definitions of uid and euid

[Paul Smith]

On Tue, 2021-02-02 at 11:04 -0600, Peng Yu via Libc-help wrote:
> Could anybody explain the definitions of uid and euid, and provide a
> minimum working example demonstrating when they are different?

Google is helpful here I think; this is a UNIX/POSIX operating system
thing not a libc thing.

For example this answer from the first hit I got seems informative and
pretty accurate

https://stackoverflow.com/a/32456814/939557

and other answers to that question go into more details.