Multidimensional array with unequal second dimension size using malloc()

Question

I am playing around with multidimensional array of unequal second dimension size. Lets assume that I need the following data structure:

[&ptr0]->[0][1][2][3][4][5][6][7][8][9]

[&ptr1]->[0][1][2]

[&ptr2]->[0][1][2][3][4]

int main()
{
 int *a[3];
 int *b;
 int i;

 a[0] = (int *)malloc(10 * sizeof(int));
 a[1] = (int *)malloc(2 * sizeof(int));
 a[2] = (int *)malloc(4 * sizeof(int));

 for(i=0; i<10; i++) a[0][i]=i;

 for(i=0; i<2; i++) a[1][i]=i;

 for(i=0; i<4; i++) a[2][i]=i;
}

I did some tests and it seems like I can store a value at a[1][3]. Does it mean that rows in my array are of equal size 10?

Answer 1

No, The address a[1][3] does not "officially exist". It is a memory which is not define in your program and accessing it result in a undefined behavior.

It can lead to the following error:

• Segmentation fault (access a restricted memory)
• Used a memory already used by other variable (other allocation memory) (so possible overwrite)
• It can be an uninitialized value (unsused memory address)

Answer 2

It is undefined behavior of your code. You are accessing something that you don't own. It may work, it may not, but it is always wrong.