Question

size = (size_in_bytes + sizeof(int) - 1) / sizeof(int);

This is more of a math questions than real programming question…
Is it because C always round down?

Answer 1

Integer division truncates in C, yes. (i.e. it’s round towards zero, not round down.)

By adding “divisor – 1” you make sure that any number which isn’t an exact multiple of the divisor gets rounded up. So for example, dividing by 3:

 (0 + 2) / 3 == 0 (0 is an exact multiple)
 (1 + 2) / 3 == 1 (round up)
 (2 + 2) / 3 == 1 (round up)
 (3 + 2) / 3 == 2 (3 is an exact multple)

etc