Passing NULL to a C function

[kumar_vikram]

I am new to GoLang and writing a C-GO binding.

The C Function takes an argument which is a const char *. This works fine if I pass NULL for the argument in C.

Now in my GO implementation I need to pass NULL in the C call.

func AvformatMyfunc(ctx **Context, o *OutputFormat, fo, fi string) int {
	Cformat_name := C.CString(fo)
	defer C.free(unsafe.Pointer(Cformat_name))

	Cfilename := C.CString(fi)
	defer C.free(unsafe.Pointer(Cfilename))

	return int(C.myfunc((**C.struct_AVFormatContext)(unsafe.Pointer(ctx)), (*C.struct_AVOutputFormat)(o),  Cformat_name, Cfilename))
}

This is how I am calling it in my main package.

if AvformatMyfunc(&ofmt_ctx, nil, "", os.Args[2]) != 0 {
		fmt.Printf("Unable to open output file %s\n", os.Args[2]);
		os.Exit(1);
	}

The code I have pasted above passes empty string and my c code fails. It expects NULL or some valid string to be passed as an argument. Can you please suggest how?
Thanking you in advance

[skillian (Sean Killian)]

Go can’t have null strings. If you’re OK with an empty string always being a null C pointer, you could do this:

func AvformatMyfunc(ctx **Context, o *OutputFormat, fo, fi string) int {
	var Cformat_name *C.char
	if fo != "" {
		Cformat_name = C.CString(fo)
		defer C.free(unsafe.Pointer(Cformat_name))
	}

	var Cfilename *C.char
        if fi != "" {
                Cfilename = C.CString(fi)
        	defer C.free(unsafe.Pointer(Cfilename))
        }
	return int(C.myfunc((**C.struct_AVFormatContext)(unsafe.Pointer(ctx)), (*C.struct_AVOutputFormat)(o),  Cformat_name, Cfilename))
}

[kumar_vikram]

No. Need to pass NULL only not empty string.

I solved it using *(C.char)(nil)