Getting errno after a write operation

Question

I have the following Go code which will eventually fill the disk and fail with ENOSPC (just a proof of concept). How can I determine from the err returned by os.Write that it indeed failed because of ENOSPC (so I need a way to grab errno after the write operation) ?

package main

import (
    "log"
    "os"
)

func main() {
    fd, _ := os.Create("dump.txt")

    defer fd.Close()

    for {
        buf := make([]byte, 1024)

        _, err := fd.Write(buf)
        if err != nil {
            log.Fatalf("%T %v", err, err)
        }
    }
}

EDIT: Updated the program as @FUZxxl suggested:

package main

import (
    "log"
    "os"
    "syscall"
)

func main() {
    fd, _ := os.Create("dump.txt")

    defer fd.Close()

    for {
        buf := make([]byte, 1024)

        _, err := fd.Write(buf)
        if err != nil {

            log.Printf("%T %v\n", err, err)
            errno, ok := err.(syscall.Errno)

            if ok {
                log.Println("type assert ok")
                if errno == syscall.ENOSPC {
                    log.Println("got ENOSPC")
                }
            } else {
                log.Println("type assert not ok")
            }
            break
        }
    }
}

However, I'm not getting the expected result. Here is the output:

2015/02/15 10:13:27 *os.PathError write dump.txt: no space left on device
2015/02/15 10:13:27 type assert not ok

Answer 1

File operations generally return an *os.PathError; cast err to os.PathError and use the Err field to examine the underlying cause, like this:

patherr, ok := err.(*os.PathError)
if ok && patherr.Err == syscall.ENOSPC {
    log.Println("Out of disk space!")
}