undirected Graph adjacency list implementation using C - graph

Question

Code :
#include <stdio.h>
#include <stdlib.h>
#define N 5
typedef struct list
{
int data;
struct list * next;//self referenced structure
}slist;
void displayList(slist * start)
{
slist * temp;
if(start==NULL)
{
printf("Empty Linked List");
return;
}
for(temp=start;temp!=NULL;temp=temp->next)
printf("%d ",temp->data);
return;
}
void insertLast(slist * * start,slist * node)
{
slist * temp;
if(start==NULL){
(* start)=node;
return;
}
temp=(* start);
while((temp->next)!= NULL)
temp=temp->next;
temp->next=node;
return;
}
int main()
{
int i,j;
//slist * node;
char Ans;
/*printf("Write the number of vertices\n");
scanf("%d",&N);*/
slist * start[N];
for(i=0;i<N;i++)
start[i]=NULL;
for(i=0;i<N;i++)
{
for(j=i+1;j<N;j++)
{
printf("Is there a connection between V[%d] and V[%d]\n",(i+1),(j+1));
scanf(" %c",&Ans);
if(Ans=='y'||Ans=='Y')
{
slist * node1=(slist *)malloc(sizeof(slist));
node1->data=(j+1); node1->next=NULL;
insertLast(&start[i],node1);enter code here
slist * node2=(slist *)malloc(sizeof(slist));
node2->data=(i+1); node2->next=NULL;
insertLast(&start[j],node2);
}
}
}
for(i=0;i<N;i++)
{
displayList(start[i]);
printf("\n");
}
return 0;
}
The above code is showing segmentation fault at the line where while((temp->next)!=NULL) is written whereas while creation of linked lists, the same insertLast worked just fine. What is the fault in the code?

Answer 1

Your program crashed as you are checking if start is NULL or not. But that does not guarantee that *start is also not NULL. In such situation, temp gets NULL and in while loop temp->next actually trying to access next element of NULL pointer and that is why the crash.
Changing this line -
if(start==NULL)
to
if(*start==NULL)
in insertLast() will fix the crash.
I also recommend to use a debugger like gdb to debug such issues.