Same behaviour with go version go1.2.1 linux/amd64 on Ubuntu 14.04.

Same behavior on go1.4 windows/amd64

As I understand, you have to give the \n in the format string if a newline is expected. Then it will work regardless of the line ending.

From the fmt package documentation: Scanf, Fscanf and Sscanf require newlines in the input to match newlines in the format

However, I think the documentation is misleading: In all the scanning functions, a carriage return followed immediately by a newline is treated as a plain newline (\r\n means the same as \n).

Always, and then there is a bug, or only when the formatting string has a \n?

Working as intended, but the documentation needs to be improved.

There have been several fixes to fmt.Scanf in the recent flurry. Your example behaves consistently for me now if I rewrite it as follows, making sure the newlines appear in the format to match the input.

package main

import (
    "fmt"
    "strings"
)

func main() {
    do("2\r\n2 5\r\n4           8\r\n")
    do("2\n2 5\n4           8\n")
}

func do(s string) {
    file := strings.NewReader(s)
    var steps int
    fmt.Fscanf(file, "%d\n", &steps)
    fmt.Println("steps:", steps)


    var a, b int
    for i := 0; i < steps; i++ {
        a = -1
        b = -1
        n, err := fmt.Fscanf(file, "%d", &a)
        fmt.Println("first", n, err)
        n, err = fmt.Fscanf(file, "%d\n", &b)
        fmt.Println("second", n, err)
        fmt.Println(a, b, a+b)
    }


}