CGo error : Cannot use GoCallback (type func(_Ctype_int, unsafe.Pointer)) as type *[0]byte in function argument

[Francesco Bochicchio]

Hi all,

I'm trying to call a Go function from C using CGo. The function shall accept a char** argument. 

The following code exemplifies what I'm trying to do:

 

package main

//

// typedef void (*Callback)(int argnum, char **args   );

// void CallMe( Callback f ) { 

//      char *args[2] = { "Ciao", "mondo" };

//       f(2, args); 

//           

//}

import "C"

import "unsafe"

func GoCallback( argnum C.int, args unsafe.Pointer ) {

// Convert args and do stuff

}

func main() {

C.CallMe(GoCallback)

}

I get the following error :

Cannot use GoCallback (type func(_Ctype_int, unsafe.Pointer)) as type *[0]byte in function argument

How comes a C function pointer becomes  a *[0]byte ( which should translate to "pointer to a zero-length array of bytes" ) ??

P.S : Doing  C.CallMe(C.Callback(GoCallback)) gives the following error, which is less puzzling but does not explain much:

cannot convert GoCallback (type func(_Ctype_int, unsafe.Pointer)) to type C.Callback 

Thanks in advance for any help 

Ciao

--- 

FB

[minux]

you cannot pass a Go func to C function.

what you can do is exporting a Go function, and let a C function call it.

read golang.org/cmd/cgo for details.

[Malcolm Smith]

Correct, you can't pass a Go func to a C function.

But in cgo you get the same error if you pass a C function to a C function!

C.Mix_HookMusicFinished(C.musicFinished)

fails with "cannot use _Cfpvar_fp_musicFinished (type unsafe.Pointer) as type *[0]byte in function argument"

while

C.Mix_HookMusicFinished((*[0]byte)(C.musicFinished))

works fine.

So my question is also How come a C function pointer becomes  a *[0]byte ?

[Francesco Bochicchio]

 

Yes, I forgot to add the //export  directive ... which in turn forced me to put the C code in a separate file ... but at the

end it seems to work.

Thanks Again 

[Francesco Bochicchio]

[minux]

unsupported C pointer types are translated to *[0]byte.

[minux]

On Jan 30, 2014 7:48 AM, "Francesco Bochicchio" <bief...@gmail.com> wrote:
> Yes, I forgot to add the //export  directive ... which in turn forced me to put the C code in a separate file ... but at the
> end it seems to work.

if your C code contains only tiny wrapper functions, you can still put in the same Go file and label them as static inline.

[Malcolm Smith]

On Thursday, January 30, 2014 4:09:28 PM UTC, minux wrote:

unsupported C pointer types are translated to *[0]byte.

Right, so you are saying they are unsupported by Go, hence the need for the ugly (*[0]byte) type conversion. But as I am passing the C pointer type to a C function this doesn't matter and the code works.

[minux]

if you cast the C function pointer to *[0]byte then it could be passed to a C function that takes a function pointer.

the choice of *[0]byte is not arbitrary, it means the same as void * in C.

[Malcolm Smith]

On Thursday, January 30, 2014 8:48:55 PM UTC, minux wrote:

 the choice of *[0]byte is not arbitrary, it means the same as void * in C.

Thanks, having to use (*[0]byte) was making me nervous, but if it means void * then it makes perfect sense.