将uint8_t与C ++中的十六进制进行比较

Question

Let's say I have uint8_t bytes[maxBytes]; .

Now I want to compare the lower 6 bits of the first byte ( bytes[0] ) with 0x3c .

I tried to do it like this:

bytes[0] & 0x3f == 0x3c

Unfortunately, this did not produce the expected result. (ie it's always false, even though when I print out bytes[0] & 0x3f , it is 0x3c )

I've played around with this some more and found out that

bytes[0] & 0x00 == 0x00

is sometimes true, and sometimes false. (Same with bytes[0] & 0x0 == 0x0 and bytes[0] & 0x00 == 0x0 ). Shouldn't it always be true?

What is going on here? How can I make my 0x3c comparison work?

Sitenote: I'm running this code on an arduino w/ atmega328pb MCU.

Answer 1

You need parentheses:

(bytes[0] & 0x3f) == 0x3c

This is because of the weird precedences of & and | which were inherited from C, which inherited them from B (Dennis Ritchie described this precedence problem in https://www.bell-labs.com/usr/dmr/www/chist.html ).