Difference between Re-entrant and Thread-Safe function

Question

What is the difference between a re-entrant function and a thread safe function?

http://stackoverflow.com/questions/2274254/what-kind-of-code-can-be-called-re-entrant/2274289#2274289 — Carlos Gutiérrez, Feb 17 '10 at 06:55
very similar if not identical: http://stackoverflow.com/questions/856823/threadsafe-vs-re-entrant — Hawkeye Parker, Aug 07 '14 at 03:53

score 2 · Answer 1 · answered Feb 17 '10 at 06:54

2

Re-entrant means no global state (local only).

Thread safe means it is not possible for 2 (or more) threads to conflict with each other (by writing conflicting values).

answered Feb 17 '10 at 06:54

Mitch Wheat

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score 2 · Answer 2 · edited Jun 20 '20 at 09:12

A thread-safe function can be called simultaneously from multiple threads, even when the invocations use shared data, because all references to the shared data are serialized.

A reentrant function can also be called simultaneously from multiple threads, but only if each invocation uses its own data.

Hence, a thread-safe function is always reentrant, but a reentrant function is not always thread-safe.

The difference can be cottoned on with the example,

A class is said to be reentrant if its member functions can be called safely from multiple threads, as long as each thread uses a different instance of the class. The class is thread-safe if its member functions can be called safely from multiple threads, even if all the threads use the same instance of the class.

Source: Qt

Brian Rasmussen · Accepted Answer · 2016-12-08T17:07:55.297

Did you check the wiki article on the subject. It explains it well so please see that for a full discussion.

A few relevant bits from the article:

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.

and

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).

Please post an actual answer that summarizes the link in case it goes cold (as is the case here and now). — nedR, Mar 26 '15 at 06:13

Difference between Re-entrant and Thread-Safe function

3 Answers3