Why are same addresses sent into function? [duplicate] - c

Question

This question already has answers here:
Copy one pointer content to another
(4 answers)
Closed 1 year ago.
I'm working on doubly linked list and I came across the problem. I will paste parts of code step by step I try to explain what's happening.
So I have defined datatype:
typedef struct node {
void *data;
struct node *prev, *next;
} NODE;
In this part of add function might be something wrong. I've checked all scenarios but I don't want to paste unnecessary code.
void add(NODE **phead, NODE **ptail, void *data, int (*cmp)(const void*, const void*)){
NODE *p, *q, *new = (NODE*)calloc(1, sizeof(NODE));
new->data = data;
if(*phead == 0)
*phead = *ptail = new;
else if((*cmp)((*phead)->data, data) > 0){
new->next = *phead;
(*phead)->prev = new;
*phead = new;
}
When I was debugging code I saw that when function pointer is called same addresses were sent as arguments.
NODE *search(NODE *head, NODE *tail, const void *data, int (*cmp)(const void*, const void*)){
if(head == 0)
return 0;
while ((*cmp)(head->data, data) < 0 && (*cmp)(tail->data, data) > 0)
{
head = head->next;
tail = tail->prev;
}
if((*cmp)(head->data, data) == 0)
return head;
else if((*cmp)(tail->data, data) == 0)
return tail;
else
return 0;
}
Compare function which is argument of add and search function is:
int cmp_str(const void *a, const void *b){
return strcmp((const char*)a, (const char*)b);
}
Part of main function where both search and add are called:
int main(){
NODE *head = 0, *tail = 0;
char c, *data = (char*)calloc(20, sizeof(char));
do
{
printf("Add [A], delete [D], write [W], search [S], end [0]: ");
scanf("\n%c", &c);
if(c == 'A'){
get_string(&data);
NODE *p = search(head, tail, data, &cmp_str);
if(p){
p->data = data;
printf("Data updated!\n");
}
else{
add(&head, &tail, data, &cmp_str);
printf("Data added.\n");
}
}
So basically the thing that goes wrong is that only one data is saved. Here I work with strings but the parameters and variables of data is void*. So when I input add two nodes only the data of the last typed is saved. Also in main, every time except first p is found by search function even if it doesn't exist. As I told, debugger's saying that cmp_str receives two arguments of same address, which might be hint to find where the error is.

Answer 1

You are setting data pointer as same for each add operation. It seems you allocated once and using same memory for every cycle. If I'm wrong please correct me.
You need to allocate new memory and deep copy data pointer's content into new->data.
void add(NODE **phead, NODE **ptail, void *data, int (*cmp)(const void*, const void*)){
NODE *p, *q, *new = (NODE*)calloc(1, sizeof(NODE));
new->data = data;
...

Answer 2

The answer is very simple. It is quite common misunderstanding when people learn C.
The data pointer points always to the same memory location.
In add functions you simply assign this reference to all nodes. In C = does not copy the memory referenced by the pointers only assigns the reference.
You need to allocate it at every interaction (in main) or copy it in the add function.
void add(NODE **phead, NODE **ptail, void *data, size_t data_size, int (*cmp)(const void*, const void*)){
NODE *p, *q, *new = calloc(1, sizeof(*new));
//check if calloc did not return NULL
new -> data = calloc(1, data_size);
//check if calloc did not return NULL
memcpy(new->data, data, data_size);
// ...
Some additional remarks:
Use objects not types in sizeof.
Do not cast the result of malloc/calloc. If compiler is giving you errors the it is an indication that you compile the C code using C++ compiler which is not a good idea.