Linked list always show empty - c

Question

REQUIREMENT / QUESTION 2
In computing, the process identifier (normally referred to as the process ID or PID), is a number used by most operating system kernels, to uniquely identify an active process. The PIDs are usually allocated on a sequential basis, beginning with 0 and rising to a maximum value that varies from system to system. Create a link list to store the PIDs. To create a new PID, createPID() function is used.
Each PID is inserted into the list at the beginning of the list, using insertPID() function. Once a process is completed, that particular PID is deleted, using the deletePID() function.
The assessment will be done based on the following criteria:
A proper writing of C codes and its structure
The ability of program to be compiled and executed
Implementation of correct programming techniques
Complete documentation and correct submission
Note: You must write C programming codes for this assignment.
I already create a code:
#include<stdio.h>
#include<stdlib.h>
struct node
{
int info;
struct node *link;
};
void createPID ();
void insertPID (struct node *start,int x);
struct node * deletePID (struct node * start,int x);
void displayPID(struct node *start);
struct node * start;
int main()
{
createPID();
insertPID(start,0);
insertPID(start,1);
insertPID(start,2);
displayPID(start);
start=deletePID(start, 3);
displayPID(start);
}
void createPID(){
struct node *start = NULL;
}
void displayPID(struct node *start)
{
struct node *p;
if(start == NULL)
{
printf("List is empty\n");
return;
}
printf("List is :");
p=start;
while(p!=NULL)
{
printf("%d\t",p->info);
p=p->link;
}
printf("\n");
}
/*End of displayList()*/
void insertPID(struct node * start, int data)
{
struct node *temp,*p;
p=start;
while (p!=NULL)
{
if(p->link==NULL)
break;
p=p->link;
}
temp=(struct node *)malloc (sizeof (struct node));
temp->info=data;
if(p==NULL)
start=temp;
else
{
temp->link= p->link;
p->link= temp;
}
}
struct node* deletePID(struct node * start,int x){
struct node *temp, *p;
if(start == NULL)
{
printf("List is empty\n");
return start;
}
/*Deletion of first node */
if(start->info == x)
{
temp=start;
start= start->link;
free(temp);
return start;
}
/*Deletion in between or at the end */
p=start;
while (p->link != NULL)
{
if (p-> link -> info== x)
break;
p=p-> link;
}
if(p->link==NULL)
printf("Element %d not in list \n\n",x);
else
{
temp=p->link;
p->link=temp->link;
free (temp);
}
return start;
}
The result always empty list. Need help to figure out what is the problem ?

Answer 1

Problem is in insertPID function. start as argument of function is local variable, it hides global start and the line below
start = temp;
only modifies local start, global start is not affected and it is always NULL.
If you want to update start pointer you need to pass it by pointer-to-pointer:
void insertPID (struct node **start,int x);
//...
insertPID(&start,0);
insertPID(&start,1);
insertPID(&start,2);
//...
void insertPID(struct node ** start, int data)
{
//...
p=*start;
//...
if(p==NULL)
*start=temp;
//...
}
void createPID(){
struct node *start = NULL;
}
I assume this function wants to set start (global) to 0 ? So it should be:
void createPID(){
start = NULL;
}
Your version introduces local start variable, so global is not affected.
But the call of createPID is superfluous here because start as global variable is initialized to 0 before main starts executing.