Segmentation Fault in Control Flow Analysis - data-structures

Question

I'm Getting a Segmentation fault here
if((cur->block)!=(cur->next)->block)
I Tried to fix Some on my own but didn't got it correct.
this Program is used to implement Control Flow Analysis it was having some more Segmentation faults which were identified and fixed ,but now. I don't have any alternative to try and edit this code.
struct Listnode
{
char data[50];
int leader,block,u_goto,c_goto;
struct Listnode *next;
char label[10],target[10];
}*temp,*cur,*first=NULL,*last=NULL,*cur1;
FILE *fpr;
void createnode(char code[50])
{
temp=(struct Listnode*) malloc(sizeof(struct Listnode));
strcpy(temp->data,code);
strcpy(temp->label,"\0");
strcpy(temp->target,"\0");
temp->leader=0;
temp->block=0;
temp->u_goto=0;
temp->c_goto=0;
temp->next=NULL;
if(first==NULL)
{
first=temp;
last=temp;
}
else
{
last->next=temp;
last=temp;
}
}
void main()
{
char codeline[50];
char c,dup[50],target[10];
char *substring,*token;
int i=0,j=0,block,block1;
fpr= fopen("P10-CFA.txt","r");
while((c=getc(fpr))!=EOF)
{
if(c!='\n')
{
codeline[i]=c;
i++;
}
else
{
codeline[i]='\0';
createnode(codeline);
i=0;
}
}
//create last node
codeline[i]='\0';
createnode(codeline);
fclose(fpr);
// find out leaders,conditional stmts
cur=first;
cur->leader=1;
while(cur!=NULL)
{
substring=strstr((cur->data),"if");
if(substring==NULL)
{
if((strstr((cur->data),"goto"))!=NULL)
{
cur->u_goto=1;
(cur->next)->leader=1;
}
}
else
{
cur->c_goto=1;
(cur->next)->leader=1;
}
substring=strstr((cur->data),":");
if(substring!=NULL)
{
cur->leader=1;
}
substring=strstr((cur->data),"call");
if(substring!=NULL)
{
cur->leader=1;
}
if(strstr(cur->data,"return")!=NULL)
{
cur->leader=1;
(cur->next)->leader=1;
}
cur=cur->next;
//to find labels and targets
cur=first;
while(cur!=NULL)
{
if((cur->u_goto==1)||(cur->c_goto==1))
{
substring=strstr(cur->data,":");
if(substring!=NULL)
{
token=strstr(substring,"L" );
if(token!=NULL)
strcpy(cur->target,token);
else
{
substring=strstr(cur->data,"L");
if(substring!=NULL)
strcpy(cur->target,substring);
}
}
if(strstr(cur->data,":")!=NULL)
{
strcpy(dup,cur->data);
token=strtok(dup,":");
//printf("\ntoken:%s",token);
if(token!=NULL)
strcpy(cur->label,token);
}
cur=cur->next;
//to identify blocks
cur=first;
while(cur!= NULL)
{
cur=cur->next;
if((cur->leader)==1)
{
j++;
cur->block=j;
}
else
cur->block=j;
}
printf("\n\n.....Basic Blocks \n");
cur=first;
j=0;
printf("\nBlock %d:",j);
while(cur!=NULL)
{
if ((cur->block)==j)
{
printf("%s",cur->data);
printf("\n\t");
cur=cur->next;
}
else
j++;
printf("\nBlock %d:",j);
}
}
//to output the control flow from each block
printf ("\t\t.......Control Flow.........\n\n");
cur=first;
i=0;
while(cur!=NULL)
{
if((cur->block)!=(cur->next)->block)
{
block=cur->block;
if(cur->u_goto==1)
{
strcpy(target,cur->target);
cur1=first;
while(cur1!=NULL)
{
if(strcmp(cur1->label,target)==0)
{
block1=cur1->block;
printf("\t\tBlock%d---->Block%dln",block,block1);
}
cur1=cur1->next;
}
}
else if(cur->c_goto==1)
{
strcpy(target,cur->target);
cur1=first;
while(cur1!=NULL)
{
if(strcmp(cur1->label,target)==0)
{
block1=cur1->block;
printf("lt\1Block%d---TRUE--->Block%d---FALSE--->Block%d\n",block,block1,(block+1));
}
cur1=cur1->next;
}
}
else if(strstr(cur->data,"return")==NULL)
{
printf("\t\tBlock%d--->Block%d\n",block,(block+1));
}
else
printf("lt\tBlock%d--->NULL\n",block);
}
cur=cur->next;
}
cur=last;
block= cur->block;
printf("\t\tBlock%d--->NULL",block);
}
}
}