With the last revision, I get this warning from the compiler.

gcc -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -Wstrict-prototypes    -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration   -I. -I./Include    -DPy_BUILD_CORE  -c ./Modules/_collectionsmodule.c -o Modules/_collectionsmodule.o
./Modules/posixmodule.c: In function ‘os_dup2_impl’:
./Modules/posixmodule.c:7785:9: warning: ‘res’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     int res;
         ^~~

I already reported the issue at:
https://bugs.python.org/issue32441#msg310752

I suggest to close this issue as a duplicate.

Yes, you can close it, but I don't understand why there is no fix for
this warning. my issue has a small PR, but the other issue on b.p.o, I
am not really sure about my fix... I just set the 'res' variable to 0
and avoid this kind of message. Is there an other way if I want to fix
it ?

New changeset 3d86e484de6334fe16cbab512744597bd0de4e80 by Gregory P. Smith (Stéphane Wirtel) in branch 'master':
bpo-32681: Fix an uninitialized variable in the C implementation of os.dup2 (GH-5346)
https://github.com/python/cpython/commit/3d86e484de6334fe16cbab512744597bd0de4e80