2D array dynamic memory allocation crashes

Question

Possible Duplicate:
How do I correctly set up, access, and free a multidimensional array in C?

I am trying to dynamically allocate memory for a 2D array using calloc. The columns are fixed as 2 so its only the rows that are dynamic.

Here is what I have been trying :

unsigned int **pts, rows;
int main()
{
    //some code

    pts = (unsigned int **)calloc(2*rows, sizeof (unsigned int **));
}


//The code to access the array :
for(k=1;k<=i;k++)
{
    printf("\nX%d=",k);
    scanf("%d",&pts[k][0]);
    printf("\nY%d=",k);
    scanf("%d",&pts[k][1]);
}

But the problem is, while accessing the array, the program crashes. I am using Eclipse with MinGW GCC.

Please let me know if I need to put more data here or give me some idea how I can deal with this, as this is my first post.

[How do I correctly set up, access, and free a multidimensional array in C?](http://stackoverflow.com/questions/12462615/how-do-i-correctly-set-up-access-and-free-a-multidimensional-array-in-c) — Lundin, Oct 18 '12 at 13:52

Jimmy Lu · Accepted Answer · 2012-10-18T14:05:32.580

Tudor's answer is the correct solution. But to provide a bit more insight into why your code is wrong....

What your code is really doing is just to allocate an array, of length 2 * rows, of pointer to pointer to type int.

What you are trying to create is this:

an array of int** -> int* -> int
                          -> int
                          -> int
                          -> ...more
                  -> int* -> int
                          -> int
                          -> int
                          -> ...more
                  -> int* -> int
                          -> int
                          -> int
                          -> ...more
                  -> ...more

What you have actually created is this:

an array of int** -> int* -> nothing (null address)
                  -> int* -> nothing...
                  -> ...more

You then attempt to assign an int to one of the null address pointed by one of the zero-initialized int* in your array of int** (You see, calloc has made sure that all your int*'s are zero)

When you are trying to execute

scanf("%d",&pts[k][0]);

pts[k] refers to the (k - 1)th element in your array of int**, but as shown above, though your code has allocated space for this element indeed, it has initialized it as zero. So, this pts[k] points at NULL. So scanf has obtained an address based on a zero offset from the NULL address... It should be now clear to you that this is invalid.

@Nik, you're welcome. I recall making the very same mistake when I was in my first year of university... lolz — Jimmy Lu, Oct 18 '12 at 14:11

score 1 · Answer 2 · answered Oct 18 '12 at 13:55

1

Here's the way to do it:

pts = (unsigned int **)calloc(rows, sizeof (unsigned int *));
for(int i = 0; i < rows; i++) {
    pts[i] = (unsigned int *)calloc(2, sizeof (unsigned int));
}

answered Oct 18 '12 at 13:55

Tudor

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Thanks Tudor ! One more question. Cant I write 2*rows and use calloc only once ? – Nik Oct 18 '12 at 13:57
@Nik: Not really, you need to allocate each row individually if you use pointers to pointers. – Tudor Oct 18 '12 at 14:00
Thanks again. I need to study more :) – Nik Oct 18 '12 at 14:06
In C all those cast are useless. – alk Sep 25 '16 at 09:56

2D array dynamic memory allocation crashes

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