> output, err := os.Create(o)
The short variable declaration you used to create `err` also shadows
`output`.
var err error
output, err = os.Create(o)
On Tue, Apr 12, 2022 at 8:53 AM Rory Campbell-Lange <r...@campbell-lange.net>
wrote:
> I have a command line programme that can output to either stdout or a
> named file. Output to stdout, as commented out in the small example below,
> works fine, but not to a named file.
>
> Attempting to write to a named file panics on go 1.17 on Linux with:
>
> panic: runtime error: invalid memory address or nil pointer dereference
> [signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x46123d]
>
> I'm confused since both os.Stdout and os.Create are *os.File types, which
> implement the Write(b []byte) (n int, err error) method which fulfils the
> io.Writer interface. I pass around io.Writer types in my programme to
> facilitate testing.
>
> I've chosen io.WriteString because it will use StringWriter on the writer
> if the writer supports this method, which may perform better than a simple
> Write.
>
> Clearly I am missing something fundamental, or I have a stupid error in my
> programme. Help very much appreciated.
>
> Thanks
> Rory
>
> package main
>
> import (
> "io"
> "log"
> "os"
> )
>
> func main() {
>
> // o := "-" // use stdin
> o := "/tmp/output.txt" // or a file
>
> var output io.Writer
> if o == "-" {
> output = os.Stdout
> } else {
> output, err := os.Create(o)
> if err != nil {
> log.Fatalf("Could not create file %s, %s", output, err)
> }
> defer output.Close()
> }
>
> _, err := io.WriteString(output, "hello")
> if err != nil {
> log.Fatal(err)
> }
> }
>
> --
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> .
>
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