializer before 'extern' 
>> > 96 | extern "C" { 
>> > | ^~~~~~ 
>> > jband.cpp:104:1: error: expected unqualified-id before '{' token 
>> > 104 | { 
>> > | ^ 
>> > jbandtest.f90:28:14: 
>> > 
>> > 28 | FORTJBAND = JBAND (tbar) 
>> > | 1 
>> > Error: Type mismatch in argument 'tbar' at (1); passed REAL(16) to REAL(10) 
>> > 
>> > The last error will go away together with the others.
>> Michael S has given you one way to go for that. My comments were going 
>> to be more general. You have several ways of specifying the types, and 
>> I could not tell if you really wanted all of them. In general, I would 
>> define the Fortran types from the C ones (since those are the types you 
>> need for the call and return) and leave it at that. 
>> 
>> But the big one is that the Fortran code seems to think the function is 
>> called jband, when in fact it's called iband. 
>> 
>> I went for this: 
>> 
>> module STUFF 
>> use iso_c_binding 
>> integer, parameter :: qud=real(c_long_double)
>> end module STUFF 
>> program JBAND_TEST 
>> use STUFF; implicit none
>> real(qud) :: tbar, curr, FORTJBAND
>> do 
>> read *, tbar 
>> if (tbar < 0) exit 
>> curr = FORTJBAND (tbar) 
>> print '(" curr =", f10.4)', curr 
>> enddo 
>> end program JBAND_TEST 
>> 
>> function FORTJBAND (tbar) 
>> use STUFF; implicit none 
>> interface
>> function IBAND (tbar) bind(c) 
>> use STUFF 
>> real(qud), VALUE :: tbar 
>> real(qud) :: IBAND 
>> end function iband 
>> end interface 
>
> I think, you are taking chances here. Even if it happens to work
> on x86-64 Linux, it can fail on some other target where calling
> conventions for long double return value in 'C' differ from those
> of Fortran. The method shown in one of my previous posts
> appear more robust:
>
> interface
> real(c_long_double) function IBAND (tbar) bind(c)
> import c_long_double
> real(c_long_double), value :: tbar
> end function IBAND

I think I see -- keep the C-related types in the smallest interface?  I
didn't study your example, but the OP renamed the C function and gave
the resulting Fortran function a native Fortran type.  Is that what you
advise?

The OP should have taken your Fortran and applied my C++ edits.  Maybe
they took my Fortran and it did not work on Windows.  That would be a
shame.

>> BTW, you said the C++ was written by a C++ expert. I don't think it 
>> was! 
>
> With no relationships to C, C++ or Fortran, I don't like the way they
> evaluate their polynomials from numerical perspective . But I assume
> that algorithm was copied 'as is' from 40 y.o. book so it's not a
> fault of C++ programmer.

I was just remarking on the C++ itself which has some 'non-expert'
attributes.

> Despite my disliking, their method is good enough to achieve their modest
> specified goal for relative accuracy: of 14-15 significant digits.
> With smarter method the same or better accuracy can be achieved using
> standard 'double" arithmetic (IEEE-754 binary64) thus making the routine
> portable to platforms that have no support for extended precision
> math.

Interesting.  I don't know the function being evaluated at all.

-- 
Ben.

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Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-09 10:54 +0200
  Re: Problem with calling C++ from Fortran Muttley@dastardlyhq.com - 2023-09-09 09:02 +0000
    Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-09 12:47 +0200
      Re: Problem with calling C++ from Fortran Muttley@dastardlyhq.com - 2023-09-09 10:51 +0000
      Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-09 12:52 +0200
        Re: Problem with calling C++ from Fortran Muttley@dastardlyhq.com - 2023-09-09 11:00 +0000
          Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-10 14:33 +0200
            Re: Problem with calling C++ from Fortran Paavo Helde <eesnimi@osa.pri.ee> - 2023-09-10 18:49 +0300
            Re: Problem with calling C++ from Fortran Muttley@dastardlyhq.com - 2023-09-11 08:23 +0000
        Re: Problem with calling C++ from Fortran Paavo Helde <eesnimi@osa.pri.ee> - 2023-09-09 16:56 +0300
          Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-10 14:36 +0200
          Re: Problem with calling C++ from Fortran James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-09-11 00:14 -0400
        Re: Problem with calling C++ from Fortran Bo Persson <bo@bo-persson.se> - 2023-09-10 16:08 +0200
      Re: Problem with calling C++ from Fortran "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2023-09-09 16:30 +0200
    Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-10 14:48 +0200
      Re: Problem with calling C++ from Fortran Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-10 14:22 +0100
        Re: Problem with calling C++ from Fortran Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-10 17:27 +0100
        Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-11 14:00 +0200
          Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-11 05:21 -0700
          Re: Problem with calling C++ from Fortran Muttley@dastardlyhq.com - 2023-09-11 13:33 +0000
          Re: Problem with calling C++ from Fortran Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-12 00:26 +0100
            Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-12 10:48 +0200
              Re: Problem with calling C++ from Fortran Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-12 14:14 +0100
              Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-12 07:15 -0700
            Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-12 02:00 -0700
              Re: Problem with calling C++ from Fortran Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-12 14:22 +0100
                Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-12 07:32 -0700
                Re: Problem with calling C++ from Fortran db <dieterhansbritz@gmail.com> - 2023-09-12 18:36 +0200
                Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-19 02:39 -0700
  Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-10 08:38 -0700
  Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-10 09:51 -0700
    Re: Problem with calling C++ from Fortran Michael S <already5chosen@yahoo.com> - 2023-09-11 02:49 -0700
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