[SOLVED] *** Error in `./a.out': free(): invalid next size (fast): 0x0000560569d5d830 ***

zazamama · 12-13-2017, 11:47 AM

Hello,
my program: (compiled with : gcc -lm prog1.c)

Code:

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

// Returns the exponent of the largest power of 2 less than x.

int pow2(int x)
{
	int i;
	for(i=0; pow(2, i)<=x; i++)
		;
	return i-1;
}

int main()
{
	int a, n, k, i;

	printf("give a positif integer \n");
	scanf("%d", &a);
	k = n = pow2(a);
	printf("n = %d\n", k);
	int *tab = (int *)malloc((n+1)*sizeof(int));
	for(i=0; i<=n; i++)
		tab[i]=0;

	for(i=0; i<=n; i++)
	{
		tab[n-k]=1;
		a=a-pow(2, k);
		k=pow2(a);
	}

	for(i=0; i<=n; i++)
		printf("%d ", tab[i]);
	
	printf("\n");

	free(tab);

	return 0;
}

all work fine until 31 but for 32 I get:

32
n = 5
1 0 0 0 0 0
*** Error in `./a.out': free(): invalid next size (fast): 0x0000560569d5d830 **

if in the line : int *tab = (int *)malloc((n+1)*sizeof(int));
I replace n+1 with n+2 the problem disappears:

255
n = 7
1 1 1 1 1 1 1 1

I do not understand why n + 1 does not work beyond 31 ?

Thanks in advance for your help

NevemTeve · 12-13-2017, 12:02 PM

If you allocate space for 'n' elements, the indexes are 0..n-1
If you allocate for 'n+1' elements, the indexes are 0..n
If you use an index outside the range, the results are unpredictable.

pan64 · 12-13-2017, 12:21 PM

you may try to use ddd (data display debugger) to check how your program works.

suicidaleggroll · 12-13-2017, 02:50 PM

Put some print statements in. Whenever you subscript an array, just print out what you're doing, eg: printf("tab[%d]=1\n",n-k);

In this case, this is the result for 31:

Code:

$ ./a.out 
give a positif integer                                                                              
31                                                                                                     
n = 4                                                                                                  
tab[0]=1                                                                                                  
tab[1]=1                                                                                                     
tab[2]=1                                                                                                        
tab[3]=1                                                                                                        
tab[4]=1                                                                                                           
1 1 1 1 1

And for 32:

Code:

$ ./a.out                                                                                     
give a positif integer                                                                                                 
32                                                                                                                          
n = 5                                                                                                                       
tab[0]=1                                                                                                                    
tab[6]=1                                                                                                                        
tab[6]=1                                                                                                                        
tab[6]=1                                                                                                                        
tab[6]=1                                                                                                                            
tab[6]=1                                                                                                                            
1 0 0 0 0 0

You're trying to assign values to tab[6], which doesn't exist.

astrogeek · 12-13-2017, 04:54 PM

Please place your code snippets inside [CODE]...[/CODE] tags for better readability. You may type those yourself or click the "#" button in the edit controls.

zazamama · 12-14-2017, 03:59 PM

Thank you to everyone who answered.
suicidaleggroll, your explanation is crystal clear and now I understand why n+2 did the trick.
with gdb I understood that it's at line 40 that the problem was occurring with free but I did not understand why.

Thanks again to all

PS Excuse me for forgetting

Code:

..

Gerard.