What does AND 0xFF do?

Question

In the following code:

short = ((byte2 << 8) | (byte1 & 0xFF))

What is the purpose of &0xFF? Because other somes

Answer 1

it clears the all the bits that are not in the first byte

Answer 2

if byte1 is an 8-bit integer type then it's pointless - if it is more than 8 bits it will essentially give you the last 8 bits of the value:

    0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
 &  0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
    -------------------------------
    0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1

Answer 3

Anding an integer with 0xFF leaves only the least significant byte. For example, to get the first byte in a short s, you can write s & 0xFF. This is typically referred to as "masking". If byte1 is either a single byte type (like uint8_t) or is already less than 256 (and as a result is all zeroes except for the least significant byte) there is no need to mask out the higher bits, as they are already zero.

See tristopiaPatrick Schlüter's answer below when you may be working with signed types. When doing bitwise operations, I recommend working only with unsigned types.

Answer 4

The danger of the second expression comes if the type of byte1 is char. In that case, some implementations can have it signed char, which will result in sign extension when evaluating.

signed char byte1 = 0x80;
signed char byte2 = 0x10;

unsigned short value1 = ((byte2 << 8) | (byte1 & 0xFF));
unsigned short value2 = ((byte2 << 8) | byte1);

printf("value1=%hu %hx\n", value1, value1);
printf("value2=%hu %hx\n", value2, value2);

will print

value1=4224 1080     right
value2=65408 ff80    wrong!!

I tried it on gcc v3.4.6 on Solaris SPARC 64 bit and the result is the same with byte1 and byte2 declared as char.

TL;DR

The masking is to avoid implicit sign extension.

EDIT: I checked, it's the same behaviour in C++.

Answer 5

The byte1 & 0xff ensures that only the 8 least significant bits of byte1 can be non-zero.

if byte1 is already an unsigned type that has only 8 bits (e.g., char in some cases, or unsigned char in most) it won't make any difference/is completely unnecessary.

If byte1 is a type that's signed or has more than 8 bits (e.g., short, int, long), and any of the bits except the 8 least significant is set, then there will be a difference (i.e., it'll zero those upper bits before oring with the other variable, so this operand of the or affects only the 8 least significant bits of the result).

Answer 6

& 0xFF by itself only ensures that if bytes are longer than 8 bits (allowed by the language standard), the rest are ignored.

And that seems to work fine too?

If the result ends up greater than SHRT_MAX, you get undefined behavior. In that respect both will work equally poorly.