Depending on what exactly you are trying to learn from this, I would recommend looking at the source for musl like https://git.musl-libc.org/cgit/musl/tree/src/string and trying to understand why it looks so much more complicated than the simple implementation.

Look at glibc. eg: https://github.com/lattera/glibc/blob/master/string/strcpy.c

how do you know that the implementation is correct? i don't see tests

`r_strlen` truncates the output length on 64bit systems (size_t is 64 bits while int is 32 bits).

You should be compiling with -Wall -Werror.

If you specifically want code review, consider submitting the code to the code review SE https://codereview.stackexchange.com/

I would rather see some novel, simple, easy to use c string library.

strcmp doesn’t return a bool.

Is this meant to be used in production systems, or is it just a learning exercise?