Reentrant (subroutine)

In computing, a computer program or subroutine is called reentrant if it can be interrupted in the middle of its execution, and then be safely called again ("re-entered") before its previous invocations complete execution. The interruption could be caused by an internal action such as a jump or call, or by an external action such as a hardware interrupt or signal. Once the reentered invocation completes, the previous invocations will resume correct execution.

This definition originates from single-threaded programming environments where the flow of control could be interrupted by a hardware interrupt and transferred to an interrupt service routine (ISR). Any subroutine used by the ISR that could potentially have been executing when the interrupt was triggered should be reentrant. Often, subroutines accessible via the operating system kernel are not reentrant. Hence, interrupt service routines are limited in the actions they can perform; for instance, they are usually restricted from accessing the file system and sometimes even from allocating memory.

A subroutine that is directly or indirectly recursive should be reentrant. This policy is partially enforced by structured programming languages. However a subroutine can fail to be reentrant if it relies on a global variable to remain unchanged but that variable is modified when the subroutine is recursively invoked.

This definition of reentrancy differs from that of thread-safety in multi-threaded environments. A reentrant subroutine can achieve thread-safety, but being reentrant alone might not be sufficient to be thread-safe in all situations. Conversely, thread-safe code does not necessarily have to be reentrant (see below for examples).

Other terms used for reentrant programs include "pure procedure" or "sharable code".

This is an example of a swap() function that fails to be reentrant (as well as failing to be thread-safe). As such, it should not have been used in the interrupt service routine isr():

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