Are variadic functions supported?

[craig65535]

The source has a comment about a special case for variadic functions -

// There is a special case when the last argument of fptr is a variadic interface (or []interface}
// it will be expanded into a call to the C function as if it had the arguments in that slice.
// This means that using arg ...interface{} is like a cast to the function with the arguments inside arg.
// This is not the same as C variadic.

Does this mean they are not supported? I made my own library in C and am unable to find a way to get my function to see any of the variadic parameters.

void printSomething(int a, const char *fmt, ...)
{
    va_list ap;
    printf("start %d\n", a);
    printf("fmt %s\n", fmt);
    va_start(ap, fmt);
    vprintf(fmt, ap);
    va_end(ap);
    printf("\n");
    printf("end %d\n", a);
}

Calling code:

    var printSomething func(int, string, []interface{})
    purego.RegisterLibFunc(&printSomething, libcraig, `printSomething`)
    printSomething(5, `%s`, []interface{}{`foo`})

This prints:

start 5
fmt %s
(null)
end 5

[craig65535]

I should mention I'm using macOS (darwin/arm64).

[TotallyGamerJet]

That is correct. Variadic functions are not supported and there are currently no plans to add them.

[craig65535]

Thanks for the response.

I think the documentation for RegisterFunc/RegisterLibFunc could be improved because that wasn't clear to me. But I'm not sure what to change it to, as there is probably some use for the current arrangement with []interface{} that I'm not aware of.

[TotallyGamerJet]

The current implementation is used to implement the Send method in the objc package. This way the user of the method can call any objc method without having to create a func variable, link it to the C function and then call it.

[lsy88]

How do I convert Go []string to C char **

[TotallyGamerJet]

Create a slice of byte. Convert each string to null terminated byte. Then pass the address of the slice to the function. So the signature would be &slice[0] and the type **byte

[lsy88]

Please understand that I may be a little unclear in the description, for the case of a C function parameter char **, what type should be used in Go?

[TotallyGamerJet]

**byte like I said

[lsy88]

I'm sorry that I can't solve this matter according to your suggestion C：

int test(char *path, char **result, int *len)

Go:

var test func(path string, result **byte, rlen *int) int32
...
purego.RegisterLibFunc(&test, lic, "test")
...
func Test(path string) {
  var lenRes int
  result := make([]*byte, 0)
  _ = test(path, &result[0], &lenRes)
}
...

I'm not sure if that's what you said, but I still haven't gotten the correct data return, The correct result data should be a string, but I only got one byte, what should I do to get the returned string

[TotallyGamerJet]

You actually got all the code right so far. You just need to turn the *byte back into a string. There's no easy way to do this atm. There's discussion in https://github.com/ebitengine/purego/issues/121 about making it more accessible. You can also just copy the code that's inside purego/internal/strings. Ofc you'll have to do that for each pointer in the slice

EDIT: Oh it's a result no a parameter. So instead of using a slice just create a *byte and take the address of that.

---

I don't really want to busy up this issue more as your question doesn't relate to variadic functions. If you have more questions about this please ask on the discord.

[JupiterRider]

@craig65535 @TotallyGamerJet My libffi wrapper supports variadic functions now. See ffi.PrepCifVar.

Unfortunately ffi doesn't support MacOS yet.