Segmentation fault but can't find the bug - c

Question

#include<stdio.h>
#include<stdlib.h>
struct node {
char str[200];
int nn;
struct node* next;
};
int number;
struct node* start=NULL;
struct node* current;
//function to insert into the list
void insert() {
struct node* n;
n=(struct node*)malloc(sizeof(struct node));
n->str=malloc(sizeof(char) * 1000);
printf("please enter the data that you would like to insert: ");
gets(n->str);
printf("asdasdasdasd");
n->next=NULL;
if( start==NULL ) {
start->next=n;
current=n;
}
else {
current->next=n;
current=n;
}
printf("done\n");
}
void display() {
current=start;
int i=0;
while( current->next!=NULL ) {
printf("node%d= %s\n",++i,current->str);
current=current->next;
}
printf("this the end");
}
int main() {
char c;
int input;
do {
printf("Select from the following options:\n"
"1.Display list\n"
"2.Add to list\n"
"3.delete from list\n");
scanf("%d",&input);
switch (input) {
case 1: display(); break;
case 2: insert(); break;
// case 3: delete(); break;
default : printf("Please select 1 , 2 or 3\n");
}
printf( "would you like to continue?(y/n)\n");
scanf("%s",&c);
} while(c=='y');
return 0;
}
This is giving me an error in the insert function, A SEGMENTATION FAULT !
I Tried things but I just don't get the clear picture. I am a bit weak with pointers, actually confused !
Please help me out by telling me what I am doing wrong. Forget about my linked list logic, let it be wrong. I just wanna know why a segmentation fault is occurring !

Answer 1

you have many mistakes, but your crash is because you don't allocate start
n->next=NULL;
if( start==NULL ) {
start->next=n;
current=n;
}
else {
current->next=n;
current=n;
you ask if start is null, and if it is you try to read it by calling start->next it. you need to allocate it first.

Answer 2

Since you did not tell us the line in which your segfault happened, I can only guess.
At least these lines sounds bogus to me:
if( start==NULL ) {
start->next=n;
current=n;
}
Are you sure that you want to dereference start when it is NULL? That is what you are doing with start->next. Dereferencing NULL causes a segfault.
Maybe this will work the way you expect it:
if( start==NULL ) {
start=n;
current=n;
}
So basically, if start was not defined yet, define it and it also becomes your current item.

Answer 3

You should use debugger (not us) in order to find more information about the problem.
For example gdb (GNU Debuger).

Answer 4

Compilation Error
n->str=malloc(sizeof(char) * 1000);
and this is address reassign on a constant!!!!!You are changing the address of array???
Dangerous Function with 25% probability to execute.
gets(n->str);
Setting value at null pointer!!!
struct node* start=NULL;....
start->next=n;
Example of working Code
#include<stdio.h>
#include<stdlib.h>
struct node {
char *str;
int nn;
struct node* next;
};
int number;
struct node* start=NULL;
struct node* current;
//function to insert into the list
void insert() {
struct node* n;
n=(struct node*)malloc(sizeof(struct node));
n->str=(char*)malloc(sizeof(char) * 1000);
printf("please enter the data that you would like to insert: ");
scanf("%s",n->str);
printf("asdasdasdasd");
n->next=NULL;
if( start==NULL ) {
//start->next=n;
current=n;
}
else {
current->next=n;
current=n;
}
printf("done\n");
}
void display() {
current=start;
int i=0;
while( current->next!=NULL ) {
printf("node%d= %s\n",++i,current->str);
current=current->next;
}
printf("this the end");
}
int main() {
char c;
int input;
do {
printf("Select from the following options:\n"
"1.Display list\n"
"2.Add to list\n"
"3.delete from list\n"
);
scanf("%d",&input);
switch (input) {
case 1: display(); break;
case 2: insert(); break;
// case 3: delete(); break;
default : printf("Please select 1 , 2 or 3\n");
}
printf( "would you like to continue?(y/n)\n");
scanf("%s",&c);
}while(c=='y');
return 0;
}

Answer 5

if( start==NULL ) {
start->next=n;
current=n;
}
I think you have a little problem here ;-)

Answer 6

the next line is not safe:
scanf("%s",&c);
scanf adds terminating null character '\0' that corrupts stack in your case.