CODEWITHSUNDEEP
cmallocruntime-errorfree
sps
sps

Reputation: 2730

why free(pointer) is giving runtime error?

I have below C program. It asks for number of coordinates from user. Then uses a malloc to allocate memory, stores the coordinates (integers) in the allocated memory, and then later frees the memory.

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

int main(int argc, char *argv[]) /* Arguments to main() not necessary but used to keep with convention.*/
{
    int num_of_coordinates;
    printf("How many co-ordinates: ");
    scanf("%d", &num_of_coordinates);

    int *coordinate_array = malloc(num_of_coordinates * 2);
    int i, j;

    /* Below for loop takes the x and y coordinate of all points. */
    /* These coordinates are stored in coordinate_aaray[]. */
    for (i=0; i < num_of_coordinates*2; i++)
    {
            j = (i / 2) + 1 ;
            if (i % 2 != 0)
            {
                    printf("Enter x coordinate of point %d: ",j);
                    scanf("%d",&coordinate_array[i]);
            }
            else
            {
                    printf("Enter y-coordinate of point %d: ",j);
                    scanf("%d",&coordinate_array[i]);
            }
    }

    for (i=0; i < num_of_coordinates*2; i++)
    {
            printf("%d ",coordinate_array[i]);
    }
    printf("\n");

    /* Free the allocated memory. */
    free (coordinate_array);

    return 0;
}

When I run these program I get no issues till number_of_coordinates is equal or less than 3.

-bash-4.1$ ./a.out
How many co-ordinates: 3
Enter y-coordinate of point 1: 1
Enter x coordinate of point 1: 2
Enter y-coordinate of point 2: 3
Enter x coordinate of point 2: 4
Enter y-coordinate of point 3: 5
Enter x coordinate of point 3: 6
1 2 3 4 5 6
-bash-4.1$

However when I give num_of_coordinates a value of 4 or more I get a runtime error (most probably because of free(coordinate_array)).

-bash-4.1$ ./a.out
How many co-ordinates: 4
Enter y-coordinate of point 1: 1
Enter x coordinate of point 1: 2
Enter y-coordinate of point 2: 3
Enter x coordinate of point 2: 4
Enter y-coordinate of point 3: 5
Enter x coordinate of point 3: 6
Enter y-coordinate of point 4: 7
Enter x coordinate of point 4: 8
1 2 3 4 5 6 7 8
*** glibc detected *** ./a.out: free(): invalid next size (fast):    0x000000000185b010 ***

Actually the runtime error message is a long one, so I just showed the first line of that error.

Why is this error happening when num_of_coordinatesgreater or equal to 4 in this case ?

Thanks.

Upvotes: 1

Views: 933

Answers (3)

amahfouz
amahfouz

Reputation: 2398

Make sure to allocate the exact number of bytes required. As another user mentioned above you should use:

malloc(num_of_coordinates * sizeof(int) * 2)

Also, just to double check, how about you log values to test your code. For instance, log the num_of_coordinates to make sure that you scanned the right value.

To debug, you can try something like this: http://dmalloc.com/

Upvotes: 0

user3629249
user3629249

Reputation: 16540

there are a number of problems with this line:

int *coordinate_array = malloc(num_of_coordinates * 2);

1) the amount of allocated memory is 1byte * num_of_coordinates * 2
That is not large enough to hold num_of_coordinates*2 integers
use:

int *coordinate_array = malloc(num_of_coordinates * 2 * sizeof int);

2) always check the returned value from malloc (and family) 
   to assure the operation was successful

int *coordinate_array = NULL;

if( NULL == (coordinate_array = malloc(num_of_coordinates * 2 * sizeof int) ) )
{ // then, malloc failed
    perror( "malloc for coordinate array failed" );
    exit( EXIT_FAILURE );
}

// implied else, malloc successful

Upvotes: 1

sps
sps

Reputation: 2730

I had made a silly mistake.

I had allocated insufficient amount of memory using below statement.

int *coordinate_array = malloc(num_of_coordinates * 2);

However as each number is an int I had to use below statement.

int *coordinate_array = malloc(num_of_coordinates * 2 * sizeof(int));

After making this change, the program works with no runtime errors.

-bash-4.1$ ./a.out
How many co-ordinates: 4
Enter y-coordinate of point 1: 1
Enter x coordinate of point 1: 2
Enter y-coordinate of point 2: 3
Enter x coordinate of point 2: 4
Enter y-coordinate of point 3: 5
Enter x coordinate of point 3: 6
Enter y-coordinate of point 4: 7
Enter x coordinate of point 4: 8
1 2 3 4 5 6 7 8
-bash-4.1$

Upvotes: 1

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