What exactly is a reentrant function?

Question

1. How is safely defined?

Semantically. In this case, this is not a hard-defined term. It just mean "You can do that, without risk".

2. If a program can be safely executed concurrently, does it always mean that it is reentrant?

No.

For example, let's have a C++ function that takes both a lock, and a callback as a parameter:

#include <mutex>

typedef void (*callback)();
std::mutex m;

void foo(callback f)
{
    m.lock();
    // use the resource protected by the mutex

    if (f) {
        f();
    }

    // use the resource protected by the mutex
    m.unlock();
}

Another function could well need to lock the same mutex:

void bar()
{
    foo(nullptr);
}

At first sight, everything seems ok… But wait:

int main()
{
    foo(bar);
    return 0;
}

If the lock on mutex is not recursive, then here's what will happen, in the main thread:

1. main will call foo.
2. foo will acquire the lock.
3. foo will call bar, which will call foo.
4. the 2nd foo will try to acquire the lock, fail and wait for it to be released.
5. Deadlock.
6. Oops…

Ok, I cheated, using the callback thing. But it's easy to imagine more complex pieces of code having a similar effect.

3. What exactly is the common thread between the six points mentioned that I should keep in mind while checking my code for reentrant capabilities?

You can smell a problem if your function has/gives access to a modifiable persistent resource, or has/gives access to a function that smells.

(Ok, 99% of our code should smell, then… See last section to handle that…)

So, studying your code, one of those points should alert you:

1. The function has a state (i.e. access a global variable, or even a class member variable)
2. This function can be called by multiple threads, or could appear twice in the stack while the process is executing (i.e. the function could call itself, directly or indirectly). Function taking callbacks as parameters smell a lot.

Note that non-reentrancy is viral : A function that could call a possible non-reentrant function cannot be considered reentrant.

Note, too, that C++ methods smell because they have access to this, so you should study the code to be sure they have no funny interaction.

4.1. Are all recursive functions reentrant?

No.

In multithreaded cases, a recursive function accessing a shared resource could be called by multiple threads at the same moment, resulting in bad/corrupted data.

In singlethreaded cases, a recursive function could use a non-reentrant function (like the infamous strtok), or use global data without handling the fact the data is already in use. So your function is recursive because it calls itself directly or indirectly, but it can still be recursive-unsafe.

4.2. Are all thread-safe functions reentrant?

In the example above, I showed how an apparently threadsafe function was not reentrant. OK, I cheated because of the callback parameter. But then, there are multiple ways to deadlock a thread by having it acquire twice a non-recursive lock.

4.3. Are all recursive and thread-safe functions reentrant?

I would say "yes" if by "recursive" you mean "recursive-safe".

If you can guarantee that a function can be called simultaneously by multiple threads, and can call itself, directly or indirectly, without problems, then it is reentrant.

The problem is evaluating this guarantee… ^_^

5. Are the terms like reentrance and thread safety absolute at all, i.e. do they have fixed concrete definitions?

I believe they do, but then, evaluating a function is thread-safe or reentrant can be difficult. This is why I used the term smell above: You can find a function is not reentrant, but it could be difficult to be sure a complex piece of code is reentrant

6. An example

Let's say you have an object, with one method that needs to use a resource:

struct MyStruct
{
    P * p;

    void foo()
    {
        if (this->p == nullptr)
        {
            this->p = new P();
        }

        // lots of code, some using this->p

        if (this->p != nullptr)
        {
            delete this->p;
            this->p = nullptr;
        }
    }
};

The first problem is that if somehow this function is called recursively (i.e. this function calls itself, directly or indirectly), the code will probably crash, because this->p will be deleted at the end of the last call, and still probably be used before the end of the first call.

Thus, this code is not recursive-safe.

