Re: 'guix build --target=' handling questions

[ludo@gnu.org (Ludovic Courtès)]

Sergei Trofimovich <slyfox@inbox.ru> skribis:

>> Question time:
>> 
>> - Is there a way to run 'guix environment --target=' in the same way as 'guix build --target='
>>   sets it up? I'd like to see how both compilers are supposed to be present in there.
>>
>> - Why default g++ in PATH is the host g++ and not target g++?
>>   Target seems to make most sense if no explicit compiler is specified.
>> 
>> - How to actually set CXX to point to target g++?
>>   It looks like implicitly there already both host (through native-inputs)
>>   and target (through build-inputs) compilers available.
>>   I would expect something like
>>       #:make-flags (list (string-append "CXX=" <.?.>)) 
>>   What should be in place of that "<.?.>" to refer to target g++?
>
> I think I've found a workaround at least for my third question.
>
> Both host and target compilers are available as g++ and ${target}-g++.
>
> Thus the following workaround seems to work:
>
> diff --git a/gnu/packages/regex.scm b/gnu/packages/regex.scm
> index f04cba706..a8fa689ab 100644
> --- a/gnu/packages/regex.scm
> +++ b/gnu/packages/regex.scm
> @@ -20,11 +20,13 @@
>
>  (define-module (gnu packages regex)
>    #:use-module ((guix licenses) #:prefix license:)
>    #:use-module (guix packages)
>    #:use-module (guix download)
> -  #:use-module (guix build-system gnu))
> +  #:use-module (guix build-system gnu)
> +  #:use-module (guix utils) ; for %current-target-system
> +  )
>
>  (define-public re2
>     (package
>       (name "re2")
>       (version "2017-01-01")
> @@ -40,11 +42,15 @@
>                   "0yij1ajh66h3pj3kfz7y0ldrsww8rlpjzaavyr5lchl98as1jq74"))))
>       (build-system gnu-build-system)
>       (arguments
>        `(#:test-target "test"
>          ;; There is no configure step, but the Makefile respects a prefix.
> -        #:make-flags (list (string-append "prefix=" %output))
> +        #:make-flags (list (string-append "prefix=" %output)
> +                           (string-append "CXX=" ,(string-append (if (%current-target-system)
> +                                                                     (string-append (%current-target-system) "-")
> +                                                                     "")
> +                                                                 "g++")))

As John wrote, this is the right fix for this package.

If you can send it with ‘git send-email’ (see
<https://www.gnu.org/software/guix/manual/html_node/Submitting-Patches.html>),
I’ll apply it right away.  Otherwise I can do it on your behalf.

Thanks!

Ludo’.