Reentrant

A 'reentrant' process, function, or code segment is designed to be safely called (or re-entered) while it is already in progress, usually in a multi-threaded or interrupt-driven environment. It means that the function's execution does not depend on shared resources or modify global variables in a way that would cause problems if another invocation of the function were to interrupt its execution. A reentrant function can be interrupted and restarted from any point without loss of data or causing errors, ensuring data integrity and safe concurrency. Proper synchronization is paramount to a reentrant code.

Reentrant meaning with examples

• In a real-time operating system (RTOS), interrupt service routines (ISRs) are often designed to be reentrant. This allows an ISR to handle an event even if another ISR is already running, ensuring timely responses to critical events, as long as its not dependent on the prior context.
• Consider a recursive function calculating the factorial of a number. It can be made reentrant by passing all necessary data as arguments and using only local variables; therefore, supporting concurrent calculations on different threads without interference.
• Libraries for numerical analysis often implement reentrant functions for common mathematical operations. This is important in multithreaded applications, so one thread can safely call an operation while another is performing the same or a different calculation.
• A graphics rendering engine, supporting asynchronous rendering, may include reentrant functions for its primitive drawing operations. This allows for user interface updates or other operations without blocking the ongoing rendering process.