What is a reentrant code with respect to thread safety?

What I have understood by now:

A code is considered reentrant if the process/thread executing it can be interrupted in the middle of its execution and then safely be resumed again. The interruption could be caused by an internal action such as a `goto` statement, by an external action such as process scheduling by the kernel, or by a hardware interrupt. Also, a code which can be accessed by the multiple threads at the same time without any kind of curruption in the shared data structures is called reentrant code. . Making a code reentrant requires the saving of state information in variables local to each execution, usually on a stack, rather than in a static variable, global variables, or other non-local state.

Is this understanding completely correct?

Originally Posted by Anisha Kaul What I have understood by now: Is this understanding completely correct?

I think that "process scheduling by the kernel, or by a hardware interrupt" is not very clearly put.

If a user program runs at the moment, and if we have just one core, kernel does not run at the same time. I.e. simply no other program runs at the time.

I.e. switching to kernel will be caused by an external interrupt anyway - if nothing else happens, it will be interrupt from timer.

Or, to put it differently, external interrupts are hidden from user by kernel. Unless one decides to implement something really sophisticated in which there are HW interrupts with ISRs (Interrupt Service Routines) implemented outside the kernel.

...

I hope I've sufficiently confused you .