What's the difference between these two memset?

Question

int color[1001][1001];
int m,n;
m=10;
n=10;
memset(color,0,sizeof(color));
memset(color,0,sizeof(color[0][0])*m*n );

What's the difference between these two memset statements?

Any answer will be highly appreciated. Thanks in advance.

Answer 1

What's the difference between these two memset statements?

The memset function takes, the destination, value and count. The count is sizeof(color) which would be sizeof(int) * 1001 * 1001 for the first call.

For the second it will be sizeof(int) * 10 * 10 .

The former clears the complete array with zeros, while the latter does it only partially, starting from color[0][0] to color[0][99] , which relies on the fact that arrays are laid out in a row-major fashion. Relevant excerpt from the C11 standard (draft n1570), §6.5.2.1 Array subscripting :

[…] It follows from this that arrays are stored in row-major order (last subscript varies fastest).

Alternatively, if m = n = 1001 ie m and n actually denote the array's dimensions, the two calls then are the same, just two different ways of writing it .