Question

I am currently experimenting with C, and I am having a really hard time understanding how to use pointers in functions. I understand what is happening when I initialize a pointer then dereferencing it, however, I get a little confused when I have to use them in functions as follows:

I have three functions here, but I do not know exactly when to use which. I am still relatively new to C.

int returnSomething(int a, int b)
int returnSomething(int *ptrA, int *ptrB)
int* returnSomething(int *ptrA, int *ptrB);

edit:

Is there a major difference between the three?

Answer 1

You need to adapt your usage to every situation.

The first case, you take two ints by value as parameters and return an int. Because your parameters are by value, any changes applied to them will only have function scope.

For example:

int returnSomething(int a, int b)
{
    a = 0;
    b = 0;
    return 0;
}

//....

int x = 3;
int y = 4;
returnSomething(a,b);
// x will still be 3, y will still be 4

In the second case, because you pass parameters as pointers, you will be able to change the values.

int returnSomething(int* a, int* b)
{
    *a = 0;
    *b = 0;
    return 0;
}

//....

int x = 3;
int y = 4;
returnSomething(&a,&b);
// x and y will be 0 here

The third case, besides passing parameters by their pointer, you return a pointer to an int. This means inside the function you have to allocate memory and free it when you are done. I don’t recommend using this, there usually are workarounds to doing it.

int* returnSomething(int* a, int* b)
{
   int* x = malloc(sizeof(int));
   *x = 1;
   return x;
}

//....

int x = 3;
int y = 4;
int* z = returnSomething(&a,&b);
free(z);

The answer is, it depends on what you want to do. If you need to change the parameters value in the method, pass by reference or by pointer. I wouldn’t recommend using the last method.

Also, this applies because you’re dealing with POD types. If you have your own struct, it will be more efficient passing it by pointer or returning a pointer, since a new copy won’t have to be made.