Pierre Neidhardt <ambrevar@gmail.com> writes:

>> guix package -I local
> glibc-utf8-locales	2.26.105-g0890d5379c	out	/gnu/store/3k6hl20c3b7big8ngrsl6mj9k8xav99d-glibc-utf8-locales-2.26.105-g0890d5379c
>
>> guix package -I emacs
> emacs	25.3	out	/gnu/store/y335nx4r08m6kg0yrna7spfwr4s05n36-emacs-25.3
>
> How do I check which glibc Emacs is using?
> I can think of `ldd emacs` but... Where is ldd? :p

"ldd" is in "glibc" :-)

You can also use `guix gc -R /gnu/store/...-emacs-25.3 | grep glibc`.

> A more general question: How do I find to which non-installed package a
> filename belongs?

Guix does not currently know anything about the files inside each
package, I typically do a web search...

> If Emacs happens to be using glibc 2.25, how could such a sitution occur
> in the first place?  Why is glibc 2.25 needed at all?

This situation can occur when you've installed emacs built against glibc
2.25 (which was the glibc in Guix until ~February), and then later
updated "glibc-utf8-locales" to 2.26 which has incompatible locale data.

Updating emacs would fix it in that case, since it would be built
against the new glibc.

On GuixSD, you can use the 'locale-libc' operating-system parameter to
install locale data for multiple glibc versions to ease transition
between glibc updates.