Reentrancy

Reentrancy
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Two Related Definitions

Origin of the Term "Reentrant":
- A function is reentrant if its execution can be interrupted and it can be called again (before the previous invocation is completed)
Origin of the Term "Sharable Code":
- A function is reentrant if its instructions are invariant and, hence, can be shared across instances

Explicitly Reentrant

The Concept:
- The reentrancy of the function can be asserted regardless of how it is called
Ensuring Explicit Reentrancy:
- Accept all arguments by value
- Do not accept pointers as arguments
- Use only local variables (i.e., in C use only automatic variables, no static variables or variables with file scope; in Java use only variables with local scope)

Implicitly Reentrant

The Concept:
- The reentrancy of the function depends on the caller (as well as the callee)
Relaxing Explicit Reentrancy:
- Accept arguments by reference or pointer arguments but ensure that the caller only passes referenecs/pointers to appropriate variables

Common Causes of Non-Reentrancy

Modification of "Global" Variables:
- Consider the example of a function that updates a "global" linked list (e.g., malloc() in C) -- what might happen if one call is interrupted by another?
Return of Statically Allocated Memory:
- One call would "overwrite" the information being returned by another
Internal Bookeeping Using Statically Allocated Memory:
- Consider what happens when a function uses a statically allocated buffer (e.g., printf() in C)