Are my functions reentrant?

This is more of a general C/C++ question (I'm assuming that's what you're using) and it's hard to answer without more specifics. Here are some general points:

- Local variables and arguments are either on the stack or in registers so they are safe for reentrancy. Each call of your function has its own stack area (by definition) so it will have it's own storage for these values and they don't overwrite each other.

- Global variables and static variables are in the data segment so they are not automatically safe for reentrancy. Two calls to a function that accesses a global variable will (generally) access the same global variable. A common way of creating non-local storage for each thread is to create "thread-local-storage" (TLS) which is maintained either somewhere near the top of the stack or somewhere in the heap and pointed to by a pointer in the stack of the thread.

- Read and write operations to shared variables or other shared resources may be interrupted somewhere in the middle of the operation so you need to protect yourself against that (e.g. temporarily turn off interrupts or add semaphores) to make your accesses "atomic".

Andrew