What reentrancy is
Reentrancy describes code that can be interrupted partway through its execution and then safely entered again before the first execution finishes. A piece of reentrant code can have multiple overlapping invocations in progress without them interfering with one another. This property is important in situations where code might be re-entered, such as when interrupts or concurrency are involved.
Being safely re-entered
The key idea of reentrancy is that a function can be called again while a previous call is still in progress, and both will behave correctly. For this to work, the code must not rely on shared state in a way that would be corrupted by the overlapping calls. Reentrant code is written so that each invocation works independently, even if they overlap in time.
Why it matters for interrupts
Reentrancy is particularly relevant when code can be interrupted, for example by a signal or an interrupt that then runs the same code again. If the code is reentrant, this re-entry is safe. If it is not, the interruption could leave things in an inconsistent state. This is why reentrancy is an important consideration in low-level and system programming.
Reentrancy and concurrency
Reentrancy also relates to concurrency, where multiple threads might execute the same code at once. Code that is reentrant can be executed by overlapping invocations without conflict, which supports its safe use in concurrent settings. While reentrancy and thread safety are related, the core idea of reentrancy is about safe overlapping execution of the same code.
Writing reentrant code
To be reentrant, code generally must avoid depending on shared, modifiable state that could be disturbed by another invocation. Instead, it works with data that is local to each call or otherwise handled carefully. Understanding reentrancy clarifies what makes code safe to re-enter, which is important for reliable behavior when code can overlap with itself.
Why it matters
Reentrancy is an important concept in systems and concurrent programming. Understanding what it is - code that can be safely re-entered before it finishes - clarifies what is needed for code to behave correctly when interrupted or overlapped. For anyone interested in low-level or concurrent programming, it is a valuable concept to understand.
Related on Skillo
See also: What is thread safety? Explained, What is an interrupt in computing? Explained.
Sources
Published date reflects the original event date (2026-10-18). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.
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