Question

I’d like to find out how C will allocate a the data items of a multidimensional array, and if their allocation is consistent across machines.

I know that, at the lowest level, the data items are neighbours, but I don’t know how they’re arranged further up.

For example, if I allocate a 3D array as int threeD[10][5][6], can I assume that &(threeD[4][2][5]) + 1 == &(threeD[4][3][0])? On all machines?

Thanks in advance for your help.

Answer 1

The elements are stored in Row Major order. So Elements along the last dimension are contiguous. However, elements between rows (as indicated by your example) aren’t guaranteed to be contiguous. It depends on how the initial memory has been allocated.

#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>

// only elements in a single row are guaranteed to be
// contiguous because of the multiple mallocs
void main(void)
{
// 3 rows, 4 columns
int *a[3];

for ( int row = 0; row < 3; row++ )
  a[row] = (int *)malloc(4*sizeof(int));
}


// all elements are guaranteed to be contiguous
// in a row major order.
void main(void)
{
// 3 rows, 4 columns
int *a[3];

int *buf = (int *)malloc(3*4*sizeof(int));

for ( int row = 0; row < 3; row++ )
  a[row] = buf+4*row;

assert( (&a[1][3] + 1) == &a[2][0] );
}