Is appropriate to use setuid() over setresuid()/setreuid()/seteuid()?

I have an executable binary which was compiled from a C source file The executable has the setuid permission on I noticed that, if the owner of the executable is **root**, I can use

setuid(geteuid());

when compiling the file to set the *real UID* of my the process running the executable to be **root**. Then, anyone who runs the executable can run it as **root**. However, I noticed that only happens when the owner of the executable is **root**. It did not work when I tried to give **test_user** ownership of the executable (and fixing permissions to contain setuid again). After reading these documentation pages ((https://man7.org/linux/man-pages/man2/setuid.2.html) , (https://man7.org/linux/man-pages/man2/setreuid.2.html) , (https://man7.org/linux/man-pages/man2/setresuid.2.html)) and reading [this post](https://unix.stackexchange.com/questions/548480/why-doesnt-setuid-work-with-non-root-users) , I noticed that setuid(new_euid) is meant to change the *effective UID* instead of the *real UID* of the process running the exectuable. It just happens that, under particular circumstances (*effective UID* is root), setuid(new_euid) also sets the *real UID* and *saved UID* of the process running the executable to new_euid. I solved the issue by using setreuid instead of setuid, as follows:

setreuid(geteuid(), geteuid());

Which allowed me to set the *real UID* of the process to be the *effective UID* (owner of the executable) and reset *effective UID* to it's value (redundant). I understand that setuid() will work under certain conditions, but is not less confusing and more appropriate to just use setreuid(), setresuid(), or seteuid() when changing *real UID*, *saved UID*, or *effective UID* is desired since they always work? Moreover: I understand that seteuid() appears to be doing the same as setuid() with the difference explained [here](https://stackoverflow.com/questions/33076543/setuid-vs-seteuid-function) (*effective UID* is root). This is supposed to not allow root priviledged programs regain priviledges after dropping them (because all 3 UIDs would be changed to the same value using setuid())? So should I just use setuid() for root priviledged programs even when it is not as clear compared to setresuid() for example? I see that setuid() can be secure since it doesn't allow root priviledged programs regain priviledges once droped, but that behavior can be implemented using the other mentioned functions with less confussion. Another thing getuid() returns the *real UID* of the process while setuid() is meant for modifying *effective UID* (unless priviledged), which is also confusing.