即使不满足条件，C 'for' 循环也会继续

Question

I have a simple function in C that calculates days between two given dates.

It works well when both dates are valid. When a date is not valid I set the date to "0/0/0".

This is one my for loops:

for(int i = 0; i < date2.month - 1; i++)
    d2Days += monthDays[i];

Supposedly if a date is invalid the date2.month would be equal to 0 and the loop would not iterate at all but it does and it keeps on going indefinitely until i surpasses the size of the array and it seg faults.

You can see there i = 13 even when the month = 0

Can someone explain why is that such behavior?

Answer 1

I think @pmg nailed it in his comment above. If your date2.month member is unsigned, then when it is set to zero, decrementing it by one in the loop causes the integer overflow and returns INT_MAX . When this occurs your loop condition will evaluate to true until you exceed the bounds of monthDays