Thank you for the PR. Will leave comments in-line.

Aside from the in-line comments, what is the expected behavior if more than one source files are specified together with -c option? (e.g. -c foo.c bar.c).

I would appreciate it if you could describe the expected behavior and confirm that the PR works correspondingly.

Thank you for your comment.

The rewrite should not be limited to .c
Yes, you are right. I have no idea about that cases.

what is the expected behavior if more than one source files are specified together with -c option?
I expected foo.o,bar.o.

Now I think that It's better to change my logic.

Like this :
all processing is done,then add the following logic.
mv .o .o
and We don't need .i .
so rm .i

I think the filenames can be saved using hash table.

What do you think?

thank you.