How to memcpy a part of a two dimensional array in C?

Question

How to memcpy the two dimensional array in C:

I have a two dimensional array:

int a[100][100];

int c[10][10];

I want to use memcpy to copy the all the values in array c to array a, how to do this using memcpy?

int i;
for(i = 0; i<10; i++)
{
    memcpy(&a[i][10], c, sizeof(c));
}

is this correct?

That depends on where you want the values to go. It looks like not quite what you probably want. — Daniel Fischer, Jun 03 '13 at 12:00
You have 100 times the capacity in `a`. Which elements of `a` do you want to hold a copy of `c`? — simonc, Jun 03 '13 at 12:00

score 13 · Accepted Answer · answered Jun 03 '13 at 12:01

13

That should work :

int i;
for(i = 0; i<10; i++)
{
    memcpy(&a[i], &c[i], sizeof(c[0]));
}

answered Jun 03 '13 at 12:01

Fabien

12,486
9
44
62

score 3 · Answer 2 · answered Jun 03 '13 at 12:02

3

It should actually be:

for(i = 0; i < 10; ++ i)
{
  memcpy(&(a[i][0]), &(c[i][0]), 10 * sizeof(int));
}

answered Jun 03 '13 at 12:02

cgledezma

612
6
11

when I try to do &(c[i][0]), it tells me that error: no match for 'operator[]' in '*(c+ 924u)[0]' – user2131316 Jun 03 '13 at 12:49
You sure you typed it right? My compiler tells me nothing.. Which compiler are you using? – cgledezma Jun 03 '13 at 13:00

score 1 · Answer 3 · answered Jun 03 '13 at 12:00

I don't think it's correct, no.

There's no way for memcpy() to know about the in-memory layout of a and "respect" it, it will overwrite sizeof c adjacent bytes which might not be what you mean.

If you want to copy into a "sub-square" of a, then you must do so manually.

How to memcpy a part of a two dimensional array in C?

3 Answers3

Linked