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From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.lang.c++
Subject: Re: Problem with calling C++ from Fortran
Date: Sun, 10 Sep 2023 14:22:12 +0100
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db <dieterhansbritz@gmail.com> writes:

> On 09.09.2023 11.02, Muttley@dastardlyhq.com wrote:
>> On Sat, 9 Sep 2023 10:54:33 +0200
>> db <dieterhansbritz@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?

-- 
Ben.