Error while freeing memory in C after program giving correct answer

Question

So here's the code for primes sieve, and it gives correct output, does exactly what I want it to do, except ugly errors after printing correct result.

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

void prime_gen(int *, int);
void show_primes(unsigned long, unsigned long, int *, int);

int main(void)
{
  int no_testcases=0;
  int count=0;
  unsigned long lower=0,upper=0;
  unsigned long size;

  size=sqrt(1000000000);
  printf("%lu\n", size);
  int * primes, *primes_temp;;
  primes=(int *)calloc(size+1,sizeof(int));
  assert (primes!=NULL);

  prime_gen(primes, size+1); /* generates array of 0 and 1's indicating if index is a prime number */

  scanf("%d", &no_testcases); /* no of test cases */
  while (count<no_testcases)
  {
    scanf("%lu %lu", &lower, &upper);
    show_primes(lower, upper, primes, size+1); /* shows all prime numbers in given range */
    count++;
    if (count!=no_testcases)
      putchar('\n');
  }

  free(primes);

  return 0;
}

And the output:

31622
size=31623, set_index=63248, index=31625
1
1
10  
2
3
5
7
No of primes = 4
*** glibc detected *** ./a.out: double free or corruption (out): 0x09d49008 ***
a.out: malloc.c:2451: 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.
Przerwane (core dumped)

Here's code for primes generator and show_primes:

void prime_gen(int * tab, int size) /* 0== is prime, 1== is not prime */
{
  tab[0]=1;
  tab[1]=1;
  unsigned long index=2;
  unsigned long set_index;


  for(index=2;index<=size;index++)
  {
    while (index<=size && tab[index]!=0) /* goes to next prime number */
    {
      index++;
    }
    for(set_index=index+index;set_index<=size;set_index+=index)
      tab[set_index]=1;
  }
  printf("size=%d, set_index=%lu, index=%lu\n", size, set_index, index);
}

void show_primes(unsigned long l, unsigned long u, int * array, int size)
{
  int index = 0;
  int is_prime=1;
  int primes_count=0;

  for(l;l<=u;l++)
  {
    if (l<=size) /* if l is valid index of array with primes */
    {
      if(array[l]==0)
      {
    printf("%lu\n", l);
    primes_count++;

      }
    }
    else /* if checked number is bigger than last index of array */
    {
      is_prime=1;
      for (index=2;index<=size;index++)
      {
    if (array[index]==0)
    {
      if (l%index==0)
      {
        is_prime=0;
        break;
      }
    }
      }
      if (is_prime)
      {
    printf("%lu\n", l);
    primes_count++;

      }
    }

  }
  printf("No of primes = %d\n", primes_count);
}

I narrowed it to free(primes) as the problem, because when removed error disappears and program terminated as supposed to. And I know array would be better here, but dynamic arrays are new to me and it's supposed to be my lesson on them. Many thanks for answers.

Answer 1

My guess:

void prime_gen(int *, int);
void show_primes(unsigned long, unsigned long, int *, int);

int main(void)
{
  int no_testcases=0;
  int count=0;
  unsigned long lower=0,upper=0;
  unsigned long size;

  size=sqrt(1000000000);
  printf("%lu\n", size);
  int * primes, *primes_temp; *primes_head;
  primes=(int *)calloc(size+1,sizeof(int));
  primes_head = primes;
  assert (primes!=NULL);

  /* NOTE CHANGE BELOW */
  prime_gen(primes, size); /* generates array of 0 and 1's indicating if index is a prime number */

  scanf("%d", &no_testcases); /* no of test cases */
  while (count<no_testcases)
  {
    scanf("%lu %lu", &lower, &upper);
    /* NOTE CHANGE HERE */
    show_primes(lower, upper, primes, size); /* shows all prime numbers in given range */
    count++;
    if (count!=no_testcases)
      putchar('\n');
  }

  free(primes_head);

  return 0;
}

Answer 2

In prime_gen function, you have this:

for(set_index=index+index;set_index<=size;set_index+=index)
  tab[set_index]=1;

You are overflowing your array. On the last iteration you write to element tab[set_index] with set_index value greater than size.

Try to change your for loop stop condition to: set_index < size

Answer 3

You are calling primes_gen and show_primes with the primes array and the size of the array. Inside those functions, you are reading from and writing to one past the bounds of the array:

for(set_index=index+index;set_index<=size;set_index+=index)
  tab[set_index] = 1;

This is corrupting your array and so your call to free() is failing.

Answer 4

try moving scanf("%lu %lu", &lower, &upper); outside of the loop