Create image kernel in C freeing memory error - c

Question

I am trying to create a Gaussian filter kernel in C to do some image processing. I am using a 2d float array on the heap, but when I call free() on the rows I keep getting an free(): invalid pointer error. I have printed out memory locations and values of the filter to standard output and everything seems to be what I expect
//kernel->kernel = float **
//kernel->row_len = kernel->col_len = 5
float total_weight = 0.0;
//build the holding col
kernel->kernel = malloc(sizeof(float *) * kernel->col_len);
//get mem for each row and set the values
for (int j = 0; j < kernel->col_len; j++)
{
kernel->kernel[j] = malloc(sizeof(float) * kernel->row_len);
for (int i = 0; i < kernel->row_len; i++)
{
kernel->kernel[j][i] = ken_ComputeGuassianVal(i, j, sigma, size);
total_weight += kernel->kernel[j][i];
}
//print debugging info
printf("Create - %p\n", (kernel->kernel + j));
for (int i = 0; i < kernel->row_len; i++)
{
printf("%d, %d - %f \n", i, j, kernel->kernel[j][i]);
printf("%p\n", (*(kernel->kernel + j) + i));
}
printf("\n");
}
//Normalise the kernel otherwise brightness will be added to the image
for (int j = 0; j < kernel->col_len; j++)
{
for (int i = 0; i < kernel->row_len; i++)
{
kernel->kernel[j][i] /= total_weight;
}
}
for (int j = 0; j < kernel->col_len; j++)
{
printf("Attempting to free memory at location %p\n", (kernel->kernel + j));
free(kernel->kernel + j);
printf("\n");
}
free(kernel->kernel);
Here is the output I am getting to standard output
Create - 0x55aa2a80d4e0
0, 0 - 0.000000
0x55aa2a7ed8d0
1, 0 - 0.000000
0x55aa2a7ed8d4
2, 0 - 0.000001
0x55aa2a7ed8d8
3, 0 - 0.000000
0x55aa2a7ed8dc
4, 0 - 0.000000
0x55aa2a7ed8e0
Create - 0x55aa2a80d4e8
0, 1 - 0.000000
0x55aa2a84c6a0
1, 1 - 0.001083
0x55aa2a84c6a4
2, 1 - 0.034551
0x55aa2a84c6a8
3, 1 - 0.001083
0x55aa2a84c6ac
4, 1 - 0.000000
0x55aa2a84c6b0
Create - 0x55aa2a80d4f0
0, 2 - 0.000001
0x55aa2a7f96a0
1, 2 - 0.034551
0x55aa2a7f96a4
2, 2 - 1.102181
0x55aa2a7f96a8
3, 2 - 0.034551
0x55aa2a7f96ac
4, 2 - 0.000001
0x55aa2a7f96b0
Create - 0x55aa2a80d4f8
0, 3 - 0.000000
0x55aa2a80d510
1, 3 - 0.001083
0x55aa2a80d514
2, 3 - 0.034551
0x55aa2a80d518
3, 3 - 0.001083
0x55aa2a80d51c
4, 3 - 0.000000
0x55aa2a80d520
Create - 0x55aa2a80d500
0, 4 - 0.000000
0x55aa2a7eddf0
1, 4 - 0.000000
0x55aa2a7eddf4
2, 4 - 0.000001
0x55aa2a7eddf8
3, 4 - 0.000000
0x55aa2a7eddfc
4, 4 - 0.000000
0x55aa2a7ede00
Destroy - 0x55aa2a7ed8d0
Attempting to free memory at location 0x55aa2a80d4e0
Destroy - 0x55aa2a84c6a0
Attempting to free memory at location 0x55aa2a80d4e8
free(): invalid pointer
[1] 13936 abort ./dipcw
I have tried both array notation kernel->kernel[j] and (kernel->kernel + j). I am using elementary linux 5.0 and gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04)
Edit: Changed the stop condition variable in the normalization loop to use the same variables as the other loops. Added free statement for the double pointer at the end

Answer 1

The rule is: there should be a corresponding free() for every malloc()
struct matrix {
unsigned nrow;
unsigned ncol;
float ** ptrs;
} data;
/* allocate */
unsigned row, col;
data.ptrs = malloc (data.nrow * sizeof *data.ptrs); // <<< [A]
for (row=0; row < data.nrow; row++) { // <<< [B]
data.ptrs[row] = malloc (data.ncol * sizeof *data.ptrs[0]); // <<< [C]
}
/* there should be a corresponding free for every malloc
, but in the "inside-out" order :
*/
unsigned row, col;
for (row=0; row < data.nrow; row++) { // <<< [B]
free( data.ptrs[row] ); // <<< [C]
}
free(data.ptrs); // <<< [A]
Note: for simplicity, I swapped rows/columns , and I used matrix.field instead of pointer->field.