Segmentation fault with linked list

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January 3rd, 2013, 04:53 PM

so.very.tired

Contributing User

Join Date: Nov 2012

Posts: 112

Time spent in forums: 1 Day 4 h 12 m 48 sec
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Segmentation fault with linked list

Hi.

i'm currently taking my first steps with linked list, and i'm having a problem:

whenever i run the program it terminates and mumbles somthing about segmentaion fault.
from what i've read, it means the program tried to access an unauthorized memory block.
i just couldn't figure out what's wrong with the code, and i can really use some help.

here's the code:

Code:

struct numbers
{
	int num;
	struct numbers *next;
};


void Append(struct numbers **headRef, int num)
{
	struct numbers *current;
	struct numbers *newNum;

	current=*headRef;

	newNum=(struct numbers *)malloc(sizeof(struct numbers));
	newNum->num=num;
	newNum->next=NULL;

        //case of first node
	if (current==NULL)
		*headRef=newNum;
	else
	{
		while (current!=NULL)
			current=current->next;
		current->next=newNum;  //it always fails here, at the second call to Append
	}
}

int main()
{
	struct numbers *head=NULL;
	struct numbers *conductor;
	int i;

	for (i=1; i<6 ;i++)
		Append(&head, i);
	conductor=head;

	while (conductor->next!=NULL)
		printf("%d", conductor->num);
}

it always fails at the same code line (marked in red).

thanks in advanced!

January 3rd, 2013, 05:06 PM

jakotheshadows

Contributing User

Join Date: Aug 2009

Posts: 149

Time spent in forums: 3 Days 12 h 1 m 16 sec
Reputation Power: 35

Code:

		while (current!=NULL)
			current=current->next;
		current->next=newNum;

When you start the loop, you already know that current != NULL or you wouldn't have made it into the else to begin with. You should loop on current->next != NULL instead.

The likely reason you're segfaulting is because of your incorrect loop condition. It fails immediately on the second call because with your loop implementation, current will always be null after the loop has finished so current->next is dereferencing a NULL pointer.

Comments on this post

salem agrees!

January 4th, 2013, 03:10 AM

so.very.tired

Contributing User

Join Date: Nov 2012

Posts: 112

Time spent in forums: 1 Day 4 h 12 m 48 sec
Reputation Power: 1

Quote:

Originally Posted by jakotheshadows

Code:

		while (current!=NULL)
			current=current->next;
		current->next=newNum;

brilliant. thank you so much!

Comments on this post

jakotheshadows agrees!

Segmentation fault with linked list

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