How do i dynamically allocate memory to a 2D array in C avoiding the stack overflow issue?

Question

pascal = (int **) malloc(no_of_rows * sizeof(int));

for (i = 0; i < no_of_rows; i++)
{
    *(pascal + i) = (int*) malloc(col * sizeof(int));
}

Can someone tell me what's wrong in this code as I am beginner to this language. I keep on getting problem of Stack Overflow? What could be its possible reasons and how could it be avoided?

Please use some whitespace. Do you hate your readers so much? — Kerrek SB, Jun 19 '15 at 10:03
Remove the casts and also change `malloc(no_of_rows * sizeof(int));` to `malloc(no_of_rows * sizeof(int*));`. Also, check the return value of those `malloc`s — Spikatrix, Jun 19 '15 at 10:03
To avoid errors such as `sizeof(int)` instead of `sizeof(int*)`and keep your code easy to change, you should write `p = malloc(number_of_things * sizeof(*p));` — Eregrith, Jun 19 '15 at 10:07
Please pick one of the two different languages: C **or** C++. — too honest for this site, Jun 19 '15 at 11:19

score 8 · Accepted Answer · answered Jun 19 '15 at 10:02

8

pascal=(int **)malloc(no_of_rows * sizeof(int));

should be

pascal=malloc(no_of_rows * sizeof(int*));

Notice the additional * I added. In general, you can write it better:

pascal=malloc(no_of_rows * sizeof *pascal);

Note that casting mallocs results is unnecessary in C.

answered Jun 19 '15 at 10:02

P.P

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I actually tend to strongly discourage casting `void *` (in C). – too honest for this site Jun 19 '15 at 11:22
Nice one. :-) However, I thought of adding an elaboration as an answer, can you review it once? – Sourav Ghosh Jun 19 '15 at 11:27
@SouravGhosh I don't see any problem. Your answer is quite good :) – P.P Jun 19 '15 at 16:07

score 2 · Answer 2 · edited May 23 '17 at 12:22

You already got the right answer from Mr. Blue Moon. However, to clarify about the error in your case, I want to add an elaboration.

For the code

pascal = (int **) malloc(no_of_rows * sizeof(int));

it looks like, your pascal is of type int **, that is, pointer to a pointer-to-int.

So, while allocating a valid memory to pascal, you need to allocate the size equal to the element it points to, and it points to another integer pointer.

So, your allocation statement should be

pascal=malloc(no_of_rows * sizeof(int*));

Point to notice:

See why not to cast the return value of malloc() and family in C.
The allocation size is calculated as sizeof(int *) (size of a pointer to int) instead of sizeof(int).

Now, to make this statement more robust, we can (should) re-write it as

pascal=malloc(no_of_rows * sizeof*pascal);

Here, once again, two things to notice,

sizeof is an operator, it does not need the parenthesise around its operand unless it is a type name.
This statement, is independent of the type of pascal. even if you change the type of pascal, you don't need to change the above statement.

That said, always check for the success of malloc() before using the returned pointer.

Hope this helps.

How do i dynamically allocate memory to a 2D array in C avoiding the stack overflow issue?

2 Answers2