io.WriteString to a file panics

[Rory Campbell-Lange]

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)
}
}

[Sean Liao]

> 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)

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[Jan Mercl]

On Tue, Apr 12, 2022 at 9:53 AM Rory Campbell-Lange
<ro...@campbell-lange.net> wrote:

> 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]

Not providing the backtrace means other have to guess where the
program actually failed.

The 'output` variable is nil in the 'io.WriteString' statement, the
panic is legitimate. The variable 'output', declared within the block
after 'else' is a different variable because it is not assigned, using
'=', but declared, using ':='. Go has blocks scope very much like C
and most other languages.

-j

[Rory Campbell-Lange]

On 12/04/22, 'Sean Liao' via golang-nuts (golan...@googlegroups.com) wrote:
> > output, err := os.Create(o)
> The short variable declaration you used to create `err` also shadows
> `output`.

Thanks very much for pointing out this shadowing problem.

>
> var err error
> output, err = os.Create(o)
>

> > 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)

...