How to check if overflow occured? [duplicate]

Question

Possible Duplicate:
Best way to detect integer overflow in C/C++

This is probably a rookie question, but how can I check some overflow affected the value of my numbers in C. For example, when multiplying integers, and waiting for an integer result, if actual result was bigger than max-integer value, actual result is altered(right?). So how can I tell if something like this occured?

Answer 1

Signed integer overflow is like division by zero - it leads to undefined behaviour, so you have to check if it would occur before executing the potentially-overflowing operation. Once you've overflowed, all bets are off - your code could do anything.

The *_MAX and _MIN macros defined in <limits.h> come in handy for this, but you need to be careful not to invoke undefined behaviour in the tests themselves. For example, to check if a * b will overflow given int a, b;, you can use:

if ((b > 0 && a <= INT_MAX / b && a >= INT_MIN / b) ||
    (b == 0) ||
    (b == -1 && a >= -INT_MAX) ||
    (b < -1 && a >= INT_MAX / b && a <= INT_MIN / b))
{
    result = a * b;
}
else
{
    /* calculation would overflow */
}

(Note that one subtle pitfall this avoids is that you can't calculate INT_MIN / -1 - such a number isn't guaranteed to be representable and indeed causes a fatal trap on common platforms).

Answer 2

The C99 standard has this section explaining what undefined behavior is:

3.4.3
undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).
EXAMPLE
An example of undefined behavior is the behavior on integer overflow.

So you're pretty much out of luck, there is no portable way of detecting that in the general case, after the fact.
Your compiler/implementation might have extensions/support for it though, and there are techniques to avoid these situations.
See this question for excellent advice: Best way to detect integer overflow in C/C++.