"Re-entrant" means that the function can "re-enter". A re-entrant function can be correctly executed even if an attempt is made in another process to execute that function while the function is being executed. For example, in an application using a real-time OS, this function is correctly executed even if dispatching to another task is triggered by an interrupt while a certain task is executing this function, and even if the function is executed in that task. A function that must use RAM as a temporary area may not necessarily be re-entrant.