Are static member functions reentrant?

Question

Like the title says:

If I have a class with a static member function, which by itself contains no static variables, can I consider the member function reentrant?

Answer 1

Static member functions are no different from namespace-scope functions (or even member functions) with respect to reentrancy. They are not predisposed either way, it depends entirely on what the function does inside.

Answer 2

The rules for Reentrancy laid out in the Wikipedia article on Reentrancy are as follows:

• Reentrant code may not hold any static (or global) non-constant data.
• Reentrant code may not modify its own code.
• Reentrant code may not call non-reentrant computer programs or routines.

so as long as the member function follows those rules it should be ok.

The article Use reentrant functions for safer signal handling I feel gives a slightly better desciption:

A reentrant function is one that can be used by more than one task concurrently without fear of data corruption. Conversely, a non-reentrant function is one that cannot be shared by more than one task unless mutual exclusion to the function is ensured either by using a semaphore or by disabling interrupts during critical sections of code. A reentrant function can be interrupted at any time and resumed at a later time without loss of data. Reentrant functions either use local variables or protect their data when global variables are used.

A reentrant function:

• Does not hold static data over successive calls
• Does not return a pointer to static data; all data is provided by the caller of the function
• Uses local data or ensures protection of global data by making a local copy of it
• Must not call any non-reentrant functions

and then goes on to explain how this differs from thread-safety before going the article fully.

Answer 3

If your static member function contains only auto variables you could consider it re-entrant.