We could use a reference counter to correct this:

struct MyStruct
{
    size_t c;
    P * p;

    void foo()
    {
        if (c == 0)
        {
            this->p = new P();
        }

        ++c;
        // lots of code, some using this->p
        --c;

        if (c == 0)
        {
            delete this->p;
            this->p = nullptr;
        }
    }
};

This way, the code becomes recursive-safe… But it is still not reentrant because of multithreading issues: We must be sure the modifications of c and of p will be done atomically, using a recursive mutex (not all mutexes are recursive):

#include <mutex>

struct MyStruct
{
    std::recursive_mutex m;
    size_t c;
    P * p;

    void foo()
    {
        m.lock();

        if (c == 0)
        {
            this->p = new P();
        }

        ++c;
        m.unlock();
        // lots of code, some using this->p
        m.lock();
        --c;

        if (c == 0)
        {
            delete this->p;
            this->p = nullptr;
        }

        m.unlock();
    }
};

And of course, this all assumes the lots of code is itself reentrant, including the use of p.

And the code above is not even remotely exception-safe, but this is another story… ^_^

7. Hey 99% of our code is not reentrant!

It is quite true for spaghetti code. But if you partition correctly your code, you will avoid reentrancy problems.

7.1. Make sure all functions have NO state

They must only use the parameters, their own local variables, other functions without state, and return copies of the data if they return at all.

7.2. Make sure your object is "recursive-safe"

An object method has access to this, so it shares a state with all the methods of the same instance of the object.

So, make sure the object can be used at one point in the stack (i.e. calling method A), and then, at another point (i.e. calling method B), without corrupting the whole object. Design your object to make sure that upon exiting a method, the object is stable and correct (no dangling pointers, no contradicting member variables, etc.).

7.3. Make sure all your objects are correctly encapsulated

No one else should have access to their internal data:

    // bad
    int & MyObject::getCounter()
    {
        return this->counter;
    }

    // good
    int MyObject::getCounter()
    {
        return this->counter;
    }

    // good, too
    void MyObject::getCounter(int & p_counter)
    {
        p_counter = this->counter;
    }

Even returning a const reference could be dangerous if the user retrieves the address of the data, as some other portion of the code could modify it without the code holding the const reference being told.

7.4. Make sure the user knows your object is not thread-safe

Thus, the user is responsible to use mutexes to use an object shared between threads.

The objects from the STL are designed to be not thread-safe (because of performance issues), and thus, if a user want to share a std::string between two threads, the user must protect its access with concurrency primitives;

7.5. Make sure your thread-safe code is recursive-safe

This means using recursive mutexes if you believe the same resource can be used twice by the same thread.

"Safely" is defined exactly as the common sense dictates - it means "doing its thing correctly without interfering with other things". The six points you cite quite clearly express the requirements to achieve that.

The answers to your 3 questions is 3× "no".

---

Are all recursive functions reentrant?

NO!

Two simultaneous invocations of a recursive function can easily screw up each other, if they access the same global/static data, for example.

---

Are all thread-safe functions reentrant?

NO!

A function is thread-safe if it doesn't malfunction if called concurrently. But this can be achieved e.g. by using a mutex to block the execution of the second invocation until the first finishes, so only one invocation works at a time. Reentrancy means executing concurrently without interfering with other invocations.

---

Are all recursive and thread-safe functions reentrant?

NO!

See above.

The common thread:

Is the behavior well defined if the routine is called while it is interrupted?

If you have a function like this:

int add( int a , int b ) {
  return a + b;
}

Then it is not dependent upon any external state. The behavior is well defined.

If you have a function like this:

int add_to_global( int a ) {
  return gValue += a;
}

The result is not well defined on multiple threads. Information could be lost if the timing was just wrong.

The simplest form of a reentrant function is something that operates exclusively on the arguments passed and constant values. Anything else takes special handling or, often, is not reentrant. And of course the arguments must not reference mutable globals.

Now I have to elaborate on my previous comment. @paercebal answer is incorrect. In the example code didn't anyone notice that the mutex which as supposed to be parameter wasn't actually passed in?

