Here's some code you can compile, it shows the problem: #include <stdio.h> #include <stdint.h> int main() { printf("size_t bytes: %u\n", sizeof(si

Here's some code you can compile, it shows the problem: #include <stdio.h> #include <stdint.h> int main() { printf("size_t bytes: %u\n", sizeof(size_t)); size_t high = SIZE_MAX; size_t low = high-1; size_t mid_correct = low+(high-low)/2; size_t mid_incorrect = (low+high)/2; printf("low: %.ju\n", low); printf("high: %.ju\n", high); printf("low+high: %.ju\n", low+high); printf("(low+high)/2 -- incorrect: %.ju\n", mid_incorrect); printf("low+(high-low)/2 -- correct: %.ju\n", mid_correct); } On my 64-bit machine, I get: size_t bytes: 8 low: 18446744073709551614 high: 18446744073709551615 low+high: 18446744073709551613 (low+high)/2 -- incorrect: 9223372036854775806 low+(high-low)/2 -- correct: 18446744073709551614