CODEWITHSUNDEEP
cpointersstructnull
Justin Lloyd
Justin Lloyd

Reputation: 123

Pointer not being set to NULL (C)

I'm trying to figure out how to set a struct pointer to NULL within a function. Here is my code. 

struct object
{
    char* name;
    int type;
};

struct object* createObject(char* new_name, int new_type)
{
    struct object* new_object = (struct object*) malloc(sizeof(struct object));

    new_object->name = (char*) malloc(sizeof(char) * (strlen(new_name) + 1));

    strcpy(new_object->name, new_name);
    new_object->type = new_type;

    return new_object;
}

void freeData(struct object* clearedObject)
{
    free(clearedObject->name);
    clearedObject->name = NULL;
    clearedObject->type = 0;
}

void freeObject(struct object* deadObject)
{
    free(deadObject);
    deadObject = NULL;
}

int main(void)
{
    struct object* my_object = createObject("Test", 1234);

    printf("Pointer to Object: %p\n", my_object);
    printf("Pointer to Name  : %p\n", my_object->name);
    printf("Name: %s\n", my_object->name);
    printf("Type: %d\n", my_object->type);
    printf("\n");

    freeData(my_object);

    printf("Pointer to Object: %p\n", my_object);
    printf("Pointer to Name  : %p\n", my_object->name);
    printf("Name: %s\n", my_object->name);
    printf("Type: %d\n", my_object->type);
    printf("\n");

    freeObject(my_object);

    printf("Pointer to Object: %p\n", my_object);
}

This is what the code outputs:

Pointer to Object:  005E17B8
Pointer to Name  :  005E17D8
Name:  Test
Type:  1234

Pointer to Object:  005E17B8
Pointer to Name  :  00000000
Name:  (null)
Type:  0

Pointer to Object:  005E17B8

Why can I set the pointer to the struct's name to NULL within a function, but not the actual struct pointer itself? How can this be done?
I need to set my_object to NULL from within the function, how would this be possible? Thanks.

Upvotes: 0

Views: 566

Answers (1)

Vlad from Moscow
Vlad from Moscow

Reputation: 310950

Define the function the following way

void freeObject(struct object ** deadObject)
{
    free( *deadObject);
    *deadObject = NULL;
}

And call it like

freeObject( &my_object );

The problem with your original function is that the parameter is a local variable of the function. Any changes of a local variable does not influence on the original argument.

Upvotes: 3

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