UNIX for Dummies Questions & Answers

I saw somewhere that describe read() as a primitive. But when I lean signals, it says the read() may be interrupted by a signal.
My Question:
1, What is the diffence between primitive and reentrant?
2, Is read() a primitive or reentrant?
3, Are all system calls primitive or reentrant?

I think -
You want the word atomic - that means the operation completes before any interrupt can be delivered, like a signal.

Primitive does not mean a call cannot be interrupted by some type of interrupt or signal.
It does mean that they are syscalls, which are direct calls with a defined entry point into the kernel. And they are part of kernel code.

Reentrant has only a little to do with the above. You can write a reentrant function yourself in C.

Reentrant means a function call can be interrupted in the middle of its execution and then safely called again ("re-entered") before its previous invocations complete executing. This is common in threads. In other words a function is reentrant when it can be interrupted anywhere in the middle and then resumed much later - and it always completes correctly even if another instance of the function is started before the old one completes. Being "paused" while another thread runs does not break it.

Any function like localtime that creates a variable in memory that can be overwritten by a subsequent call to the same function is not reentrant.

strtok is notoriously non-reentrant, so a lot of systems have strtok_r. The "_r" terminating characters are a defacto standard for saying this is a "reentrant" function.

But what operations in linux is atomic? Can a primitive be reentrant?