[Solved] Implementing / enforcing wraparound arithmetic in C [closed]

Question

Signed overflow is undefined. Unsigned overflow wraps. Implementing signed wraparound arithmetic is mostly a matter of doing everything in unsigned math. There are a few things to be careful about, though:

unsigned short and unsigned char arithmetic works by converting the operands to either int or unsigned int first. Usually int, unless you’re on a weird setup where int doesn’t have enough range to store all unsigned short values. That means that converting short or char to unsigned short or unsigned char for arithmetic can still produce signed integer overflow and UB. You need to do your math in unsigned int or larger to avoid this.
unsigned->signed conversion is technically implementation-defined when the original value is beyond the range of the result type. This shouldn’t be a problem on most compilers and architectures.

Alternatively, if you want to go the compiler flag route, -fwrapv makes signed overflow wrap for addition, subtraction, and multiplication on GCC and Clang. It doesn’t do anything about INT_MIN / -1, though.

Accepted Answer

Signed overflow is undefined. Unsigned overflow wraps. Implementing signed wraparound arithmetic is mostly a matter of doing everything in unsigned math. There are a few things to be careful about, though:

unsigned short and unsigned char arithmetic works by converting the operands to either int or unsigned int first. Usually int, unless you’re on a weird setup where int doesn’t have enough range to store all unsigned short values. That means that converting short or char to unsigned short or unsigned char for arithmetic can still produce signed integer overflow and UB. You need to do your math in unsigned int or larger to avoid this.
unsigned->signed conversion is technically implementation-defined when the original value is beyond the range of the result type. This shouldn’t be a problem on most compilers and architectures.

Alternatively, if you want to go the compiler flag route, -fwrapv makes signed overflow wrap for addition, subtraction, and multiplication on GCC and Clang. It doesn’t do anything about INT_MIN / -1, though.