Two's complement and fmt.Printf

Question

So computers use Two\'s complement to internally represent signed integers. I.e., -5 is represented as ^5 + 1 = \"1111 1011\".

However, trying to print the binary re

Answer 1

I believe the answer lies in how the fmt module formats binary numbers, rather than the internal format.

If you take a look at fmt.integer, one of the very first actions that the function does is to convert the negative signed integer to a positive one:

   165      negative := signedness == signed && a < 0
   166      if negative {
   167          a = -a
   168      }

There's then logic to append - in front of the string that's output here.

IOW -101 really is - appended to 5 in binary.

Note: fmt.integer is called from pp.fmtInt64 in print.go, itself called from pp.printArg in the same function.

Answer 2

Unsafe pointers must be used to correctly represent negative numbers in binary format.

package main

import (
    "fmt"
    "strconv"
    "unsafe"
)

func bInt8(n int8) string {
    return strconv.FormatUint(uint64(*(*uint8)(unsafe.Pointer(&n))), 2)
}

func main() {
    fmt.Println(bInt8(-5))
}

Output

11111011