How to know the segmentation faults in C? The code is below(seeing to the word limit its incomplete)

Question

#include<stdio.h>
#include<stdlib.h>

struct node
{
int rno;
struct node *next;
};

void main()
{
int n,i,r;
struct node *head;
head=(struct node *)malloc(sizeof(struct node));
head=NULL;
scanf("%d",&n);
for(i=1;i<=n;++i)
{
scanf("%d",&r);
if(head==NULL)
{
head->rno=r;
head->next=NULL;
}
struct node *tmp;
tmp=head;
int f=0;
while(tmp!=NULL)
{
if(tmp->rno==r)
{
f=1;
break;
}
tmp=tmp->next;
}
if(f=0)
{
tmp->next=head;
head=tmp;
}
if(f=1)

Answer 1

in line
head=(struct node *)malloc(sizeof(struct node));
you allocate memory for a node, but in the next line:
head=NULL;
you overwrite the address of allocated memory with NULL.
Remove this line.

segmentation fault arises in the next lines:
if(head==NULL)
{
head->rno=r; // in this line segmentation fault arises because head is NULL
head->next=NULL;
}

You check head on NULL and write data to head if it is NULL.
But you have to do the opposite: if head is NULL you shouldn't write to it, you should allocate memory for it.