Mysterious linked list crashes

I have to currently build a program that removes the forth student from a circle until here are none left. I have done the following code however when running it crashes with no explanation of error. Any help would be grateful and any possible explanations of the error caused.

Code:

---


#include <stdio.h>
#include <stdlib.h>
int main()
    {
    /*Declarations*/
    int i,n,j;
    struct node
        {
        int num;
        struct node * next;
        };
    struct node *header, *last, *temp, *present,*previous, *first;     
    header = NULL;
    printf("Enter how many students in the circle\n");
    scanf("%d",&n);
    /*create the circle of students*/
    for ( i=1;i<=n;i++)
        {
        temp = malloc(sizeof(struct node));
        temp->num=i;
        if (i==1)
            header = temp;
        else
            last -> next = temp;
        last = temp;
        }
    /*prints of list and removes students*/
    present = header;
    first = present;
   
    while (n>0)
        {
        for (j=0;j<3;j++)
            {
            if (present == NULL)
                present = first;
            if (j==1)
                previous = present;
            present = present -> next;
            }
        if (present == first)
            first = first -> next;
        /*print off each student before removing them*/
        printf("%d ",present -> num);
        temp = present;
        previous->next = present -> next;
        free(temp);
        n--;
        }
    }

---

At the end of the while loop, you free(temp) and thereby make "present" point to freed memory. You should make present point to something that exists before the next iteration of the while loop.

Thanks for that, Ive added an extra line as now seen above, how ever now I get the wrong output, for example with 20 students I get 4,8,12,16,20,6,14,2,18,10,1072626 etc

Your code for removing every fourth node is off:

Code:

---

$ gdb -q ./foo
Reading symbols from ./foo...done.
(gdb) b 46
Breakpoint 1 at 0x80485d5: file foo.c, line 46.
(gdb) r
Starting program: ./foo
Enter how many students in the circle
4

Breakpoint 1, main () at foo.c:46
46              previous->next = present -> next;
(gdb) p *previous
$1 = {num = 2, next = 0x804b028}
(gdb) p *present
$2 = {num = 4, next = 0x0}
(gdb) n
47              free(temp);
(gdb) p *previous
$3 = {num = 2, next = 0x0}

---

As you can see you change the next-pointer of the wrong node ("2" isn't the previous node of "4") thus destroying the list.

I would recommend implementing a circular list for your task.

Bye, Andreas