Dynamic memory array crash the executable

Question

I have this code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{

    int num,i,k,tamano,cont=0;

    int *prim;
    scanf("%d",&num);
    num=num+1;
    prim = malloc(num * sizeof(int));


    for(i=1;i<num;i++)
        prim[i] = 1;

    for(i=2;i<=sqrt(num);i++){
        if((prim[i])!=0 ){
        for(k=2;i*k<num;k++)
                prim[k*i]=0;
        }
    }

    for(i=2;i<num;i++)
        if(prim[i])
            printf("%5d",i);

free(num);
}

This code is for determinate the prime numbers from 2 to num, using the Sieve of Eratosthenes algorithm.

But I have a problem, when I put a number like 15 the executable crash, I don't know what is the problem. If I use a static array the program works perfect, but when I put the dynamic thing the executable crash with no apparent reason.

What can it be?

free(num); On no. Are you sure it's not a typo for free(prim);? — Yu Hao
– Yu Hao, Commented Aug 15, 2014 at 3:01
I'm surprised your compiler didn't give you at least a warning. — T.C.
– T.C., Commented Aug 15, 2014 at 3:09

metacubed · Accepted Answer · 2014-08-15 03:06:13Z

3

You are freeing the integer num instead of the pointer prim. Your last statement should be:

free(prim);

Freeing the integer will cause the run-time to interpret the integer value as a memory address and try to access it. This causes unknown behavior to execute, most probably a protected memory violation, which crashes the program.

answered Aug 15, 2014 at 3:06

metacubed

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Comments

paxdiablo · Accepted Answer · 2014-08-15 03:32:35Z

2

You are allocating prim then freeing num.

You should only attempt to free things that have been allocated.

The final line should be changed to:

free (prim);

A good compiler will generally warn you about this, along the lines of "Trying to free a variable that isn't a pointer".

edited Aug 15, 2014 at 3:32

answered Aug 15, 2014 at 3:05

paxdiablo

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2 Comments

exsnake Over a year ago

is code::blocks, there is another one better than that one?

paxdiablo Over a year ago

Code blocks is an IDE, I believe it uses gcc under the covers which is pretty good.

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