C unsigned int比较-环绕自定义位数

Question

I am reading a 24-bit value, presently into a uint32_t variable.

After bashing my head against the wall calculating the difference between two such variables, and thinking "it's unsigned! Why is this overflow a problem?!", I realised the values are overflowing short of the size of the variable containing them.

This is a counter, so one is known to be 'larger' than the other, though may have overflowed.

Thus, 0x1 - 0x2 = 0xFFFFFFFF , but should be 0x00FFFFFF .

How should I best deal with this?

• Define a type uint24 ;
• if / else on which is larger before doing the appropriate arithmetic;
• Something else I haven't thought of?

'best' should be interpreted as 'best practices'/'most readable'/'safest'.

Answer 1

To subtract (or add) two numbers and have the result wrap around the range of an unsigned 24-bit number, do a binary-and of the result with 0xFFFFFF , ie (xy) & 0xFFFFFF . For example:

(0x1 - 0x2) & 0xFFFFFF == 0xFFFFFF

Answer 2

At first, you can use compare operator != , == , > , >= , < and <= . you don't need do such as num1 - num2 if you want to compare...

Anyway, if it's really necesaary, just use bitwise AND

uint32_t n1 = 1, n2 = 5;
printf("0x%08x", (n1 - n2) & 0x00ffffff);

(live example)

Output:

0x00fffffc

As you know, 0xfffffc means -4 in 24-bit signed integer.

(Notice that 2's complement is not specified by C standard; my code may not work in non-2's complement system.)