如何在C中调用具有void参数的Void函数

Question

I am a beginner in C. For now I have a function like this

void mark_function(void *obj, void (*mark_obj)(void *));

To my understand, this a void function, and it has two parameter, first one is a void pointer, and the second one is a another void function with a void parameter.

I tried a lot ways to call it, but it seems not work properly,either give me back the segmentation false or the pointer type not the same warning.

So What exactly means for this function? what parameters should pass in? and how to call it? Any help?

Thanks alot!

Answer 1

Let's make it more clear first. You have a void function, called mark_function , which takes 2 parameters. The first parameter is a void pointer and the second parameter is a pointer to a function that returns void and takes as a parameter a void pointer. Let's create a function that will be apropiate to pass as parameter to mark_function .

void param(void *p) {
    // function body
}

Assume a and b are 2 void pointers. I will not enter into details about their scope, but you need to pay attention to it: they must be available in the scope they are used. Then, the mark_function will be called as:

mark_function(a, param);

Inside mark_function body you can have something like:

param(b);

, which is a call to the function passed as a parameter.

---

Long story short: the function pointers used as parameters are meant to make it possible for the function that requires them to perform different activities by calling different functions. The value of a pointer function is simply the name of a function that has the appropiate signature (return value and parameter list). Your function can use this to call the function provided as parameter as needed.

Answer 2

You should call the function with one pointer to an object, and one pointer to a function of suitable type.

int my_int;

void mark_function(void *obj, void (*mark_obj)(void *));

void my_func (void *vp)
{
  /* Convert vp to a pointer to a real type */
  int *ip = vp;

  /* Do something with ip or *ip */
}

mark_function (&my_int, my_func);

A pointer to object ( &my_int ) can safely be convert to a void * and back to the same type. The resulting pointer is guaranteed to be identical to the original pointer. This allows mark_function() to perform some task regardless of the actual type that the pointer points to.

Typical examples of such tasks include sorting of arrays with a custom compare function provided by the caller, or maintaining linked lists without knowing the types of the object stored inside, an so on.