Function interposition: about the use of a static function variable

Question

I want to interpose a library function from a third party, using LD_PRELOAD, and I have seen the following code pattern in a lot of places:

void interposed_func()
{
    static void(*real_func)() = NULL;

    if (real_func == NULL)
       real_func = (void(*)())dlsym(RTLD_NEXT, "interposed_func");

    real_func();
}

It calls to my attention that if at each call of the function. What's wrong with just:

void interposed_func()
{
    static void(*real_func)() = (void(*)())dlsym(RTLD_NEXT, "interposed_func");
    real_func();
}

Since real_func is a static function local variable it will only be initialized once right? I don't understand why all of the examples I have faced uses an additional but (in my view) unnecesary if. Is there any else to consider that I'm not aware of?

Answer 1

Objects with static storage duration (i.e. those declared at file scope or with the static keyword) may only be initialized with a constant expression, as such initialization effectively happens at compile time.

Your second example does not qualify because a function call is not a constant expression. It would be the same as trying to initialize a file scope variable with a function call.

The first example is required to satisfy the requirement of a constant initializer. The overhead of a NULL check on each access is minimal.

Also, the conversion on this line:

real_func = (void(*)())dlsym(RTLD_NEXT, "interposed_func");

Is technically not allowed by the C standard, as it converts an object pointer to a function pointer. The proper conversion would be this:

*(void **)(&real_func) = dlsym(RTLD_NEXT, "interposed_func");

As noted in the man page for dlsym in an example:

   /* Writing: cosine = (double (*)(double)) dlsym(handle, "cos");
       would seem more natural, but the C99 standard leaves
       casting from "void *" to a function pointer undefined.
       The assignment used below is the POSIX.1-2003 (Technical
       Corrigendum 1) workaround; see the Rationale for the
       POSIX specification of dlsym(). */

   *(void **) (&cosine) = dlsym(handle, "cos");

Answer 2

None is correct.

1. The second code will not compile in C.

2. The code doesn't check if the symbol exists. If dlsym does not exist you will be calling a NULL pointer converted to a function pointer which is UB

void interposed_func()
{
    static void(*real_func)() = NULL;

    if (real_func == NULL)
       real_func = dlsym(0, "interposed_func");

    if(real_func) real_func();
}