All objects (blocks of memory, etc) should be measured in terms of a
size_t type. Therefore any iterations (e.g. for(i=0; i<N;
i++)) should also use an index of type size_t. If you need to
write a descending loop you have to be careful because size_t is
unsigned, so instead of
for (i = N - 1; i >= 0; i--) { ... }
use something like
for (i = N; i > 0 && i--;) { ... }
to avoid problems with wrap-around at i=0.
If you really want to avoid confusion use a separate variable to invert the loop order,
for (i = 0; i < N; i++) { j = N - i; ... }