freeing an allocation in a struct inside a struct - c

Question

When I try to free an allocation in a struct inside a struct, I get an error.
How can I fix it?
typedef struct card
{
char* sign;
char* color;
int number;
char* name;
}card;
typedef struct deck
{
card data;
deck* next;
}deck;
deck* deleteHead(deck* head)
{
deck* curr = head;
if (head==NULL)
return head;
curr=curr->next;
if(head->data.color!=NULL)
free(head->data.color);//error
if(head->data.name!=NULL)
free(head->data.name);//error
if(head->data.sign!=NULL)
free(head->data.sign);//error
free(head);//ok
return curr;
}
when I'll delete the errors and only freeing the head - it'll work, but when I'll try to delete the allocations inside the head, I'll get a run time error.
How can I solve this?
Thank you in advance.

Answer 1

You probably did not initialize the pointers in the card structure. These should either be initialized to NULL or to a pointer to memory allocated by malloc, calloc or strdup.
Also note that you don't need to test pointers against NULL before calling free(). free(NULL); will gracefully return immediately, it is legal to call free with NULL. Incidentally it is also legal in C++ to delete a null pointer.
The function can be further simplified this way:
deck *deleteHead(deck *head) {
deck *next = NULL;
if (head != NULL) {
next = head->next;
free(head->data.color);
free(head->data.name);
free(head->data.sign);
free(head);
}
return next;
}

Answer 2

The function free can only de-allocate a block of memory previously allocated by a call to malloc, calloc or realloc. Your code will run without any runtime error if you initialize it properly. Here's a sample code:
int main()
{
deck* root = (deck*)malloc(sizeof(struct deck));
root->card.color = strdup("color");
root->card.name = strdup("name");
root->card.sign = strdup("sign");
root->card.number = 2;
root->next = NULL;
root = deleteHead(root);
return 0;
}
And also there is a slight correction in your code:
typedef struct deck
{
card data;
struct deck* next;
}deck;