Golang: Parse bit values from a byte

Question

I need to parse a network packet which consists of two bytes: first one consists of 8 bits that set certain flags depending on their order (for instance) and the second one is uint8 (which is simple)

1 - online
0 - not active
1 - isPretty
1 - isHandsome
0 - bald
0 - deaf
0 - mute
0 - blind

How do I parse it from a byte primitive?

There's no automatic way to "parse" bits from a byte, but it sounds like what you're looking for is a simple bitmask: yourbasic.org/golang/bitmask-flag-set-clear — Adrian
– Adrian, Commented Feb 21, 2019 at 14:22
See Difference between some operators “|”, “^”, “&”, “&^”. Golang; and Extract bits into a int slice from byte slice. — icza
– icza, Commented Feb 21, 2019 at 14:28

Eli Bendersky · Accepted Answer · 2019-02-21 14:34:18Z

4

Some useful Go standard library packages for dealing with binary:

For extracting single bits from a byte, you ought to use bitwise operators - |, & and >>.

For example:

package main

import (
    "fmt"
)

func main() {
    v := byte(0xB2)
    if (v >> 4) & 1 == 1 {
        fmt.Println("bit 4 (counting from 0) is set")
    }
}

This checks bit 4 (with bit 0 being the lowest bit in the byte) by shifting the byte value 4 positions to the right and AND-ing with 1. You can check the other bits in your flag value similarly. Feel free to write a function that replaces the 4 in the sample above with an argument N to check bit number N.

You can find more examples in other SO answers like this one or the ones @icza linked to in a comment.

answered Feb 21, 2019 at 14:34

Eli Bendersky

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1 Comment

Henry Over a year ago

Does probably not matter in this example, but v & (1<<4) != 0 would be a bit more efficient because the shift can then be done at compile time.

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