Correct way to free double pointers

Question

What is the correct way to free a float ** like below.

e.g. float ** someArray

for(int i = 0; i < numberOfDimensions; i++)
    {
        somearray[i] = (float *) malloc(numberOfDimensions * sizeof(float));
    }

Man, you changed your question after receiving 6 answer… That will cause some trouble — sidyll, Aug 07 '12 at 22:06

score 4 · Accepted Answer · edited May 23 '17 at 11:59

4

If you have malloc'ed another round of memory and assigned it to each float pointer in the original array, then you should free them as well beforehand:

int i;
for (i = 0; i < numberOfDimensions; i++)
    free(someArray[i]);

// and free the container array only now

free(someArray);

P. s.: don't cast the return value of malloc.

edited May 23 '17 at 11:59

Community

1
1

answered Aug 07 '12 at 22:05

1

I changed my question, while you answered, sorry. – some_id Aug 07 '12 at 22:07
2

I changed my answer to reflect the new question while you were changing the question. Sorry. :P – Aug 07 '12 at 22:09

score 1 · Answer 2 · answered Aug 07 '12 at 22:08

1

I believe you're expected to free the elements, and then free the array.

answered Aug 07 '12 at 22:08

rsegal

401
3
11

score 1 · Answer 3 · answered Aug 07 '12 at 22:10

1

Go backwards:

for(int i = 0; i < numberOfDimensions; i++)
{
    free(somearray[i]);
}

free(somearray);

answered Aug 07 '12 at 22:10

SheetJS

22,470
12
65
75

An equal and opposite reaction, got it. ;) – some_id Aug 07 '12 at 22:12
@Helium3 the approach generalizes to arbitrary dimensions – SheetJS Aug 07 '12 at 22:14

score 0 · Answer 4 · answered Aug 07 '12 at 22:04

0

Well, in this case you simply free(someArray). Now, if you malloc'd more memory and added that to someArray, then you need to walk that array and free each object.

in other words if you:

for(int i=0; i< whatever; ++i) {
  someArray[i] = malloc(...
}

Then you need to walk it again and do frees before you free someArray.

answered Aug 07 '12 at 22:04

David H

40,852
12
92
138

I changed my question, while you answered, sorry. – some_id Aug 07 '12 at 22:07
Well @Nirk gave the complete answer a bit after I told you how to do it. – David H Aug 07 '12 at 22:58

score 0 · Answer 5 · answered Aug 07 '12 at 22:04

0

Just a simple 1-free() call:

free(someArray);

answered Aug 07 '12 at 22:04

MByD

135,866
28
264
277

I changed my question, while you answered, sorry. – some_id Aug 07 '12 at 22:07

Tarun · Answer 6 · 2016-12-20T10:22:09.867

-1

for(int i = 0; i < numberOfDimensions; i++)
{
    free(somearray[i]);
    somearray[i] = NULL;
}

free(somearray);
somearray=NULL;

(somearray[i] = NULL) this line will break the link of each array element and finally (somearray=NULL) will break the link to the array. These two lines will free up the array of dynamic memory and the OS allocate this freed memory to some other process

edited Dec 20 '16 at 10:22

answered Nov 07 '16 at 13:31

Tarun

11
4

can you please improve the post? Instead of only posing a sample code, can you try to explain the reason behind writing the code? – Sourav Ghosh Nov 07 '16 at 13:32

Correct way to free double pointers

6 Answers6