Non-reentrant

Describing a software function, subroutine, or block of code designed to be executed by only one thread or process at a time. This means that if a non-reentrant piece of code is currently executing, any subsequent attempt to call or re-enter it, either by the same thread or a different one, before the first execution is complete, can lead to unpredictable behavior, data corruption, or program crashes. non-reentrant code often relies on global variables or shared resources that must be protected from simultaneous modification. The use of locks, mutexes, and other synchronization primitives is crucial to make code reentrant and thread-safe.

Non-reentrant meaning with examples

• The legacy financial software contained several non-reentrant functions. If multiple transactions tried to update account balances simultaneously, data corruption was inevitable. To resolve this, developers had to redesign the core modules using mutexes to synchronize access to shared data, transforming them into reentrant functions and improving system stability. This process reduced the occurrence of errors and increased the reliability of the transactions processing.
• In a multi-threaded game engine, the render function was initially non-reentrant. If a UI update triggered a re-render before the previous frame was complete, the engine crashed. To fix this, the developers used a double-buffering technique along with a mutex to prevent the re-entrant call. The rendering system's performance was improved as data was safely read and displayed while preventing data corruption.
• The operating system's device driver for the printer was non-reentrant. If multiple applications sent print jobs simultaneously, the data would become corrupted. The solution included the introduction of a locking mechanism to ensure that only one process could interact with the printer driver at a time, preventing conflicts. This change guaranteed the correct printing of documents and maintained data integrity.
• The audio processing library was non-reentrant, causing intermittent audio glitches in the video editing software. The library used global variables for temporary buffers which were corrupted when the audio was reprocessed. To address this issue, the engineers modified the library, introducing local variables or allocating memory as needed to avoid the conflict that causes these audio drop-outs. This improved the overall stability of the software.
• The database connection manager used by the web server was non-reentrant. Several concurrent web requests tried to acquire database connections through this manager simultaneously. When this occurred the server would crash, or the data integrity would be compromised. Developers improved the performance of their web server by making the connection manager reentrant, using a pool of pre-established connections and a mutex that would synchronize them to mitigate concurrency problems.