I dispute the conclusion, I assert: for a function to be safe in the presence of concurrency it must be re-entrant. Therefore concurrent-safe (usually written thread-safe) implies re-entrant.

Neither thread safe nor re-entrant have anything to say about arguments: we're talking about concurrent execution of the function, which can still be unsafe if inappropriate parameters are used.

For example, memcpy() is thread-safe and re-entrant (usually). Obviously it will not work as expected if called with pointers to the same targets from two different threads. That's the point of the SGI definition, placing the onus on the client to ensure accesses to the same data structure are synchronised by the client.

It is important to understand that in general it is nonsense to have thread-safe operation include the parameters. If you've done any database programming you will understand. The concept of what is "atomic" and might be protected by a mutex or some other technique is necessarily a user concept: processing a transaction on a database can require multiple un-interrupted modifications. Who can say which ones need to be kept in sync but the client programmer?

The point is that "corruption" doesn't have to be messing up the memory on your computer with unserialised writes: corruption can still occur even if all individual operations are serialised. It follows that when you're asking if a function is thread-safe, or re-entrant, the question means for all appropriately separated arguments: using coupled arguments does not constitute a counter-example.

There are many programming systems out there: Ocaml is one, and I think Python as well, which have lots of non-reentrant code in them, but which uses a global lock to interleave thread acesss. These systems are not re-entrant and they're not thread-safe or concurrent-safe, they operate safely simply because they prevent concurrency globally.

A good example is malloc. It is not re-entrant and not thread-safe. This is because it has to access a global resource (the heap). Using locks doesn't make it safe: it's definitely not re-entrant. If the interface to malloc had be design properly it would be possible to make it re-entrant and thread-safe:

malloc(heap*, size_t);

Now it can be safe because it transfers the responsibility for serialising shared access to a single heap to the client. In particular no work is required if there are separate heap objects. If a common heap is used, the client has to serialise access. Using a lock inside the function is not enough: just consider a malloc locking a heap* and then a signal comes along and calls malloc on the same pointer: deadlock: the signal can't proceed, and the client can't either because it is interrupted.

Generally speaking, locks do not make things thread-safe .. they actually destroy safety by inappropriately trying to manage a resource that is owned by the client. Locking has to be done by the object manufacturer, thats the only code that knows how many objects are created and how they will be used.

The "common thread" (pun intended!?) amongst the points listed is that the function must not do anything that would affect the behaviour of any recursive or concurrent calls to the same function.

So for example static data is an issue because it is owned by all threads; if one call modifies a static variable the all threads use the modified data thus affecting their behaviour. Self modifying code (although rarely encountered, and in some cases prevented) would be a problem, because although there are multiple thread, there is only one copy of the code; the code is essential static data too.

Essentially to be re-entrant, each thread must be able to use the function as if it were the only user, and that is not the case if one thread can affect the behaviour of another in a non-deterministic manner. Primarily this involves each thread having either separate or constant data that the function works on.

All that said, point (1) is not necessarily true; for example, you might legitimately and by design use a static variable to retain a recursion count to guard against excessive recursion or to profile an algorithm.

A thread-safe function need not be reentrant; it may achieve thread safety by specifically preventing reentrancy with a lock, and point (6) says that such a function is not reentrant. Regarding point (6), a function that calls a thread-safe function that locks is not safe for use in recursion (it will dead-lock), and is therefore not said to be reentrant, though it may nonetheless safe for concurrency, and would still be re-entrant in the sense that multiple threads can have their program-counters in such a function simultaneously (just not with the locked region). May be this helps to distinguish thread-safety from reentarncy (or maybe adds to your confusion!).

The answers your "Also" questions are "No", "No" and "No". Just because a function is recursive and/or thread safe it doesn't make it re-entrant.

Each of these type of function can fail on all the points you quote. (Though I'm not 100% certain of point 5).