is this function reentrant?

Question

void reverse_string(char* string, int str_size) {
    char tmp;
    int i = 0;
    int j = str_size - 1;
    while (i < j) {
        tmp = string[i];
        string[i] = string[j];
        string[j] = tmp;
        ++i;
        --j;
    }
}

I think this function is reentrant, since it doesn't use any global variable. It only modifies the arguments.

My question is: is this function reentrant? if it is, is my argument good enough?

thanks in advance

I believe the equality guard if (i == j) is unnecessary. You have already required i to be strictly less than j. — dmckee --- ex-moderator kitten
Purely for reference, you could achieve the same effect using std::reverse(foo, foo + strlen(foo)) where "foo" is a mutable string. AFAIK, most std::reverse() implementations are thread-safe and reentrant when using such a basic iterator, i.e. the pointers in this case. The strlen() comments other have posted, still apply of course. — Void
@Yves asked a duplicate question to wonder if the function is really reentrant because it will mess up string if called twice in parallel with the same arguments. So I'm making a NB here: as the answers here (and the linked Wikipedia page) all state, this is not one of the requirements for reentrancy. — Amadan

Adam Wright Adam Wright · Accepted Answer · 2009-09-16T21:26:30

Yes, this is a reentrant function. Reentrant functions are defined as those that can be called whilst they are themselves executing (either due to recursion, or concurrency). In this case, recursion is moot, and you are concurrently safe (assuming differing parameters).

Your argument is fine - there's no global or shared state being accessed either explicitly or implicitly, so reentrancy is ensured. This is a combination of both your explicit code and the semantics of C. Other languages and APIs may not have this property.

Edit: On double checking, the ISO C standard doesn't seem to force the thread safety of strlen. As such, there is a tiny possibility that you could be using a C standard library with a non-thread safe strlen and as such, inherit non-reentrancy from it.

is this function reentrant?

4 Answers