6.3.1 Overflow with Unsigned Integers

Unsigned arithmetic in C ignores overflow; it produces the true result modulo the nth power of 2, where n is the number of bits in the data type. We say it “truncates” the true result to the lowest n bits.

A true result that is negative, when taken modulo the nth power of 2, yields a positive number. For instance,

unsigned int x = 1;
unsigned int y;

y = -x;

causes overflow because the negative number -1 can’t be stored in an unsigned type. The actual result, which is -1 modulo the nth power of 2, is one less than the nth power of 2. That is the largest value that the unsigned data type can store. For a 32-bit unsigned int, the value is 4,294,967,295. See Maximum and Minimum Values.

Adding that number to itself, as here,

unsigned int z;

z = y + y;

ought to yield 8,489,934,590; however, that is again too large to fit, so overflow truncates the value to 4,294,967,294. If that were a signed integer, it would mean -2, which (not by coincidence) equals -1 + -1.