Everything about Reentrant totally explained
» For other uses of the term, see Reentrant (disambiguation)A
computer program or
routine is described as
reentrant if it can be safely executed
concurrently; that is, the routine can be re-entered while it's already running. To be reentrant, a function must:
- Hold no static (global) non-constant data.
- Must not return the address to static (global) non-constant data.
- Must work only on the data provided to it by the caller.
- Must not rely on locks to singleton resources.
- Must not call non-reentrant functions.
Multiple levels of 'user/object/process
priority' and/or
multiprocessing usually complicate the control of reentrant code. Also, IO code is usually not reentrant because it relies on shared, singleton resources such as disks.
Reentrancy is a key feature of
functional programming.
Examples
In the following piece of
C code, neither functions
f nor
g are reentrant.
int g_var = 1;
int f
Relation to thread-safety
Both concepts of reentrance and
thread-safety relate to the way functions handle resources. However, they're not the same. While the concept of reentrance can affect the external interface of a function, thread-safety only concerns the implementation of the function and not its external interface.
In most cases, to make a non-reentrant function reentrant, its external interface must be modified such that all data is provided by the caller of the function.
To make a thread-unsafe function thread-safe, only the implementation needs to be changed, usually by adding synchronization blocks to protect shared resources from concurrent accesses by different threads.
Therefore, reentrance is a stronger property than thread-safety and by definition results in it: every reentrant function is thread-safe, but not every thread-safe function is reentrant.
Further Information
Get more info on 'Reentrant'.
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