Supplied parameter to the function becomes NULL? - c

Question

I wrote a recursive function to reverse a linked list as follows:
struct node{
int val;
struct node *next;
};
//Global pointer to structure
struct node *start=NULL,*head=NULL;
//*Function to input node*
void create(int data){
struct node *temp;
temp=(struct node *)malloc(sizeof(struct node));
if(start == NULL){
temp->val=data;
temp->next=NULL;
start=temp;
head=temp;
}
else{
temp->val=data;
temp->next=NULL;
head->next=temp;
head=temp;
}
}
*Function to reverse the linked list*
void* rev(struct node *prev,struct node *cur){
if(cur!=NULL){
printf("Works");
rev(cur,cur->next);
cur->next=prev;
}
else{
start=prev;
}
}
And the related code in main is:
main(){
struct node *temp;
temp=start;
/*Code to insert values*/
rev(NULL,temp);
}
Now the code takes input and prints it perfectly, but after I call rev() function the same traversal function prints nothing.
I did run the code on debugger line by line n it gave me the following output:
rev (prev=0x0, cur=0x0)
Also since cur is somehow NULL, the if part of rev() never gets executed and only the else executes once.
When I take input in my create() function I do update start to the first element of the linked list and even in main a print statement proves it is so.
But then why the function rev() always receives input parameters as NULL?
Please comment if any extra information is required.

Answer 1

Specific problems with your code: your main() function lacks sufficient code to test the reversal functionality (e.g. it doesn't create any nodes!); your create() routine really needs a head and tail pointer to work correctly, not the current head and start; your reversal function maintaines the head/start pointer but doesn't handle a tail pointer; you've redundant code in your if and else clauses that can be pulled out of the conditional; you declared rev() a void * instead of simply a void.
I've reworked your code below with addressing the above changes along with some style issues:
#include <stdlib.h>
#include <stdio.h>
struct node {
int value;
struct node *next;
};
// Global pointers to structure
struct node *head = NULL, *tail = NULL;
// Function to add node
void create(int data) {
struct node *temporary = malloc(sizeof(struct node));
temporary->value = data;
temporary->next = NULL;
if (head == NULL) {
head = temporary;
} else {
tail->next = temporary;
}
tail = temporary;
}
// Function to reverse the linked list
void reverse(struct node *previous, struct node *current) {
if (current != NULL) {
reverse(current, current->next);
current->next = previous;
} else {
head = previous;
}
if (previous != NULL) {
tail = previous;
}
}
void display(struct node *temporary) {
while (temporary != NULL) {
printf("%d ", temporary->value);
temporary = temporary->next;
}
printf("\n");
}
// And the related code in main is:
int main() {
/* Code to insert values */
for (int i = 1; i <= 10; i++) {
create(i);
}
display(head);
reverse(NULL, head);
display(head);
create(0);
display(head);
return 0;
}
OUTPUT
> ./a.out
1 2 3 4 5 6 7 8 9 10
10 9 8 7 6 5 4 3 2 1
10 9 8 7 6 5 4 3 2 1 0
>
You should add a routine to free the nodes in the linked list.