Integer overflow in C: standards and compilers

Question

Edited to include proper standard reference thanks to Carl Norum.

The C standard states

If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined.

Are there compiler switches that guarantee certain behaviors on integer overflow? I'd like to avoid nasal demons. In particular, I'd like to force the compiler to wrap on overflow.

For the sake of uniqueness, let's take the standard to be C99 and the compiler to be gcc. But I would be interested in answers for other compilers (icc, cl) and other standards (C1x, C89). In fact, just to annoy the C/C++ crowd, I'd even appreciate answers for C++0x, C++03, and C++98.

Note: International standard ISO/IEC 10967-1 may be relevant here, but as far as I could tell it was mentioned only in the informative annex.

Answer 1

Take a look at -ftrapv and -fwrapv:

-ftrapv

This option generates traps for signed overflow on addition, subtraction, multiplication operations.

-fwrapv

This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables other. This option is enabled by default for the Java front-end, as required by the Java language specification.

Answer 2

For your C99 answer, I think 6.5 Expressions, paragraph 5 is what you're looking for:

If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined.

That means if you get an overflow, you're out of luck - no behaviour of any kind guaranteed. Unsigned types are a special case, and never overflow (6.2.5 Types, paragraph 9):

A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.

C++ has the same statements, worded a bit differently:

• 5 Expressions, paragraph 4:

  If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, the behavior is undefined. [Note: most existing implementations of C++ ignore integer overflows. Treatment of division by zero, forming a remainder using a zero divisor, and all floating point exceptions vary among machines, and is usually adjustable by a library function. —endnote]

• 3.9.1 Fundamental types, paragraph 4:

  Unsigned integers, declared unsigned, shall obey the laws of arithmetic modulo 2^n where n is the number of bits in the value representation of that particular size of integer.