Is this a good way to free memory?

Question

The function for freeing an instance of struct Foo is given below:

void DestroyFoo(Foo* foo)
{
    if (foo) free(foo);
}

A

Answer 1

void destroyFoo(Foo** foo)
{
    if (!(*foo)) return;
    Foo *tmpFoo = *foo;
    *foo = NULL;
    memset(tmpFoo, 0, sizeof(Foo));
    free(tmpFoo);
}

Your colleague code is bad because

• it will crash if foo is NULL
• there's no point in creating additional variable
• there's no point in setting the values to zeros
• freeing a struct directly doesn't work if it contain things that has to be freed

I think what your colleague might have in mind is this use-case

Foo* a = NULL;
Foo* b = createFoo();

destroyFoo(NULL);
destroyFoo(&a);
destroyFoo(&b);

In that case, it should be like this. try here

void destroyFoo(Foo** foo)
{
    if (!foo || !(*foo)) return;
    free(*foo);
    *foo = NULL;
}

---

First we need to take a look at Foo, let's assume that it looks like this

struct Foo
{
    // variables
    int number;
    char character;

    // array of float
    int arrSize;
    float* arr;

    // pointer to another instance
    Foo* myTwin;
};

Now to define how it should be destroyed, let's first define how it should be created

Foo* createFoo (int arrSize, Foo* twin)
{
    Foo* t = (Foo*) malloc(sizeof(Foo));

    // initialize with default values
    t->number = 1;
    t->character = '1';

    // initialize the array
    t->arrSize = (arrSize>0?arrSize:10);
    t->arr = (float*) malloc(sizeof(float) * t->arrSize);

    // a Foo is a twin with only one other Foo
    t->myTwin = twin;
    if(twin) twin->myTwin = t;

    return t;
}

Now we can write a destroy function oppose to the create function

Foo* destroyFoo (Foo* foo)
{
    if (foo)
    {
        // we allocated the array, so we have to free it
        free(t->arr);

        // to avoid broken pointer, we need to nullify the twin pointer
        if(t->myTwin) t->myTwin->myTwin = NULL;
    }

    free(foo);

    return NULL;
}

Test try here

int main ()
{
    Foo* a = createFoo (2, NULL);
    Foo* b = createFoo (4, a);

    a = destroyFoo(a);
    b = destroyFoo(b);

    printf("success");
    return 0;
}