mjhjmtu

I have this situation

$ ls -la
-rw-r--r-- 1 user user 123 Mar 5 19:32 file-a
-rwx---rwx 1 user user 987 Mar 5 19:32 file-b

I would like to overwrite file-b with file-a but I would like to preserver all permissions and ownership of file-b.

This does not work, because it uses permissions of file-a

cp file-a file-b # << edit: this works as expected! My fault!
mv file-a file-b

This works, but it can be called only from shell. Imagine the situation I can call only execve or similar function.

cat file-a > file-b

I know, that I can execute something like

sh -c "cat file-a > file-b"

but this introduce difficulties with escaping filenames so I don't want to use this.

Is there some common command that can do this or should I write my own helper c program pro this task?

A simple command to copy a file without copying the mode is

dd if=file-a of=file-b

but then you get dd’s verbose status message written to stderr. 
You can suppress that
by running the command in a shell and adding 2> /dev/null,
but then you’re back to square 1. 
If you have GNU dd, you can do

dd if=file-a of=file-b status=none

rsync --inplace file-a file-b should do what you want.

https://linux.die.net/man/1/rsync

--inplace
This option changes how rsync transfers a file when the file's data needs to be updated: instead of the default method of creating a new copy of the file and moving it into place when it is complete, rsync instead writes the updated data directly to the destination file.