Yes! -> https://stackoverflow.com/questions/65554461/reasons-a-c-compiler-ignores-register-declaration

fixed

Sometimes random tests generate strings of identic length, whereas the contrary is stated in the description:

for strings:
"1@mx@wii$1xryh@52lr"
"pfxvn067yhan8@rjz~j"
expected:
"1@mx@wii$1xryh@52lrpfxvn067yhan8@rjz~j1@mx@wii$1xryh@52lr"
but got:
"pfxvn067yhan8@rjz~j1@mx@wii$1xryh@52lrpfxvn067yhan8@rjz~j"

I did it in c++ because I gave up in C.

In C you can't just concatenate that way as you said, this is a really weird challenge.

Undeclared identifiers everywhere: strlen and others.
Missing tests with empty strings.
cr_assert_str_eq should be used for strings.
How about random tests?

dont most modern C compilers just ignore register ?

CHEATING!!! TRIGGERED!!!

nice Kata