Question

I have written a code to add a new member to a list . Its working fine when I take add two members. However, as soon as I add third and compile and run the code gives me an error. The code and error are as follow:

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

struct list_no {
struct list_no *prev,*nest;
struct linked_list *fnl;
int seq_no;
};
static int counter = 0;

struct list_no *lists,*first;

int add_list()
{
   struct list_no *temp;

    counter++;
    if(lists == '\0')
    {
         lists = (struct list_no *)malloc(sizeof(struct list_no));
         lists->prev = '\0';
         lists->nest = '\0';
         lists->fnl  = '\0';
         lists->seq_no = 1;
         first = lists;
    }
    else
    {
         temp = lists;
         lists->nest = (struct list_no *)malloc(sizeof(struct linked_list));
         lists = lists->nest;
         lists->fnl = '\0';
         lists->prev = temp;
         lists->nest = NULL;
         lists->seq_no = counter;
    }
    return 0;
}

int main()
{
   int i=0,lcount;
   int i=0,lcount;

   char ch = 'y';
   first = '\0';
   lists = first;
   int w;

   while(i != 3)
   {
       add_list();
       printf("\n the val ::%d\n",lists->seq_no);
       i++;
   }

   return 0;
}

the error message coming after ./a.out is :

the val ::1
the val ::2

a.out: malloc.c:3097: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.
Aborted

Why its not able to add third member?

Answer 1

You overwrite your pointer

  temp = lists;
  lists->nest = (struct list_no *)malloc(sizeof(struct linked_list));
  lists = lists->nest;

So temp points to the same memory as lists. Then you allocate memory for lists->neste. And after that, you let lists point to that memory, which means, also temp points to it.

Try

memcpy((void*)temp, (*void) lists, sizeof(lists));