cocoa button with Golang callback

c.h

#include <Cocoa/Cocoa.h>

extern void ButtonClick(void);

@interface GoPasser : NSObject
+ (void)buttonClick:(id)sender; // this should call the cgo function defined in main.go
@end

void StartApp(void);

c.m

#include "c.h"

@implementation GoPasser
+ (void)buttonClick:(id)sender{
    ButtonClick(); // this should call the cgo function defined in main.go
}
@end

// GoPasser's buttonClick: gets called in here
void StartApp(void){
    [NSAutoreleasePool new];
    [NSApplication sharedApplication];
    [NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];

    id menubar = [[NSMenu new] autorelease];
    id appMenuItem = [[NSMenuItem new] autorelease];
    [menubar addItem:appMenuItem];
    [NSApp setMainMenu:menubar];

    id appMenu = [[NSMenu new] autorelease];
    id appName = [[NSProcessInfo processInfo] processName];
    id quitTitle = [@"Quit " stringByAppendingString:appName];
    id quitMenuItem = [[[NSMenuItem alloc] initWithTitle:quitTitle
        action:@selector(terminate:) keyEquivalent:@"q"]
            autorelease];
    [appMenu addItem:quitMenuItem];
    [appMenuItem setSubmenu:appMenu];

    NSRect buttonFrame = NSMakeRect(59, 33, 82, 32);
    NSButton *button = [[NSButton alloc] initWithFrame:buttonFrame];
    button.bezelStyle = NSRoundedBezelStyle;
    button.title = @"Click Me!";
    [button setTarget:[GoPasser class]];
    [button setAction:@selector(buttonClick:)];

    NSWindow *window = [[NSWindow alloc] initWithContentRect:NSMakeRect(0, 0, 200, 100)
        styleMask:NSTitledWindowMask backing:NSBackingStoreBuffered defer:NO];
    [window cascadeTopLeftFromPoint:NSMakePoint(20,20)];
    [window setTitle:appName];
    [window makeKeyAndOrderFront:nil];
    [window.contentView addSubview:button];

    [NSApp activateIgnoringOtherApps:YES];
    [NSApp run];
    return;
}

log.md

# command-line-arguments
duplicate symbol _StartApp in:
    $WORK/command-line-arguments/_obj/_cgo_export.o
    $WORK/command-line-arguments/_obj/main.cgo2.o
duplicate symbol _OBJC_METACLASS_$_GoPasser in:
    $WORK/command-line-arguments/_obj/_cgo_export.o
    $WORK/command-line-arguments/_obj/main.cgo2.o
duplicate symbol _OBJC_CLASS_$_GoPasser in:
    $WORK/command-line-arguments/_obj/_cgo_export.o
    $WORK/command-line-arguments/_obj/main.cgo2.o
ld: 3 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

main.go

package main

/*
#cgo CFLAGS: -x objective-c
#cgo LDFLAGS: -framework Cocoa
#include "c.m"
*/
import "C"
import "fmt"

//export ButtonClick
func ButtonClick() {
	fmt.Println("Button Click!")
}

func main() {
	C.StartApp()
}