cmd/cgo: cannot call functions with variable-argument lists.

[gopherbot]

by akidan:

A package such as the following will not compile with cgo. 
(printf is a contrived example, but this would be useful for bridging objective c
through id objc_msgSend(id self, SEL op, ...);)

--- (file foo.go)
package foo

// #include <stdio.h>
import "C"

func PrintStuff() {
    C.printf(C.CString("Hello"))
}

---

$ cgo foo.go

What is the expected output?
Program compiles and is able to call function with variable-argument list.

What do you see instead?
unexpected type: ...

What is your $GOOS?  $GOARCH?
darwin amd64

Which revision are you using?  (hg identify)
43fd9bb529f0+ tip

[rsc]

Comment 1:

I'm not sure this will ever work.  It's very hard to type check and do correctly.

Labels changed: added priority-low, removed priority-medium.

Owner changed to r...@golang.org.

Status changed to LongTerm.

[gopherbot]

Comment 2 by akidan:

Thank you for the update all the same - transparency is appreciated!

[rsc]

Comment 3:

Labels changed: added priority-someday.

[rsc]

Comment 5:

Labels changed: added repo-main.

[rsc]

Comment 6:

Adding Release=None to all Priority=Someday bugs.

Labels changed: added release-none.

[LionNatsu]

I think this should be documented as it is a long-term issue.

While wiki says:

... That is, if you want to call the C function printf() from Go code, you write C.printf(). Since variable argument methods like printf aren't supported yet (issue 975), we will wrap it in the C method "myprint":...

The documentation of cgo should be more explicit for the issue, but now it not even mention this.

[bradfitz]

@LionNatsu, please file a new bug just for documenting that. It's true that I see nothing at https://golang.org/cmd/cgo/ about variadic args.

[gopherbot]

Change https://golang.org/cl/90415 mentions this issue: cgo: Update documentation on calling C variadic functions

[bradfitz]

This would help with @progrium's MacDriver:

https://dev.to/progrium/use-mac-apis-and-build-mac-apps-with-go-ap6

/cc @ianlancetaylor

[ianlancetaylor]

n order to implement this, we need to be able to rewrite

C.printf(C.CString("%d %s"), 1, C.CString("2"))

into a call to a generated Go function

GoWrapper(C.CString("%d %s"), 1, C.CString("2"))

which means that we need to generate

func GoWrapper(p1 *C.char,  p2 int, p3 *C.char)

and similarly for a C wrapper function.

That is all fine, but in order to write these functions we need to know the types of the Go arguments. Normally we can simply use the types of the C function parameters, which we can get from the C debug info. But of course for a C variadic function we have no type information for the variadic arguments. That means that we need to use the types of the Go arguments. But currently cgo does not know those types.

So I think this has to wait for #16623.