Reentrancy (computing)

A computer program or subroutine is called reentrant if multiple invocations can safely run concurrently on multiple processors, or if on a single-processor system its execution can be interrupted and a new execution of it can be safely started (it can be "re-entered"). The interruption could be caused by an internal action such as a jump or call, or by an external action such as an interrupt or signal, unlike recursion, where new invocations can only be caused by internal call. This definition originates from multiprogramming environments, where multiple processes may be active concurrently and where the flow of control could be interrupted by an interrupt and transferred to an interrupt service routine (ISR) or "handler" subroutine. Any subroutine used by the handler that could potentially have been executing when the interrupt was triggered should be reentrant. Similarly, code shared by two processors accessing shared data 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 and sometimes even from allocating memory. Reentrancy is neither necessary not sufficient for thread-safety in multi-threaded environments. In other words, a reentrant subroutine can be thread-safe, but . Conversely, thread-safe code need be reentrant (see below for examples). Other terms used for reentrant programs include "sharable code". Reentrant subroutines are sometimes marked in reference material as being "signal safe". Reentrant programs are often "pure procedures". Reentrancy is not the same thing as idempotence, in which the function may be called more than once yet generate exactly the same output as if it had only been called once. Generally speaking, a function produces output data based on some input data (though both are optional, in general). Shared data could be accessed by any function at any time.

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