using arrays as hashTables - c

Question

i am trying to use arrays as hashtable, each array ponits to its own linked list
the size is to check that the number of nodes of the linked list is 32.
my problem here is that i get segmentation fault, but i cant see any error in my pointers, here is the full code.
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <time.h>
typedef int bool;
enum { false, true };
void main(int argc, char *argv[])
{
// create linked list---------------------------
struct node
{
int num;
struct node *ptr;
};
typedef struct node NODE;
NODE *first, *last, *temp, *newNode=0 ;
int count = 0;
first = NULL;
last=NULL;
temp=NULL;
newNode=0;
//-----------------------------------------------------------------------
//filling the text file with billions of integers------------------------------------------------
FILE *f = fopen("file.txt", "w");
if (f == NULL)
{
printf("Error opening file!\n");
exit(1);
}
int i = 0;
unsigned long long randomvalue;
for (i = 0; i < 10000000; i++)
{
randomvalue = random();
randomvalue <<= 16; // just picked 16 at random
randomvalue ^= random(); // you could also use + but not "or";
randomvalue %= 10000000000ULL;
fprintf(f,"%lld \n",randomvalue);
}
fclose(f);
NODE* array[312500];
first = (NODE *)malloc(sizeof(NODE));
last= (NODE *)malloc(sizeof(NODE));
temp = (NODE *)malloc(sizeof(NODE));
newNode = (NODE *)malloc(sizeof(NODE));
FILE *file = fopen ("file.txt", "r");
int x=0;
for ( x=0; x<=312500; x++)
{
while (count <=32)
{
fscanf (file, "%d", &temp->num);
temp->ptr=NULL;
newNode->num=temp->num;
newNode->ptr=NULL;
if (first != 0)
{
last->ptr=newNode;
last=newNode;
count=count+1;
}
else
{
first = newNode;
last = newNode;
count=count+1;
}
fflush(stdin);
newNode->ptr=0;
newNode=NULL;
}
count =0;
array[x]->ptr=first;
first->ptr=0;
first=NULL;
last->ptr=0;
last=NULL;
}
fclose (file);
temp->ptr = 0;
temp=NULL;
}

Answer 1

According to gdb:
Program received signal SIGSEGV, Segmentation fault. 0x00401618 in
main (argc=1, argv=0x8d0ce0) at tryy.c:77 77
newNode->num=temp->num;
(gdb) p newNode
$10 = (NODE *) 0x0
So you are getting a SIGSEGV because you are trying to access memory at 0x0.
Problems I see with your code(they may not be directly related to why newNode becomes NULL):
first = (NODE *)malloc(sizeof(NODE));
0x0 is one of the valid return values from malloc. Almost always I prefer to have a if check after a malloc call. Similarly for other api calls where a null ptr can be returned
You have declared NODE* array[312500]; but the value of x goes upto 312500 so you may end up accessing array[312500] which is undefined behavior. (Often causes memory corruption by overwriting values of other variables on stack)