How to capture stderr/stdout of cgo calls?

[Markus Zimmermann]

Given the following code https://play.golang.org/p/1_Q9SFIPjr one can see that stdout of Go and cgo are piped to two different descriptors.

a.) Is it possible to redirect stderr/stdout of cgo calls directly to os.Stderr/os.Stdout? Am I overlooking some argument or options of cgo so that is done automatically during compilation?
b.) Are there any hacks to accomplish the capturing involving only Go code?
c.) Is there a solution at all?

Either my Google-foo is not strong enough or nobody ever posted this on the mailing list or some blog.

Cheers,
Markus

[Uli Kunitz]

Your program doesn't work because changing os.Stdout and os.Stdin has no effect on stdio stdin and stdout. Stdio stdout will still reference fd 1 and stderr fd 2.

 

I recommend following three solutions:

1. The simplest approach could be not to use cgo at all and run the C code in a separate process. Capturing the output of a separate process is simple and can be done using the os/exec package.
2. Reset stdin and stdout using fdopen in the C code. This could be done permanently.
3. Swap file descriptors using dup2.

 

 

 

[Markus Zimmermann]

I feared that there would be no pure Go solution. Anyway, I used fdopen variant the code is here https://github.com/zimmski/osutil/blob/master/capture.go#L51 if anyone every stumbles over this thread.

Thanks for the help.

[Seb Binet]

On Thu, Aug 4, 2016 at 1:07 PM, Markus Zimmermann <zim...@gmail.com> wrote:

I feared that there would be no pure Go solution. Anyway, I used fdopen variant the code is here https://github.com/zimmski/osutil/blob/master/capture.go#L51 if anyone every stumbles over this thread.

seems to me like you're leaking C.CString("w") at:

https://github.com/zimmski/osutil/blob/master/capture.go#L64

it's allocated from cgo so you need to:

cw := C.CString("w")

defer C.free(unsafe.Pointer(cw))

// ...

-s

 

Thanks for the help.

On Monday, July 25, 2016 at 1:00:46 PM UTC+2, Uli Kunitz wrote:

Your program doesn't work because changing os.Stdout and os.Stdin has no effect on stdio stdin and stdout. Stdio stdout will still reference fd 1 and stderr fd 2.

 

I recommend following three solutions:

1. The simplest approach could be not to use cgo at all and run the C code in a separate process. Capturing the output of a separate process is simple and can be done using the os/exec package.
2. Reset stdin and stdout using fdopen in the C code. This could be done permanently.
3. Swap file descriptors using dup2.

 

 

 

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[mattn]

I guess calling freopen is better because cgo possibly having reference of stdin/stdout as FILE*.

[Markus Zimmermann]

You are right Sebastien, I fixed that. Thanks!

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[Markus Zimmermann]

The documentaton says that freopen closes the original stream. Wouldn't that mean, that it is not possible to restore the origianl stderr/stdout?