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c++cmemset

What's the difference between these two memset?

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.

Solution

  • 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 i.e. m and n actually denote the array's dimensions, the two calls then are the same, just two different ways of writing it.