objc_msgSend gotchas

banana.m

/*

$ gcc --version
i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ clang --version
Apple clang version 1.7 (tags/Apple/clang-77) (based on LLVM 2.9svn)
Target: x86_64-apple-darwin10
Thread model: posix

gcc   -arch x86_64 banana.m -framework Foundation && ./a.out => pear
gcc   -arch i386   banana.m -framework Foundation && ./a.out => banana
clang -arch x86_64 banana.m -framework Foundation && ./a.out => banana
clang -arch i386   banana.m -framework Foundation && ./a.out => banana

*/

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import <objc/message.h>

static int magic()
{
	return 36; // (0x0000000100000ca3 - 0x0000000100000c13) / 4
}

static id pear()
{
	__asm__("nop");
	return @"pear";
}

// The method isn't variadic itself, so the IMP also shouldn't be
static id banana(id self, SEL _cmd)
{
	return @"banana";
}

@interface Fruit : NSObject
@end

@implementation Fruit

- (id) apple
{
	return @"apple";
}

@end

int main (int argc, char const *argv[])
{
	NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
	
	Method apple = class_getInstanceMethod([Fruit class], @selector(apple));
	method_setImplementation(apple, (IMP)banana);
	
	Fruit *fruit = [[[Fruit alloc] init] autorelease];
	int n = magic();

	// The following will emit identical code
	printf("%s\n", [[fruit apple] UTF8String]);
	printf("%s\n", [(*(NSString *(*)(id, SEL))objc_msgSend)(fruit, @selector(apple)) UTF8String]);

	// This will not emit correct code
	objc_msgSend(fruit, @selector(apple));
	
	[pool drain];
	return 0;
}