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.

Answer 3

No

---

There is lots of memory in your program used for I/O buffers, library data structures, the malloc system itself, command line arguments and environment, etc. (Some of those are on the stack.)

Yes, you can clobber things out of range.

Keep in mind that x[i] is the same thing as *(x + i) . So, it's easy to calculate the address you referenced. It may overlay one of your data structures, it may overlay a part of your data structure that is a private field within malloc's mechanism, or it may overlay library data.